| Message ID | 1712969764-31039-21-git-send-email-wufan@linux.microsoft.com (mailing list archive) |
|---|---|
| State | Changes Requested |
| Delegated to: | Paul Moore |
| Headers | show |
| Series | Integrity Policy Enforcement LSM (IPE) | expand |
On Fri, Apr 12, 2024 at 05:56:03PM -0700, Fan Wu wrote: > diff --git a/Documentation/admin-guide/LSM/ipe.rst b/Documentation/admin-guide/LSM/ipe.rst > new file mode 100644 > index 000000000000..d2bdd6e5b662 > --- /dev/null > +++ b/Documentation/admin-guide/LSM/ipe.rst > @@ -0,0 +1,797 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +Integrity Policy Enforcement (IPE) > +================================== > + > +.. NOTE:: > + > + This is the documentation for admins, system builders, or individuals > + attempting to use IPE. If you're looking for more developer-focused > + documentation about IPE please see Documentation/security/ipe.rst > + > +Overview > +-------- > + > +Integrity Policy Enforcement (IPE) is a Linux Security Module that takes a > +complementary approach to access control. Unlike traditional access control > +mechanisms that rely on labels and paths for decision-making, IPE focuses > +on the immutable security properties inherent to system components. These > +properties are fundamental attributes or features of a system component > +that cannot be altered, ensuring a consistent and reliable basis for > +security decisions. > + > +To elaborate, in the context of IPE, system components primarily refer to > +files or the devices these files reside on. However, this is just a > +starting point. The concept of system components is flexible and can be > +extended to include new elements as the system evolves. The immutable > +properties include the origin of a file, which remains constant and > +unchangeable over time. For example, IPE policies can be crafted to trust > +files originating from the initramfs. Since initramfs is typically verified > +by the bootloader, its files are deemed trustworthy; "file is from > +initramfs" becomes an immutable property under IPE's consideration. > + > +The immutable property concept extends to the security features enabled on > +a file's origin, such as dm-verity or fs-verity, which provide a layer of > +integrity and trust. For example, IPE allows the definition of policies > +that trust files from a dm-verity protected device. dm-verity ensures the > +integrity of an entire device by providing a verifiable and immutable state > +of its contents. Similarly, fs-verity offers filesystem-level integrity > +checks, allowing IPE to enforce policies that trust files protected by > +fs-verity. These two features cannot be turned off once established, so > +they are considered immutable properties. These examples demonstrate how > +IPE leverages immutable properties, such as a file's origin and its > +integrity protection mechanisms, to make access control decisions. > + > +For the IPE policy, specifically, it grants the ability to enforce > +stringent access controls by assessing security properties against > +reference values defined within the policy. This assessment can be based on > +the existence of a security property (e.g., verifying if a file originates > +from initramfs) or evaluating the internal state of an immutable security > +property. The latter includes checking the roothash of a dm-verity > +protected device, determining whether dm-verity possesses a valid > +signature, assessing the digest of a fs-verity protected file, or > +determining whether fs-verity possesses a valid built-in signature. This > +nuanced approach to policy enforcement enables a highly secure and > +customizable system defense mechanism, tailored to specific security > +requirements and trust models. > + > +To enable IPE, ensure that ``CONFIG_SECURITY_IPE`` (under > +:menuselection:`Security -> Integrity Policy Enforcement (IPE)`) config > +option is enabled. > + > +Use Cases > +--------- > + > +IPE works best in fixed-function devices: devices in which their purpose > +is clearly defined and not supposed to be changed (e.g. network firewall > +device in a data center, an IoT device, etcetera), where all software and > +configuration is built and provisioned by the system owner. > + > +IPE is a long-way off for use in general-purpose computing: the Linux > +community as a whole tends to follow a decentralized trust model (known as > +the web of trust), which IPE has no support for it yet. Instead, IPE > +supports PKI (public key infrastructure), which generally designates a > +set of trusted entities that provide a measure of absolute trust. > + > +Additionally, while most packages are signed today, the files inside > +the packages (for instance, the executables), tend to be unsigned. This > +makes it difficult to utilize IPE in systems where a package manager is > +expected to be functional, without major changes to the package manager > +and ecosystem behind it. > + > +DIGLIM [#diglim]_ is a system that when combined with IPE, could be used to > +enable and support general-purpose computing use cases. > + > +Known Limitations > +----------------- > + > +IPE cannot verify the integrity of anonymous executable memory, such as > +the trampolines created by gcc closures and libffi (<3.4.2), or JIT'd code. > +Unfortunately, as this is dynamically generated code, there is no way > +for IPE to ensure the integrity of this code to form a trust basis. In all > +cases, the return result for these operations will be whatever the admin > +configures as the ``DEFAULT`` action for ``EXECUTE``. > + > +IPE cannot verify the integrity of programs written in interpreted > +languages when these scripts are invoked by passing these program files > +to the interpreter. This is because the way interpreters execute these > +files; the scripts themselves are not evaluated as executable code > +through one of IPE's hooks, but they are merely text files that are read > +(as opposed to compiled executables) [#interpreters]_. > + > +Threat Model > +------------ > + > +IPE specifically targets the risk of tampering with user-space executable > +code after the kernel has initially booted, including the kernel modules > +loaded from userspace via ``modprobe`` or ``insmod``. > + > +To illustrate, consider a scenario where an untrusted binary, possibly > +malicious, is downloaded along with all necessary dependencies, including a > +loader and libc. The primary function of IPE in this context is to prevent > +the execution of such binaries and their dependencies. > + > +IPE achieves this by verifying the integrity and authenticity of all > +executable code before allowing them to run. It conducts a thorough > +check to ensure that the code's integrity is intact and that they match an > +authorized reference value (digest, signature, etc) as per the defined > +policy. If a binary does not pass this verification process, either > +because its integrity has been compromised or it does not meet the > +authorization criteria, IPE will deny its execution. Additionally, IPE > +generates audit logs which may be utilized to detect and analyze failures > +resulting from policy violation. > + > +Tampering threat scenarios include modification or replacement of > +executable code by a range of actors including: > + > +- Actors with physical access to the hardware > +- Actors with local network access to the system > +- Actors with access to the deployment system > +- Compromised internal systems under external control > +- Malicious end users of the system > +- Compromised end users of the system > +- Remote (external) compromise of the system > + > +IPE does not mitigate threats arising from malicious but authorized > +developers (with access to a signing certificate), or compromised > +developer tools used by them (i.e. return-oriented programming attacks). > +Additionally, IPE draws hard security boundary between userspace and > +kernelspace. As a result, IPE does not provide any protections against a > +kernel level exploit, and a kernel-level exploit can disable or tamper > +with IPE's protections. > + > +Policy > +------ > + > +IPE policy is a plain-text [#devdoc]_ policy composed of multiple statements > +over several lines. There is one required line, at the top of the > +policy, indicating the policy name, and the policy version, for > +instance:: > + > + policy_name=Ex_Policy policy_version=0.0.0 > + > +The policy name is a unique key identifying this policy in a human > +readable name. This is used to create nodes under securityfs as well as > +uniquely identify policies to deploy new policies vs update existing > +policies. > + > +The policy version indicates the current version of the policy (NOT the > +policy syntax version). This is used to prevent rollback of policy to > +potentially insecure previous versions of the policy. > + > +The next portion of IPE policy are rules. Rules are formed by key=value > +pairs, known as properties. IPE rules require two properties: ``action``, > +which determines what IPE does when it encounters a match against the > +rule, and ``op``, which determines when the rule should be evaluated. > +The ordering is significant, a rule must start with ``op``, and end with > +``action``. Thus, a minimal rule is:: > + > + op=EXECUTE action=ALLOW > + > +This example will allow any execution. Additional properties are used to > +assess immutable security properties about the files being evaluated. > +These properties are intended to be descriptions of systems within the > +kernel that can provide a measure of integrity verification, such that IPE > +can determine the trust of the resource based on the value of the property. > + > +Rules are evaluated top-to-bottom. As a result, any revocation rules, > +or denies should be placed early in the file to ensure that these rules > +are evaluated before a rule with ``action=ALLOW``. > + > +IPE policy supports comments. The character '#' will function as a > +comment, ignoring all characters to the right of '#' until the newline. > + > +The default behavior of IPE evaluations can also be expressed in policy, > +through the ``DEFAULT`` statement. This can be done at a global level, > +or a per-operation level:: > + > + # Global > + DEFAULT action=ALLOW > + > + # Operation Specific > + DEFAULT op=EXECUTE action=ALLOW > + > +A default must be set for all known operations in IPE. If you want to > +preserve older policies being compatible with newer kernels that can introduce > +new operations, set a global default of ``ALLOW``, then override the > +defaults on a per-operation basis (as above). > + > +With configurable policy-based LSMs, there's several issues with > +enforcing the configurable policies at startup, around reading and > +parsing the policy: > + > +1. The kernel *should* not read files from userspace, so directly reading > + the policy file is prohibited. > +2. The kernel command line has a character limit, and one kernel module > + should not reserve the entire character limit for its own > + configuration. > +3. There are various boot loaders in the kernel ecosystem, so handing > + off a memory block would be costly to maintain. > + > +As a result, IPE has addressed this problem through a concept of a "boot > +policy". A boot policy is a minimal policy which is compiled into the > +kernel. This policy is intended to get the system to a state where > +userspace is set up and ready to receive commands, at which point a more > +complex policy can be deployed via securityfs. The boot policy can be > +specified via ``SECURITY_IPE_BOOT_POLICY`` config option, which accepts > +a path to a plain-text version of the IPE policy to apply. This policy > +will be compiled into the kernel. If not specified, IPE will be disabled > +until a policy is deployed and activated through securityfs. > + > +Deploying Policies > +~~~~~~~~~~~~~~~~~~ > + > +Policies can be deployed from userspace through securityfs. These policies > +are signed through the PKCS#7 message format to enforce some level of > +authorization of the policies (prohibiting an attacker from gaining > +unconstrained root, and deploying an "allow all" policy). These > +policies must be signed by a certificate that chains to the > +``SYSTEM_TRUSTED_KEYRING``. With openssl, the policy can be signed by:: > + > + openssl smime -sign \ > + -in "$MY_POLICY" \ > + -signer "$MY_CERTIFICATE" \ > + -inkey "$MY_PRIVATE_KEY" \ > + -noattr \ > + -nodetach \ > + -nosmimecap \ > + -outform der \ > + -out "$MY_POLICY.p7b" > + > +Deploying the policies is done through securityfs, through the > +``new_policy`` node. To deploy a policy, simply cat the file into the > +securityfs node:: > + > + cat "$MY_POLICY.p7b" > /sys/kernel/security/ipe/new_policy > + > +Upon success, this will create one subdirectory under > +``/sys/kernel/security/ipe/policies/``. The subdirectory will be the > +``policy_name`` field of the policy deployed, so for the example above, > +the directory will be ``/sys/kernel/security/ipe/policies/Ex_Policy``. > +Within this directory, there will be seven files: ``pkcs7``, ``policy``, > +``name``, ``version``, ``active``, ``update``, and ``delete``. > + > +The ``pkcs7`` file is read-only. Reading it returns the raw PKCS#7 data > +that was provided to the kernel, representing the policy. If the policy being > +read is the boot policy, this will return ``ENOENT``, as it is not signed. > + > +The ``policy`` file is read only. Reading it returns the PKCS#7 inner > +content of the policy, which will be the plain text policy. > + > +The ``active`` file is used to set a policy as the currently active policy. > +This file is rw, and accepts a value of ``"1"`` to set the policy as active. > +Since only a single policy can be active at one time, all other policies > +will be marked inactive. The policy being marked active must have a policy > +version greater or equal to the currently-running version. > + > +The ``update`` file is used to update a policy that is already present > +in the kernel. This file is write-only and accepts a PKCS#7 signed > +policy. Two checks will always be performed on this policy: First, the > +``policy_names`` must match with the updated version and the existing > +version. Second the updated policy must have a policy version greater than > +or equal to the currently-running version. This is to prevent rollback attacks. > + > +The ``delete`` file is used to remove a policy that is no longer needed. > +This file is write-only and accepts a value of ``1`` to delete the policy. > +On deletion, the securityfs node representing the policy will be removed. > +However, delete the current active policy is not allowed and will return > +an operation not permitted error. > + > +Similarly, writing to both ``update`` and ``new_policy`` could result in > +bad message(policy syntax error) or file exists error. The latter error happens > +when trying to deploy a policy with a ``policy_name`` while the kernel already > +has a deployed policy with the same ``policy_name``. > + > +Deploying a policy will *not* cause IPE to start enforcing the policy. IPE will > +only enforce the policy marked active. Note that only one policy can be active > +at a time. > + > +Once deployment is successful, the policy can be activated, by writing file > +``/sys/kernel/security/ipe/policies/$policy_name/active``. > +For example, the ``Ex_Policy`` can be activated by:: > + > + echo 1 > "/sys/kernel/security/ipe/policies/Ex_Policy/active" > + > +From above point on, ``Ex_Policy`` is now the enforced policy on the > +system. > + > +IPE also provides a way to delete policies. This can be done via the > +``delete`` securityfs node, > +``/sys/kernel/security/ipe/policies/$policy_name/delete``. > +Writing ``1`` to that file deletes the policy:: > + > + echo 1 > "/sys/kernel/security/ipe/policies/$policy_name/delete" > + > +There is only one requirement to delete a policy: the policy being deleted > +must be inactive. > + > +.. NOTE:: > + > + If a traditional MAC system is enabled (SELinux, apparmor, smack), all > + writes to ipe's securityfs nodes require ``CAP_MAC_ADMIN``. > + > +Modes > +~~~~~ > + > +IPE supports two modes of operation: permissive (similar to SELinux's > +permissive mode) and enforced. In permissive mode, all events are > +checked and policy violations are logged, but the policy is not really > +enforced. This allows users to test policies before enforcing them. > + > +The default mode is enforce, and can be changed via the kernel command > +line parameter ``ipe.enforce=(0|1)``, or the securityfs node > +``/sys/kernel/security/ipe/enforce``. > + > +.. NOTE:: > + > + If a traditional MAC system is enabled (SELinux, apparmor, smack, etcetera), > + all writes to ipe's securityfs nodes require ``CAP_MAC_ADMIN``. > + > +Audit Events > +~~~~~~~~~~~~ > + > +1420 AUDIT_IPE_ACCESS > +^^^^^^^^^^^^^^^^^^^^^ > +Event Examples:: > + > + type=1420 audit(1653364370.067:61): ipe_op=EXECUTE ipe_hook=MMAP enforcing=1 pid=2241 comm="ld-linux.so" path="/deny/lib/libc.so.6" dev="sda2" ino=14549020 rule="DEFAULT action=DENY" > + type=1300 audit(1653364370.067:61): SYSCALL arch=c000003e syscall=9 success=no exit=-13 a0=7f1105a28000 a1=195000 a2=5 a3=812 items=0 ppid=2219 pid=2241 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm="ld-linux.so" exe="/tmp/ipe-test/lib/ld-linux.so" subj=unconfined key=(null) > + type=1327 audit(1653364370.067:61): 707974686F6E3300746573742F6D61696E2E7079002D6E00 > + > + type=1420 audit(1653364735.161:64): ipe_op=EXECUTE ipe_hook=MMAP enforcing=1 pid=2472 comm="mmap_test" path=? dev=? ino=? rule="DEFAULT action=DENY" > + type=1300 audit(1653364735.161:64): SYSCALL arch=c000003e syscall=9 success=no exit=-13 a0=0 a1=1000 a2=4 a3=21 items=0 ppid=2219 pid=2472 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm="mmap_test" exe="/root/overlake_test/upstream_test/vol_fsverity/bin/mmap_test" subj=unconfined key=(null) > + type=1327 audit(1653364735.161:64): 707974686F6E3300746573742F6D61696E2E7079002D6E00 > + > +This event indicates that IPE made an access control decision; the IPE > +specific record (1420) is always emitted in conjunction with a > +``AUDITSYSCALL`` record. > + > +Determining whether IPE is in permissive or enforced mode can be derived > +from ``success`` property and exit code of the ``AUDITSYSCALL`` record. > + > + > +Field descriptions: > + > ++-----------+------------+-----------+---------------------------------------------------------------------------------+ > +| Field | Value Type | Optional? | Description of Value | > ++===========+============+===========+=================================================================================+ > +| ipe_op | string | No | The IPE operation name associated with the log | > ++-----------+------------+-----------+---------------------------------------------------------------------------------+ > +| ipe_hook | string | No | The name of the LSM hook that triggered the IPE event | > ++-----------+------------+-----------+---------------------------------------------------------------------------------+ > +| enforcing | integer | No | The current IPE enforcing state 1 is in enforcing mode, 0 is in permissive mode | > ++-----------+------------+-----------+---------------------------------------------------------------------------------+ > +| pid | integer | No | The pid of the process that triggered the IPE event. | > ++-----------+------------+-----------+---------------------------------------------------------------------------------+ > +| comm | string | No | The command line program name of the process that triggered the IPE event | > ++-----------+------------+-----------+---------------------------------------------------------------------------------+ > +| path | string | Yes | The absolute path to the evaluated file | > ++-----------+------------+-----------+---------------------------------------------------------------------------------+ > +| ino | integer | Yes | The inode number of the evaluated file | > ++-----------+------------+-----------+---------------------------------------------------------------------------------+ > +| dev | string | Yes | The device name of the evaluated file, e.g. vda | > ++-----------+------------+-----------+---------------------------------------------------------------------------------+ > +| rule | string | No | The matched policy rule | > ++-----------+------------+-----------+---------------------------------------------------------------------------------+ > + > +1421 AUDIT_IPE_CONFIG_CHANGE > +^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > + > +Event Example:: > + > + type=1421 audit(1653425583.136:54): old_active_pol_name="Allow_All" old_active_pol_version=0.0.0 old_policy_digest=sha256:E3B0C44298FC1C149AFBF4C8996FB92427AE41E4649B934CA495991B7852B855 new_active_pol_name="boot_verified" new_active_pol_version=0.0.0 new_policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD8EED7B8F4DB auid=4294967295 ses=4294967295 lsm=ipe res=1 > + type=1300 audit(1653425583.136:54): SYSCALL arch=c000003e syscall=1 success=yes exit=2 a0=3 a1=5596fcae1fb0 a2=2 a3=2 items=0 ppid=184 pid=229 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=4294967295 comm="python3" exe="/usr/bin/python3.10" key=(null) > + type=1327 audit(1653425583.136:54): PROCTITLE proctitle=707974686F6E3300746573742F6D61696E2E7079002D66002E2 > + > +This event indicates that IPE switched the active poliy from one to another > +along with the version and the hash digest of the two policies. > +Note IPE can only have one policy active at a time, all access decision > +evaluation is based on the current active policy. > +The normal procedure to deploy a new policy is loading the policy to deploy > +into the kernel first, then switch the active policy to it. > + > +This record will always be emitted in conjunction with a ``AUDITSYSCALL`` record for the ``write`` syscall. > + > +Field descriptions: > + > ++------------------------+------------+-----------+---------------------------------------------------+ > +| Field | Value Type | Optional? | Description of Value | > ++========================+============+===========+===================================================+ > +| old_active_pol_name | string | No | The name of previous active policy | > ++------------------------+------------+-----------+---------------------------------------------------+ > +| old_active_pol_version | string | No | The version of previous active policy | > ++------------------------+------------+-----------+---------------------------------------------------+ > +| old_policy_digest | string | No | The hash of previous active policy | > ++------------------------+------------+-----------+---------------------------------------------------+ > +| new_active_pol_name | string | No | The name of current active policy | > ++------------------------+------------+-----------+---------------------------------------------------+ > +| new_active_pol_version | string | No | The version of current active policy | > ++------------------------+------------+-----------+---------------------------------------------------+ > +| new_policy_digest | string | No | The hash of current active policy | > ++------------------------+------------+-----------+---------------------------------------------------+ > +| auid | integer | No | The login user ID | > ++------------------------+------------+-----------+---------------------------------------------------+ > +| ses | integer | No | The login session ID | > ++------------------------+------------+-----------+---------------------------------------------------+ > +| lsm | string | No | The lsm name associated with the event | > ++------------------------+------------+-----------+---------------------------------------------------+ > +| res | integer | No | The result of the audited operation(success/fail) | > ++------------------------+------------+-----------+---------------------------------------------------+ > + > +1422 AUDIT_IPE_POLICY_LOAD > +^^^^^^^^^^^^^^^^^^^^^^^^^^ > + > +Event Example:: > + > + type=1422 audit(1653425529.927:53): policy_name="boot_verified" policy_version=0.0.0 policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD8EED7B8F4DB auid=4294967295 ses=4294967295 lsm=ipe res=1 > + type=1300 audit(1653425529.927:53): arch=c000003e syscall=1 success=yes exit=2567 a0=3 a1=5596fcae1fb0 a2=a07 a3=2 items=0 ppid=184 pid=229 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=4294967295 comm="python3" exe="/usr/bin/python3.10" key=(null) > + type=1327 audit(1653425529.927:53): PROCTITLE proctitle=707974686F6E3300746573742F6D61696E2E7079002D66002E2E > + > +This record indicates a new policy has been loaded into the kernel with the policy name, policy version and policy hash. > + > +This record will always be emitted in conjunction with a ``AUDITSYSCALL`` record for the ``write`` syscall. > + > +Field descriptions: > + > ++----------------+------------+-----------+---------------------------------------------------+ > +| Field | Value Type | Optional? | Description of Value | > ++================+============+===========+===================================================+ > +| policy_name | string | No | The policy_name | > ++----------------+------------+-----------+---------------------------------------------------+ > +| policy_version | string | No | The policy_version | > ++----------------+------------+-----------+---------------------------------------------------+ > +| policy_digest | string | No | The policy hash | > ++----------------+------------+-----------+---------------------------------------------------+ > +| auid | integer | No | The login user ID | > ++----------------+------------+-----------+---------------------------------------------------+ > +| ses | integer | No | The login session ID | > ++----------------+------------+-----------+---------------------------------------------------+ > +| lsm | string | No | The lsm name associated with the event | > ++----------------+------------+-----------+---------------------------------------------------+ > +| res | integer | No | The result of the audited operation(success/fail) | > ++----------------+------------+-----------+---------------------------------------------------+ > + > + > +1404 AUDIT_MAC_STATUS > +^^^^^^^^^^^^^^^^^^^^^ > + > +Event Examples:: > + > + type=1404 audit(1653425689.008:55): enforcing=0 old_enforcing=1 auid=4294967295 ses=4294967295 enabled=1 old-enabled=1 lsm=ipe res=1 > + type=1300 audit(1653425689.008:55): arch=c000003e syscall=1 success=yes exit=2 a0=1 a1=55c1065e5c60 a2=2 a3=0 items=0 ppid=405 pid=441 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=) > + type=1327 audit(1653425689.008:55): proctitle="-bash" > + > + type=1404 audit(1653425689.008:55): enforcing=1 old_enforcing=0 auid=4294967295 ses=4294967295 enabled=1 old-enabled=1 lsm=ipe res=1 > + type=1300 audit(1653425689.008:55): arch=c000003e syscall=1 success=yes exit=2 a0=1 a1=55c1065e5c60 a2=2 a3=0 items=0 ppid=405 pid=441 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=) > + type=1327 audit(1653425689.008:55): proctitle="-bash" > + > +This record will always be emitted in conjunction with a ``AUDITSYSCALL`` record for the ``write`` syscall. > + > +Field descriptions: > + > ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ > +| Field | Value Type | Optional? | Description of Value | > ++===============+============+===========+=================================================================================================+ > +| enforcing | integer | No | The enforcing state IPE is being switched to, 1 is in enforcing mode, 0 is in permissive mode | > ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ > +| old_enforcing | integer | No | The enforcing state IPE is being switched from, 1 is in enforcing mode, 0 is in permissive mode | > ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ > +| auid | integer | No | The login user ID | > ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ > +| ses | integer | No | The login session ID | > ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ > +| enabled | integer | No | The new TTY audit enabled setting | > ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ > +| old-enabled | integer | No | The old TTY audit enabled setting | > ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ > +| lsm | string | No | The lsm name associated with the event | > ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ > +| res | integer | No | The result of the audited operation(success/fail) | > ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ > + > + > +Success Auditing > +^^^^^^^^^^^^^^^^ > + > +IPE supports success auditing. When enabled, all events that pass IPE > +policy and are not blocked will emit an audit event. This is disabled by > +default, and can be enabled via the kernel command line > +``ipe.success_audit=(0|1)`` or > +``/sys/kernel/security/ipe/success_audit`` securityfs file. > + > +This is *very* noisy, as IPE will check every userspace binary on the > +system, but is useful for debugging policies. > + > +.. NOTE:: > + > + If a traditional MAC system is enabled (SELinux, apparmor, smack, etcetera), > + all writes to ipe's securityfs nodes require ``CAP_MAC_ADMIN``. > + > +Properties > +---------- > + > +As explained above, IPE properties are ``key=value`` pairs expressed in IPE > +policy. Two properties are built-into the policy parser: 'op' and 'action'. > +The other properties are used to restrict immutable security properties > +about the files being evaluated. Currently those properties are: > +'``boot_verified``', '``dmverity_signature``', '``dmverity_roothash``', > +'``fsverity_signature``', '``fsverity_digest``'. A description of all > +properties supported by IPE are listed below: > + > +op > +~~ > + > +Indicates the operation for a rule to apply to. Must be in every rule, > +as the first token. IPE supports the following operations: > + > + ``EXECUTE`` > + > + Pertains to any file attempting to be executed, or loaded as an > + executable. > + > + ``FIRMWARE``: > + > + Pertains to firmware being loaded via the firmware_class interface. > + This covers both the preallocated buffer and the firmware file > + itself. > + > + ``KMODULE``: > + > + Pertains to loading kernel modules via ``modprobe`` or ``insmod``. > + > + ``KEXEC_IMAGE``: > + > + Pertains to kernel images loading via ``kexec``. > + > + ``KEXEC_INITRAMFS`` > + > + Pertains to initrd images loading via ``kexec --initrd``. > + > + ``POLICY``: > + > + Controls loading policies via reading a kernel-space initiated read. > + > + An example of such is loading IMA policies by writing the path > + to the policy file to ``$securityfs/ima/policy`` > + > + ``X509_CERT``: > + > + Controls loading IMA certificates through the Kconfigs, > + ``CONFIG_IMA_X509_PATH`` and ``CONFIG_EVM_X509_PATH``. > + > +action > +~~~~~~ > + > + Determines what IPE should do when a rule matches. Must be in every > + rule, as the final clause. Can be one of: > + > + ``ALLOW``: > + > + If the rule matches, explicitly allow access to the resource to proceed > + without executing any more rules. > + > + ``DENY``: > + > + If the rule matches, explicitly prohibit access to the resource to > + proceed without executing any more rules. > + > +boot_verified > +~~~~~~~~~~~~~ > + > + This property can be utilized for authorization of files from initramfs. > + The format of this property is:: > + > + boot_verified=(TRUE|FALSE) > + > + > + .. WARNING:: > + > + This property will trust files from initramfs(rootfs). It should > + only be used during early booting stage. Before mounting the real > + rootfs on top of the initramfs, initramfs script will recursively > + remove all files and directories on the initramfs. This is typically > + implemented by using switch_root(8) [#switch_root]_. Therefore the > + initramfs will be empty and not accessible after the real > + rootfs takes over. It is advised to switch to a different policy > + that doesn't rely on the property after this point. > + This ensures that the trust policies remain relevant and effective > + throughout the system's operation. > + > +dmverity_roothash > +~~~~~~~~~~~~~~~~~ > + > + This property can be utilized for authorization or revocation of > + specific dm-verity volumes, identified via its root hash. It has a > + dependency on the DM_VERITY module. This property is controlled by > + the ``IPE_PROP_DM_VERITY`` config option, it will be automatically > + selected when ``IPE_SECURITY`` , ``DM_VERITY`` and > + ``DM_VERITY_VERIFY_ROOTHASH_SIG`` are all enabled. > + The format of this property is:: > + > + dmverity_roothash=DigestName:HexadecimalString > + > + The supported DigestNames for dmverity_roothash are [#dmveritydigests]_ [#securedigest]_ : > + > + + blake2b-512 > + + blake2s-256 > + + sha1 > + + sha256 > + + sha384 > + + sha512 > + + sha3-224 > + + sha3-256 > + + sha3-384 > + + sha3-512 > + + md4 > + + md5 > + + sm3 > + + rmd160 > + > +dmverity_signature > +~~~~~~~~~~~~~~~~~~ > + > + This property can be utilized for authorization of all dm-verity > + volumes that have a signed roothash that validated by a keyring > + specified by dm-verity's configuration, either the system trusted > + keyring, or the secondary keyring. It depends on > + ``DM_VERITY_VERIFY_ROOTHASH_SIG`` config option and is controlled by > + the ``IPE_PROP_DM_VERITY`` config option, it will be automatically > + selected when ``IPE_SECURITY``, ``DM_VERITY`` and > + ``DM_VERITY_VERIFY_ROOTHASH_SIG`` are all enabled. > + The format of this property is:: > + > + dmverity_signature=(TRUE|FALSE) > + > +fsverity_digest > +~~~~~~~~~~~~~~~ > + > + This property can be utilized for authorization or revocation of > + specific fsverity enabled file, identified via its fsverity digest. > + It depends on ``FS_VERITY`` config option and is controlled by > + ``CONFIG_IPE_PROP_FS_VERITY``. The format of this property is:: > + > + fsverity_digest=DigestName:HexadecimalString > + > + The supported DigestNames for fsverity_roothash are [#fsveritydigest]_ [#securedigest]_ : > + > + + sha256 > + + sha512 > + > +fsverity_signature > +~~~~~~~~~~~~~~~~~~ > + > + This property is used to authorize all fs-verity enabled files that have > + been verified by fs-verity's built-in signature mechanism. The signature > + verification relies on a key stored within the ".fs-verity" keyring. It > + depends on ``CONFIG_FS_VERITY_BUILTIN_SIGNATURES`` and it is controlled by > + the Kconfig ``CONFIG_IPE_PROP_FS_VERITY``. The format of this > + property is:: > + > + fsverity_signature=(TRUE|FALSE) > + > +Policy Examples > +--------------- > + > +Allow all > +~~~~~~~~~ > + > +:: > + > + policy_name=Allow_All policy_version=0.0.0 > + DEFAULT action=ALLOW > + > +Allow only initramfs > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +:: > + > + policy_name=Allow_All_Initramfs policy_version=0.0.0 > + DEFAULT action=DENY > + > + op=EXECUTE boot_verified=TRUE action=ALLOW > + > +Allow any signed dm-verity volume and the initramfs > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +:: > + > + policy_name=AllowSignedAndInitramfs policy_version=0.0.0 > + DEFAULT action=DENY > + > + op=EXECUTE boot_verified=TRUE action=ALLOW > + op=EXECUTE dmverity_signature=TRUE action=ALLOW > + > +Prohibit execution from a specific dm-verity volume > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +:: > + > + policy_name=AllowSignedAndInitramfs policy_version=0.0.0 > + DEFAULT action=DENY > + > + op=EXECUTE dmverity_roothash=sha256:cd2c5bae7c6c579edaae4353049d58eb5f2e8be0244bf05345bc8e5ed257baff action=DENY > + > + op=EXECUTE boot_verified=TRUE action=ALLOW > + op=EXECUTE dmverity_signature=TRUE action=ALLOW > + > +Allow only a specific dm-verity volume > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +:: > + > + policy_name=AllowSignedAndInitramfs policy_version=0.0.0 > + DEFAULT action=DENY > + > + op=EXECUTE dmverity_roothash=sha256:401fcec5944823ae12f62726e8184407a5fa9599783f030dec146938 action=ALLOW > + > +Allow any signed fs-verity file > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +:: > + > + policy_name=AllowSignedFSVerity policy_version=0.0.0 > + DEFAULT action=DENY > + > + op=EXECUTE fsverity_signature=TRUE action=ALLOW > + > +Prohibit execution of a specific fs-verity file > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +:: > + > + policy_name=ProhibitSpecificFSVF policy_version=0.0.0 > + DEFAULT action=DENY > + > + op=EXECUTE fsverity_digest=sha256:fd88f2b8824e197f850bf4c5109bea5cf0ee38104f710843bb72da796ba5af9e action=DENY > + op=EXECUTE boot_verified=TRUE action=ALLOW > + op=EXECUTE dmverity_signature=TRUE action=ALLOW > + > +Additional Information > +---------------------- > + > +- `Github Repository <https://github.com/microsoft/ipe>`_ > +- Documentation/security/ipe.rst > + > +FAQ > +--- > + > +Q: > + What's the difference between other LSMs which provide a measure of > + trust-based access control? > + > +A: > + > + In general, there's two other LSMs that can provide similar functionality: > + IMA, and Loadpin. > + > + IMA and IPE are functionally very similar. The significant difference between > + the two is the policy. [#devdoc]_ > + > + Loadpin and IPE differ fairly dramatically, as Loadpin only covers the IPE's > + kernel read operations, whereas IPE is capable of controlling execution > + on top of kernel read. The trust model is also different; Loadpin roots its > + trust in the initial super-block, whereas trust in IPE is stemmed from kernel > + itself (via ``SYSTEM_TRUSTED_KEYS``). > + > +----------- > + > +.. [#diglim] https://lore.kernel.org/bpf/4d6932e96d774227b42721d9f645ba51@huawei.com/T/ > + > +.. [#interpreters] There is `some interest in solving this issue <https://lore.kernel.org/lkml/20220321161557.495388-1-mic@digikod.net/>`_. > + > +.. [#devdoc] Please see Documentation/security/ipe.rst for more on this topic. > + > +.. [#switch_root] https://man7.org/linux/man-pages/man8/switch_root.8.html > + > +.. [#fsveritydigest] These hash algorithms are based on values accepted by fsverity-utils; > + IPE does not impose any restrictions on the digest algorithm itself; > + thus, this list may be out of date. > + > +.. [#dmveritydigests] These hash algorithms are based on values accepted by dm-verity, > + specifically ``crypto_alloc_ahash`` in ``verity_ctr``; ``veritysetup`` > + does support more algorithms than the list above. IPE does not impose > + any restrictions on the digest algorithm itself; thus, this list > + may be out of date. > + > +.. [#securedigest] Please ensure you are using cryptographically secure hash functions; > + just because something is *supported* does not mean it is *secure*. > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > index 70046a019d42..7b7a24a59747 100644 > --- a/Documentation/admin-guide/kernel-parameters.txt > +++ b/Documentation/admin-guide/kernel-parameters.txt > @@ -2321,6 +2321,18 @@ > ipcmni_extend [KNL,EARLY] Extend the maximum number of unique System V > IPC identifiers from 32,768 to 16,777,216. > > + ipe.enforce= [IPE] > + Format: <bool> > + Determine whether IPE starts in permissive (0) or > + enforce (1) mode. The default is enforce. > + > + ipe.success_audit= > + [IPE] > + Format: <bool> > + Start IPE with success auditing enabled, emitting > + an audit event when a binary is allowed. The default > + is 0. > + > irqaffinity= [SMP] Set the default irq affinity mask > The argument is a cpu list, as described above. > > diff --git a/Documentation/filesystems/fsverity.rst b/Documentation/filesystems/fsverity.rst > index 362b7a5dc300..46ab280e1b13 100644 > --- a/Documentation/filesystems/fsverity.rst > +++ b/Documentation/filesystems/fsverity.rst > @@ -92,7 +92,9 @@ authenticating fs-verity file hashes include: > "IPE policy" specifically allows for the authorization of fs-verity > files using properties ``fsverity_digest`` for identifying > files by their verity digest, and ``fsverity_signature`` to authorize > - files with a verified fs-verity's built-in signature. > + files with a verified fs-verity's built-in signature. For > + details on configuring IPE policies and understanding its operational > + modes, please refer to Documentation/admin-guide/LSM/ipe.rst. > > - Trusted userspace code in combination with `Built-in signature > verification`_. This approach should be used only with great care. > @@ -508,6 +510,7 @@ be carefully considered before using them: > files with a verified fs-verity builtin signature to perform certain > operations, such as execution. Note that IPE doesn't require > fs.verity.require_signatures=1. > + Please refer to Documentation/admin-guide/LSM/ipe.rst for more details. > > - A file's builtin signature can only be set at the same time that > fs-verity is being enabled on the file. Changing or deleting the > diff --git a/Documentation/security/index.rst b/Documentation/security/index.rst > index 59f8fc106cb0..3e0a7114a862 100644 > --- a/Documentation/security/index.rst > +++ b/Documentation/security/index.rst > @@ -19,3 +19,4 @@ Security Documentation > digsig > landlock > secrets/index > + ipe > diff --git a/Documentation/security/ipe.rst b/Documentation/security/ipe.rst > new file mode 100644 > index 000000000000..674827982a72 > --- /dev/null > +++ b/Documentation/security/ipe.rst > @@ -0,0 +1,444 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +Integrity Policy Enforcement (IPE) - Kernel Documentation > +========================================================= > + > +.. NOTE:: > + > + This is documentation targeted at developers, instead of administrators. > + If you're looking for documentation on the usage of IPE, please see > + Documentation/admin-guide/LSM/ipe.rst > + > +Historical Motivation > +--------------------- > + > +The original issue that prompted IPE's implementation was the creation > +of a locked-down system. This system would be born-secure, and have > +strong integrity guarantees over both the executable code, and specific > +*data files* on the system, that were critical to its function. These > +specific data files would not be readable unless they passed integrity > +policy. A mandatory access control system would be present, and > +as a result, xattrs would have to be protected. This lead to a selection > +of what would provide the integrity claims. At the time, there were two > +main mechanisms considered that could guarantee integrity for the system > +with these requirements: > + > + 1. IMA + EVM Signatures > + 2. DM-Verity > + > +Both options were carefully considered, however the choice to use DM-Verity > +over IMA+EVM as the *integrity mechanism* in the original use case of IPE > +was due to three main reasons: > + > + 1. Protection of additional attack vectors: > + > + * With IMA+EVM, without an encryption solution, the system is vulnerable > + to offline attack against the aforementioned specific data files. > + > + Unlike executables, read operations (like those on the protected data > + files), cannot be enforced to be globally integrity verified. This means > + there must be some form of selector to determine whether a read should > + enforce the integrity policy, or it should not. > + > + At the time, this was done with mandatory access control labels. An IMA > + policy would indicate what labels required integrity verification, which > + presented an issue: EVM would protect the label, but if an attacker could > + modify filesystem offline, the attacker could wipe all the xattrs - > + including the SELinux labels that would be used to determine whether the > + file should be subject to integrity policy. > + > + With DM-Verity, as the xattrs are saved as part of the Merkel tree, if > + offline mount occurs against the filesystem protected by dm-verity, the > + checksum no longer matches and the file fails to be read. > + > + * As userspace binaries are paged in Linux, dm-verity also offers the > + additional protection against a hostile block device. In such an attack, > + the block device reports the appropriate content for the IMA hash > + initially, passing the required integrity check. Then, on the page fault > + that accesses the real data, will report the attacker's payload. Since > + dm-verity will check the data when the page fault occurs (and the disk > + access), this attack is mitigated. > + > + 2. Performance: > + > + * dm-verity provides integrity verification on demand as blocks are > + read versus requiring the entire file being read into memory for > + validation. > + > + 3. Simplicity of signing: > + > + * No need for two signatures (IMA, then EVM): one signature covers > + an entire block device. > + * Signatures can be stored externally to the filesystem metadata. > + * The signature supports an x.509-based signing infrastructure. > + > +The next step was to choose a *policy* to enforce the integrity mechanism. > +The minimum requirements for the policy were: > + > + 1. The policy itself must be integrity verified (preventing trivial > + attack against it). > + 2. The policy itself must be resistant to rollback attacks. > + 3. The policy enforcement must have a permissive-like mode. > + 4. The policy must be able to be updated, in its entirety, without > + a reboot. > + 5. Policy updates must be atomic. > + 6. The policy must support *revocations* of previously authored > + components. > + 7. The policy must be auditable, at any point-of-time. > + > +IMA, as the only integrity policy mechanism at the time, was > +considered against these list of requirements, and did not fulfill > +all of the minimum requirements. Extending IMA to cover these > +requirements was considered, but ultimately discarded for a > +two reasons: > + > + 1. Regression risk; many of these changes would result in > + dramatic code changes to IMA, which is already present in the > + kernel, and therefore might impact users. > + > + 2. IMA was used in the system for measurement and attestation; > + separation of measurement policy from local integrity policy > + enforcement was considered favorable. > + > +Due to these reasons, it was decided that a new LSM should be created, > +whose responsibility would be only the local integrity policy enforcement. > + > +Role and Scope > +-------------- > + > +IPE, as its name implies, is fundamentally an integrity policy enforcement > +solution; IPE does not mandate how integrity is provided, but instead > +leaves that decision to the system administrator to set the security bar, > +via the mechanisms that they select that suit their individual needs. > +There are several different integrity solutions that provide a different > +level of security guarantees; and IPE allows sysadmins to express policy for > +theoretically all of them. > + > +IPE does not have an inherent mechanism to ensure integrity on its own. > +Instead, there are more effective layers available for building systems that > +can guarantee integrity. It's important to note that the mechanism for proving > +integrity is independent of the policy for enforcing that integrity claim. > + > +Therefore, IPE was designed around: > + > + 1. Easy integrations with integrity providers. > + 2. Ease of use for platform administrators/sysadmins. > + > +Design Rationale: > +----------------- > + > +IPE was designed after evaluating existing integrity policy solutions > +in other operating systems and environments. In this survey of other > +implementations, there were a few pitfalls identified: > + > + 1. Policies were not readable by humans, usually requiring a binary > + intermediary format. > + 2. A single, non-customizable action was implicitly taken as a default. > + 3. Debugging the policy required manual steps to determine what rule was violated. > + 4. Authoring a policy required an in-depth knowledge of the larger system, > + or operating system. > + > +IPE attempts to avoid all of these pitfalls. > + > +Policy > +~~~~~~ > + > +Plain Text > +^^^^^^^^^^ > + > +IPE's policy is plain-text. This introduces slightly larger policy files than > +other LSMs, but solves two major problems that occurs with some integrity policy > +solutions on other platforms. > + > +The first issue is one of code maintenance and duplication. To author policies, > +the policy has to be some form of string representation (be it structured, > +through XML, JSON, YAML, etcetera), to allow the policy author to understand > +what is being written. In a hypothetical binary policy design, a serializer > +is necessary to write the policy from the human readable form, to the binary > +form, and a deserializer is needed to interpret the binary form into a data > +structure in the kernel. > + > +Eventually, another deserializer will be needed to transform the binary from > +back into the human-readable form with as much information preserved. This is because a > +user of this access control system will have to keep a lookup table of a checksum > +and the original file itself to try to understand what policies have been deployed > +on this system and what policies have not. For a single user, this may be alright, > +as old policies can be discarded almost immediately after the update takes hold. > +For users that manage computer fleets in the thousands, if not hundreds of thousands, > +with multiple different operating systems, and multiple different operational needs, > +this quickly becomes an issue, as stale policies from years ago may be present, > +quickly resulting in the need to recover the policy or fund extensive infrastructure > +to track what each policy contains. > + > +With now three separate serializer/deserializers, maintenance becomes costly. If the > +policy avoids the binary format, there is only one required serializer: from the > +human-readable form to the data structure in kernel, saving on code maintenance, > +and retaining operability. > + > +The second issue with a binary format is one of transparency. As IPE controls > +access based on the trust of the system's resources, it's policy must also be > +trusted to be changed. This is done through signatures, resulting in needing > +signing as a process. Signing, as a process, is typically done with a > +high security bar, as anything signed can be used to attack integrity > +enforcement systems. It is also important that, when signing something, that > +the signer is aware of what they are signing. A binary policy can cause > +obfuscation of that fact; what signers see is an opaque binary blob. A > +plain-text policy, on the other hand, the signers see the actual policy > +submitted for signing. > + > +Boot Policy > +~~~~~~~~~~~ > + > +IPE, if configured appropriately, is able to enforce a policy as soon as a > +kernel is booted and usermode starts. That implies some level of storage > +of the policy to apply the minute usermode starts. Generally, that storage > +can be handled in one of three ways: > + > + 1. The policy file(s) live on disk and the kernel loads the policy prior > + to an code path that would result in an enforcement decision. > + 2. The policy file(s) are passed by the bootloader to the kernel, who > + parses the policy. > + 3. There is a policy file that is compiled into the kernel that is > + parsed and enforced on initialization. > + > +The first option has problems: the kernel reading files from userspace > +is typically discouraged and very uncommon in the kernel. > + > +The second option also has problems: Linux supports a variety of bootloaders > +across its entire ecosystem - every bootloader would have to support this > +new methodology or there must be an independent source. It would likely > +result in more drastic changes to the kernel startup than necessary. > + > +The third option is the best but it's important to be aware that the policy > +will take disk space against the kernel it's compiled in. It's important to > +keep this policy generalized enough that userspace can load a new, more > +complicated policy, but restrictive enough that it will not overauthorize > +and cause security issues. > + > +The initramfs provides a way that this bootup path can be established. The > +kernel starts with a minimal policy, that trusts the initramfs only. Inside > +the initramfs, when the real rootfs is mounted, but not yet transferred to, > +it deploys and activates a policy that trusts the new root filesystem. > +This prevents overauthorization at any step, and keeps the kernel policy > +to a minimal size. > + > +Startup > +^^^^^^^ > + > +Not every system, however starts with an initramfs, so the startup policy > +compiled into the kernel will need some flexibility to express how trust > +is established for the next phase of the bootup. To this end, if we just > +make the compiled-in policy a full IPE policy, it allows system builders > +to express the first stage bootup requirements appropriately. > + > +Updatable, Rebootless Policy > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +As requirements change over time (vulnerabilities are found in previously > +trusted applications, keys roll, etcetera). Updating a kernel to change the > +meet those security goals is not always a suitable option, as updates are not > +always risk-free, and blocking a security update leaves systems vulnerable. > +This means IPE requires a policy that can be completely updated (allowing > +revocations of existing policy) from a source external to the kernel (allowing > +policies to be updated without updating the kernel). > + > +Additionally, since the kernel is stateless between invocations, and reading > +policy files off the disk from kernel space is a bad idea(tm), then the > +policy updates have to be done rebootlessly. > + > +To allow an update from an external source, it could be potentially malicious, > +so this policy needs to have a way to be identified as trusted. This is > +done via a signature chained to a trust source in the kernel. Arbitrarily, > +this is the ``SYSTEM_TRUSTED_KEYRING``, a keyring that is initially > +populated at kernel compile-time, as this matches the expectation that the > +author of the compiled-in policy described above is the same entity that can > +deploy policy updates. > + > +Anti-Rollback / Anti-Replay > +~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +Over time, vulnerabilities are found and trusted resources may not be > +trusted anymore. IPE's policy has no exception to this. There can be > +instances where a mistaken policy author deploys an insecure policy, > +before correcting it with a secure policy. > + > +Assuming that as soon as the insecure policy is signed, and an attacker > +acquires the insecure policy, IPE needs a way to prevent rollback > +from the secure policy update to the insecure policy update. > + > +Initially, IPE's policy can have a policy_version that states the > +minimum required version across all policies that can be active on > +the system. This will prevent rollback while the system is live. > + > +.. WARNING:: > + > + However, since the kernel is stateless across boots, this policy > + version will be reset to 0.0.0 on the next boot. System builders > + need to be aware of this, and ensure the new secure policies are > + deployed ASAP after a boot to ensure that the window of > + opportunity is minimal for an attacker to deploy the insecure policy. > + > +Implicit Actions: > +~~~~~~~~~~~~~~~~~ > + > +The issue of implicit actions only becomes visible when you consider > +a mixed level of security bars across multiple operations in a system. > +For example, consider a system that has strong integrity guarantees > +over both the executable code, and specific *data files* on the system, > +that were critical to its function. In this system, three types of policies > +are possible: > + > + 1. A policy in which failure to match any rules in the policy results > + in the action being denied. > + 2. A policy in which failure to match any rules in the policy results > + in the action being allowed. > + 3. A policy in which the action taken when no rules are matched is > + specified by the policy author. > + > +The first option could make a policy like this:: > + > + op=EXECUTE integrity_verified=YES action=ALLOW > + > +In the example system, this works well for the executables, as all > +executables should have integrity guarantees, without exception. The > +issue becomes with the second requirement about specific data files. > +This would result in a policy like this (assuming each line is > +evaluated in order):: > + > + op=EXECUTE integrity_verified=YES action=ALLOW > + > + op=READ integrity_verified=NO label=critical_t action=DENY > + op=READ action=ALLOW > + > +This is somewhat clear if you read the docs, understand the policy > +is executed in order and that the default is a denial; however, the > +last line effectively changes that default to an ALLOW. This is > +required, because in a realistic system, there are some unverified > +reads (imagine appending to a log file). > + > +The second option, matching no rules results in an allow, is clearer > +for the specific data files:: > + > + op=READ integrity_verified=NO label=critical_t action=DENY > + > +And, like the first option, falls short with the opposite scenario, > +effectively needing to override the default:: > + > + op=EXECUTE integrity_verified=YES action=ALLOW > + op=EXECUTE action=DENY > + > + op=READ integrity_verified=NO label=critical_t action=DENY > + > +This leaves the third option. Instead of making users be clever > +and override the default with an empty rule, force the end-user > +to consider what the appropriate default should be for their > +scenario and explicitly state it:: > + > + DEFAULT op=EXECUTE action=DENY > + op=EXECUTE integrity_verified=YES action=ALLOW > + > + DEFAULT op=READ action=ALLOW > + op=READ integrity_verified=NO label=critical_t action=DENY > + > +Policy Debugging: > +~~~~~~~~~~~~~~~~~ > + > +When developing a policy, it is useful to know what line of the policy > +is being violated to reduce debugging costs; narrowing the scope of the > +investigation to the exact line that resulted in the action. Some integrity > +policy systems do not provide this information, instead providing the > +information that was used in the evaluation. This then requires a correlation > +with the policy to evaluate what went wrong. > + > +Instead, IPE just emits the rule that was matched. This limits the scope > +of the investigation to the exact policy line (in the case of a specific > +rule), or the section (in the case of a DEFAULT). This decreases iteration > +and investigation times when policy failures are observed while evaluating > +policies. > + > +IPE's policy engine is also designed in a way that it makes it obvious to > +a human of how to investigate a policy failure. Each line is evaluated in > +the sequence that is written, so the algorithm is very simple to follow > +for humans to recreate the steps and could have caused the failure. In other > +surveyed systems, optimizations occur (sorting rules, for instance) when loading > +the policy. In those systems, it requires multiple steps to debug, and the > +algorithm may not always be clear to the end-user without reading the code first. > + > +Simplified Policy: > +~~~~~~~~~~~~~~~~~~ > + > +Finally, IPE's policy is designed for sysadmins, not kernel developers. Instead > +of covering individual LSM hooks (or syscalls), IPE covers operations. This means > +instead of sysadmins needing to know that the syscalls ``mmap``, ``mprotect``, > +``execve``, and ``uselib`` must have rules protecting them, they must simple know > +that they want to restrict code execution. This limits the amount of bypasses that > +could occur due to a lack of knowledge of the underlying system; whereas the > +maintainers of IPE, being kernel developers can make the correct choice to determine > +whether something maps to these operations, and under what conditions. > + > +Implementation Notes > +-------------------- > + > +Anonymous Memory > +~~~~~~~~~~~~~~~~ > + > +Anonymous memory isn't treated any differently from any other access in IPE. > +When anonymous memory is mapped with ``+X``, it still comes into the ``file_mmap`` > +or ``file_mprotect`` hook, but with a ``NULL`` file object. This is submitted to > +the evaluation, like any other file, however, all current trust mechanisms will > +return false as there is nothing to evaluate. This means anonymous memory > +execution is subject to whatever the ``DEFAULT`` is for ``EXECUTE``. > + > +.. WARNING:: > + > + This also occurs with the ``kernel_load_data`` hook, which is used by signed > + and compressed kernel modules. Using signed and compressed kernel modules with > + IPE will always result in the ``DEFAULT`` action for ``KMODULE``. > + > +Securityfs Interface > +~~~~~~~~~~~~~~~~~~~~ > + > +The per-policy securityfs tree is somewhat unique. For example, for > +a standard securityfs policy tree:: > + > + MyPolicy > + |- active > + |- delete > + |- name > + |- pkcs7 > + |- policy > + |- update > + |- version > + > +The policy is stored in the ``->i_private`` data of the MyPolicy inode. > + > +Tests > +----- > + > +IPE has KUnit Tests for the policy parser. Recommended kunitconfig:: > + > + CONFIG_KUNIT=y > + CONFIG_SECURITY=y > + CONFIG_SECURITYFS=y > + CONFIG_PKCS7_MESSAGE_PARSER=y > + CONFIG_SYSTEM_DATA_VERIFICATION=y > + CONFIG_FS_VERITY=y > + CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y > + CONFIG_BLOCK=y > + CONFIG_MD=y > + CONFIG_BLK_DEV_DM=y > + CONFIG_DM_VERITY=y > + CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG=y > + CONFIG_NET=y > + CONFIG_AUDIT=y > + CONFIG_AUDITSYSCALL=y > + CONFIG_BLK_DEV_INITRD=y > + > + CONFIG_SECURITY_IPE=y > + CONFIG_IPE_PROP_DM_VERITY=y > + CONFIG_IPE_PROP_FS_VERITY=y > + CONFIG_SECURITY_IPE_KUNIT_TEST=y > + > +In addition, IPE has a python based integration > +`test suite <https://github.com/microsoft/ipe/tree/test-suite>`_ that > +can test both user interfaces and enforcement functionalities. The doc LGTM, thanks! Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
On 4/15/24 5:11 AM, Bagas Sanjaya wrote: > The doc LGTM, thanks! > > Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com> Hi, Please see netiquette, section "Trim replies". Thanks.
On Mon, Apr 15, 2024 at 07:56:16AM -0700, Randy Dunlap wrote: > > On 4/15/24 5:11 AM, Bagas Sanjaya wrote: > > The doc LGTM, thanks! > > > > Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com> > > Hi, > Please see netiquette, section "Trim replies". > Thanks. > > > -- > #Randy > https://people.kernel.org/tglx/notes-about-netiquette > https://subspace.kernel.org/etiquette.html Thanks for the tip!
On Fri, Apr 12, 2024 at 05:56:03PM -0700, Fan Wu wrote: > +dmverity_roothash > +~~~~~~~~~~~~~~~~~ > + > + This property can be utilized for authorization or revocation of > + specific dm-verity volumes, identified via its root hash. It has a > + dependency on the DM_VERITY module. This property is controlled by > + the ``IPE_PROP_DM_VERITY`` config option, it will be automatically > + selected when ``IPE_SECURITY`` , ``DM_VERITY`` and > + ``DM_VERITY_VERIFY_ROOTHASH_SIG`` are all enabled. > + The format of this property is:: > + > + dmverity_roothash=DigestName:HexadecimalString > + > + The supported DigestNames for dmverity_roothash are [#dmveritydigests]_ [#securedigest]_ : > + > + + blake2b-512 > + + blake2s-256 > + + sha1 > + + sha256 > + + sha384 > + + sha512 > + + sha3-224 > + + sha3-256 > + + sha3-384 > + + sha3-512 > + + md4 > + + md5 > + + sm3 > + + rmd160 It's not the 90s anymore. Insecure algorithms like md4, md5, and sha1 should not be here. > +dmverity_signature > +~~~~~~~~~~~~~~~~~~ > + > + This property can be utilized for authorization of all dm-verity > + volumes that have a signed roothash that validated by a keyring > + specified by dm-verity's configuration, either the system trusted > + keyring, or the secondary keyring. It depends on > + ``DM_VERITY_VERIFY_ROOTHASH_SIG`` config option and is controlled by > + the ``IPE_PROP_DM_VERITY`` config option, it will be automatically > + selected when ``IPE_SECURITY``, ``DM_VERITY`` and > + ``DM_VERITY_VERIFY_ROOTHASH_SIG`` are all enabled. > + The format of this property is:: > + > + dmverity_signature=(TRUE|FALSE) > + > +fsverity_digest > +~~~~~~~~~~~~~~~ > + > + This property can be utilized for authorization or revocation of > + specific fsverity enabled file, identified via its fsverity digest. > + It depends on ``FS_VERITY`` config option and is controlled by > + ``CONFIG_IPE_PROP_FS_VERITY``. The format of this property is:: > + > + fsverity_digest=DigestName:HexadecimalString > + > + The supported DigestNames for fsverity_roothash are [#fsveritydigest]_ [#securedigest]_ : fsverity_digest, not fsverity_roothash. > +Allow any signed fs-verity file > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +:: > + > + policy_name=AllowSignedFSVerity policy_version=0.0.0 > + DEFAULT action=DENY > + > + op=EXECUTE fsverity_signature=TRUE action=ALLOW As elsewhere, ideally this would be more specific about what is meant by a signed file. The goal is not to allow *any* signed file, but rather only allow files that are signed by a particular someone/something. > +Prohibit execution of a specific fs-verity file > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +:: > + > + policy_name=ProhibitSpecificFSVF policy_version=0.0.0 > + DEFAULT action=DENY > + > + op=EXECUTE fsverity_digest=sha256:fd88f2b8824e197f850bf4c5109bea5cf0ee38104f710843bb72da796ba5af9e action=DENY > + op=EXECUTE boot_verified=TRUE action=ALLOW > + op=EXECUTE dmverity_signature=TRUE action=ALLOW This example is a bit weird because it's a denylist, not an allowlist. In general this could be trivially circumvented by creating a new binary that has fsverity disabled or that doesn't meaningfully differ from the original. > +.. [#fsveritydigest] These hash algorithms are based on values accepted by fsverity-utils; > + IPE does not impose any restrictions on the digest algorithm itself; > + thus, this list may be out of date. It's the kernel's fsverity support, not fsverity-utils, that matters here. fsverity-utils is kept up to date with the kernel, so in practice the list of algorithms is the same on both sides, but it's the kernel that matters here. > +.. [#dmveritydigests] These hash algorithms are based on values accepted by dm-verity, > + specifically ``crypto_alloc_ahash`` in ``verity_ctr``; ``veritysetup`` > + does support more algorithms than the list above. IPE does not impose > + any restrictions on the digest algorithm itself; thus, this list > + may be out of date. References to specific functions and locations in the code tend to get out of date. I think you mean something like: any hash algorithm that's supported by the Linux crypto API is supported. > + > +.. [#securedigest] Please ensure you are using cryptographically secure hash functions; > + just because something is *supported* does not mean it is *secure*. Instead of giving insecure algorithms like md4 as examples and then giving this disclaimer, how about only giving secure algorithms as examples? - Eric
On Wed, Apr 24, 2024 at 09:13:51PM -0700, Eric Biggers wrote: > > +.. [#dmveritydigests] These hash algorithms are based on values accepted by dm-verity, > > + specifically ``crypto_alloc_ahash`` in ``verity_ctr``; ``veritysetup`` > > + does support more algorithms than the list above. IPE does not impose > > + any restrictions on the digest algorithm itself; thus, this list > > + may be out of date. > > References to specific functions and locations in the code tend to get out of > date. I think you mean something like: any hash algorithm that's supported by > the Linux crypto API is supported. > Also, this scheme looks buggy because it's directly reusing the crypto API's algorithm name string as the digest name. The crypto API lets you specify the name of an algorithm, like "sha256", but it also lets you specify the name of a particular *implementation* of an algorithm, like "sha256-ni" for the SHA-NI accelerated implementation of sha256. It looks like dm-verity just passes through the name directly to the crypto API, and as a result it accepts names like sha256-ni. Since you're directly passing the same name into the security_bdev_setintegrity() LSM hook, that would result in IPE being told that the hash is "sha256-ni". That doesn't make sense. I think you want to be passing in crypto_ahash_alg_name(v->tfm), not v->alg_name. - Eric
diff --git a/Documentation/admin-guide/LSM/index.rst b/Documentation/admin-guide/LSM/index.rst index a6ba95fbaa9f..ce63be6d64ad 100644 --- a/Documentation/admin-guide/LSM/index.rst +++ b/Documentation/admin-guide/LSM/index.rst @@ -47,3 +47,4 @@ subdirectories. tomoyo Yama SafeSetID + ipe diff --git a/Documentation/admin-guide/LSM/ipe.rst b/Documentation/admin-guide/LSM/ipe.rst new file mode 100644 index 000000000000..d2bdd6e5b662 --- /dev/null +++ b/Documentation/admin-guide/LSM/ipe.rst @@ -0,0 +1,797 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Integrity Policy Enforcement (IPE) +================================== + +.. NOTE:: + + This is the documentation for admins, system builders, or individuals + attempting to use IPE. If you're looking for more developer-focused + documentation about IPE please see Documentation/security/ipe.rst + +Overview +-------- + +Integrity Policy Enforcement (IPE) is a Linux Security Module that takes a +complementary approach to access control. Unlike traditional access control +mechanisms that rely on labels and paths for decision-making, IPE focuses +on the immutable security properties inherent to system components. These +properties are fundamental attributes or features of a system component +that cannot be altered, ensuring a consistent and reliable basis for +security decisions. + +To elaborate, in the context of IPE, system components primarily refer to +files or the devices these files reside on. However, this is just a +starting point. The concept of system components is flexible and can be +extended to include new elements as the system evolves. The immutable +properties include the origin of a file, which remains constant and +unchangeable over time. For example, IPE policies can be crafted to trust +files originating from the initramfs. Since initramfs is typically verified +by the bootloader, its files are deemed trustworthy; "file is from +initramfs" becomes an immutable property under IPE's consideration. + +The immutable property concept extends to the security features enabled on +a file's origin, such as dm-verity or fs-verity, which provide a layer of +integrity and trust. For example, IPE allows the definition of policies +that trust files from a dm-verity protected device. dm-verity ensures the +integrity of an entire device by providing a verifiable and immutable state +of its contents. Similarly, fs-verity offers filesystem-level integrity +checks, allowing IPE to enforce policies that trust files protected by +fs-verity. These two features cannot be turned off once established, so +they are considered immutable properties. These examples demonstrate how +IPE leverages immutable properties, such as a file's origin and its +integrity protection mechanisms, to make access control decisions. + +For the IPE policy, specifically, it grants the ability to enforce +stringent access controls by assessing security properties against +reference values defined within the policy. This assessment can be based on +the existence of a security property (e.g., verifying if a file originates +from initramfs) or evaluating the internal state of an immutable security +property. The latter includes checking the roothash of a dm-verity +protected device, determining whether dm-verity possesses a valid +signature, assessing the digest of a fs-verity protected file, or +determining whether fs-verity possesses a valid built-in signature. This +nuanced approach to policy enforcement enables a highly secure and +customizable system defense mechanism, tailored to specific security +requirements and trust models. + +To enable IPE, ensure that ``CONFIG_SECURITY_IPE`` (under +:menuselection:`Security -> Integrity Policy Enforcement (IPE)`) config +option is enabled. + +Use Cases +--------- + +IPE works best in fixed-function devices: devices in which their purpose +is clearly defined and not supposed to be changed (e.g. network firewall +device in a data center, an IoT device, etcetera), where all software and +configuration is built and provisioned by the system owner. + +IPE is a long-way off for use in general-purpose computing: the Linux +community as a whole tends to follow a decentralized trust model (known as +the web of trust), which IPE has no support for it yet. Instead, IPE +supports PKI (public key infrastructure), which generally designates a +set of trusted entities that provide a measure of absolute trust. + +Additionally, while most packages are signed today, the files inside +the packages (for instance, the executables), tend to be unsigned. This +makes it difficult to utilize IPE in systems where a package manager is +expected to be functional, without major changes to the package manager +and ecosystem behind it. + +DIGLIM [#diglim]_ is a system that when combined with IPE, could be used to +enable and support general-purpose computing use cases. + +Known Limitations +----------------- + +IPE cannot verify the integrity of anonymous executable memory, such as +the trampolines created by gcc closures and libffi (<3.4.2), or JIT'd code. +Unfortunately, as this is dynamically generated code, there is no way +for IPE to ensure the integrity of this code to form a trust basis. In all +cases, the return result for these operations will be whatever the admin +configures as the ``DEFAULT`` action for ``EXECUTE``. + +IPE cannot verify the integrity of programs written in interpreted +languages when these scripts are invoked by passing these program files +to the interpreter. This is because the way interpreters execute these +files; the scripts themselves are not evaluated as executable code +through one of IPE's hooks, but they are merely text files that are read +(as opposed to compiled executables) [#interpreters]_. + +Threat Model +------------ + +IPE specifically targets the risk of tampering with user-space executable +code after the kernel has initially booted, including the kernel modules +loaded from userspace via ``modprobe`` or ``insmod``. + +To illustrate, consider a scenario where an untrusted binary, possibly +malicious, is downloaded along with all necessary dependencies, including a +loader and libc. The primary function of IPE in this context is to prevent +the execution of such binaries and their dependencies. + +IPE achieves this by verifying the integrity and authenticity of all +executable code before allowing them to run. It conducts a thorough +check to ensure that the code's integrity is intact and that they match an +authorized reference value (digest, signature, etc) as per the defined +policy. If a binary does not pass this verification process, either +because its integrity has been compromised or it does not meet the +authorization criteria, IPE will deny its execution. Additionally, IPE +generates audit logs which may be utilized to detect and analyze failures +resulting from policy violation. + +Tampering threat scenarios include modification or replacement of +executable code by a range of actors including: + +- Actors with physical access to the hardware +- Actors with local network access to the system +- Actors with access to the deployment system +- Compromised internal systems under external control +- Malicious end users of the system +- Compromised end users of the system +- Remote (external) compromise of the system + +IPE does not mitigate threats arising from malicious but authorized +developers (with access to a signing certificate), or compromised +developer tools used by them (i.e. return-oriented programming attacks). +Additionally, IPE draws hard security boundary between userspace and +kernelspace. As a result, IPE does not provide any protections against a +kernel level exploit, and a kernel-level exploit can disable or tamper +with IPE's protections. + +Policy +------ + +IPE policy is a plain-text [#devdoc]_ policy composed of multiple statements +over several lines. There is one required line, at the top of the +policy, indicating the policy name, and the policy version, for +instance:: + + policy_name=Ex_Policy policy_version=0.0.0 + +The policy name is a unique key identifying this policy in a human +readable name. This is used to create nodes under securityfs as well as +uniquely identify policies to deploy new policies vs update existing +policies. + +The policy version indicates the current version of the policy (NOT the +policy syntax version). This is used to prevent rollback of policy to +potentially insecure previous versions of the policy. + +The next portion of IPE policy are rules. Rules are formed by key=value +pairs, known as properties. IPE rules require two properties: ``action``, +which determines what IPE does when it encounters a match against the +rule, and ``op``, which determines when the rule should be evaluated. +The ordering is significant, a rule must start with ``op``, and end with +``action``. Thus, a minimal rule is:: + + op=EXECUTE action=ALLOW + +This example will allow any execution. Additional properties are used to +assess immutable security properties about the files being evaluated. +These properties are intended to be descriptions of systems within the +kernel that can provide a measure of integrity verification, such that IPE +can determine the trust of the resource based on the value of the property. + +Rules are evaluated top-to-bottom. As a result, any revocation rules, +or denies should be placed early in the file to ensure that these rules +are evaluated before a rule with ``action=ALLOW``. + +IPE policy supports comments. The character '#' will function as a +comment, ignoring all characters to the right of '#' until the newline. + +The default behavior of IPE evaluations can also be expressed in policy, +through the ``DEFAULT`` statement. This can be done at a global level, +or a per-operation level:: + + # Global + DEFAULT action=ALLOW + + # Operation Specific + DEFAULT op=EXECUTE action=ALLOW + +A default must be set for all known operations in IPE. If you want to +preserve older policies being compatible with newer kernels that can introduce +new operations, set a global default of ``ALLOW``, then override the +defaults on a per-operation basis (as above). + +With configurable policy-based LSMs, there's several issues with +enforcing the configurable policies at startup, around reading and +parsing the policy: + +1. The kernel *should* not read files from userspace, so directly reading + the policy file is prohibited. +2. The kernel command line has a character limit, and one kernel module + should not reserve the entire character limit for its own + configuration. +3. There are various boot loaders in the kernel ecosystem, so handing + off a memory block would be costly to maintain. + +As a result, IPE has addressed this problem through a concept of a "boot +policy". A boot policy is a minimal policy which is compiled into the +kernel. This policy is intended to get the system to a state where +userspace is set up and ready to receive commands, at which point a more +complex policy can be deployed via securityfs. The boot policy can be +specified via ``SECURITY_IPE_BOOT_POLICY`` config option, which accepts +a path to a plain-text version of the IPE policy to apply. This policy +will be compiled into the kernel. If not specified, IPE will be disabled +until a policy is deployed and activated through securityfs. + +Deploying Policies +~~~~~~~~~~~~~~~~~~ + +Policies can be deployed from userspace through securityfs. These policies +are signed through the PKCS#7 message format to enforce some level of +authorization of the policies (prohibiting an attacker from gaining +unconstrained root, and deploying an "allow all" policy). These +policies must be signed by a certificate that chains to the +``SYSTEM_TRUSTED_KEYRING``. With openssl, the policy can be signed by:: + + openssl smime -sign \ + -in "$MY_POLICY" \ + -signer "$MY_CERTIFICATE" \ + -inkey "$MY_PRIVATE_KEY" \ + -noattr \ + -nodetach \ + -nosmimecap \ + -outform der \ + -out "$MY_POLICY.p7b" + +Deploying the policies is done through securityfs, through the +``new_policy`` node. To deploy a policy, simply cat the file into the +securityfs node:: + + cat "$MY_POLICY.p7b" > /sys/kernel/security/ipe/new_policy + +Upon success, this will create one subdirectory under +``/sys/kernel/security/ipe/policies/``. The subdirectory will be the +``policy_name`` field of the policy deployed, so for the example above, +the directory will be ``/sys/kernel/security/ipe/policies/Ex_Policy``. +Within this directory, there will be seven files: ``pkcs7``, ``policy``, +``name``, ``version``, ``active``, ``update``, and ``delete``. + +The ``pkcs7`` file is read-only. Reading it returns the raw PKCS#7 data +that was provided to the kernel, representing the policy. If the policy being +read is the boot policy, this will return ``ENOENT``, as it is not signed. + +The ``policy`` file is read only. Reading it returns the PKCS#7 inner +content of the policy, which will be the plain text policy. + +The ``active`` file is used to set a policy as the currently active policy. +This file is rw, and accepts a value of ``"1"`` to set the policy as active. +Since only a single policy can be active at one time, all other policies +will be marked inactive. The policy being marked active must have a policy +version greater or equal to the currently-running version. + +The ``update`` file is used to update a policy that is already present +in the kernel. This file is write-only and accepts a PKCS#7 signed +policy. Two checks will always be performed on this policy: First, the +``policy_names`` must match with the updated version and the existing +version. Second the updated policy must have a policy version greater than +or equal to the currently-running version. This is to prevent rollback attacks. + +The ``delete`` file is used to remove a policy that is no longer needed. +This file is write-only and accepts a value of ``1`` to delete the policy. +On deletion, the securityfs node representing the policy will be removed. +However, delete the current active policy is not allowed and will return +an operation not permitted error. + +Similarly, writing to both ``update`` and ``new_policy`` could result in +bad message(policy syntax error) or file exists error. The latter error happens +when trying to deploy a policy with a ``policy_name`` while the kernel already +has a deployed policy with the same ``policy_name``. + +Deploying a policy will *not* cause IPE to start enforcing the policy. IPE will +only enforce the policy marked active. Note that only one policy can be active +at a time. + +Once deployment is successful, the policy can be activated, by writing file +``/sys/kernel/security/ipe/policies/$policy_name/active``. +For example, the ``Ex_Policy`` can be activated by:: + + echo 1 > "/sys/kernel/security/ipe/policies/Ex_Policy/active" + +From above point on, ``Ex_Policy`` is now the enforced policy on the +system. + +IPE also provides a way to delete policies. This can be done via the +``delete`` securityfs node, +``/sys/kernel/security/ipe/policies/$policy_name/delete``. +Writing ``1`` to that file deletes the policy:: + + echo 1 > "/sys/kernel/security/ipe/policies/$policy_name/delete" + +There is only one requirement to delete a policy: the policy being deleted +must be inactive. + +.. NOTE:: + + If a traditional MAC system is enabled (SELinux, apparmor, smack), all + writes to ipe's securityfs nodes require ``CAP_MAC_ADMIN``. + +Modes +~~~~~ + +IPE supports two modes of operation: permissive (similar to SELinux's +permissive mode) and enforced. In permissive mode, all events are +checked and policy violations are logged, but the policy is not really +enforced. This allows users to test policies before enforcing them. + +The default mode is enforce, and can be changed via the kernel command +line parameter ``ipe.enforce=(0|1)``, or the securityfs node +``/sys/kernel/security/ipe/enforce``. + +.. NOTE:: + + If a traditional MAC system is enabled (SELinux, apparmor, smack, etcetera), + all writes to ipe's securityfs nodes require ``CAP_MAC_ADMIN``. + +Audit Events +~~~~~~~~~~~~ + +1420 AUDIT_IPE_ACCESS +^^^^^^^^^^^^^^^^^^^^^ +Event Examples:: + + type=1420 audit(1653364370.067:61): ipe_op=EXECUTE ipe_hook=MMAP enforcing=1 pid=2241 comm="ld-linux.so" path="/deny/lib/libc.so.6" dev="sda2" ino=14549020 rule="DEFAULT action=DENY" + type=1300 audit(1653364370.067:61): SYSCALL arch=c000003e syscall=9 success=no exit=-13 a0=7f1105a28000 a1=195000 a2=5 a3=812 items=0 ppid=2219 pid=2241 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm="ld-linux.so" exe="/tmp/ipe-test/lib/ld-linux.so" subj=unconfined key=(null) + type=1327 audit(1653364370.067:61): 707974686F6E3300746573742F6D61696E2E7079002D6E00 + + type=1420 audit(1653364735.161:64): ipe_op=EXECUTE ipe_hook=MMAP enforcing=1 pid=2472 comm="mmap_test" path=? dev=? ino=? rule="DEFAULT action=DENY" + type=1300 audit(1653364735.161:64): SYSCALL arch=c000003e syscall=9 success=no exit=-13 a0=0 a1=1000 a2=4 a3=21 items=0 ppid=2219 pid=2472 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm="mmap_test" exe="/root/overlake_test/upstream_test/vol_fsverity/bin/mmap_test" subj=unconfined key=(null) + type=1327 audit(1653364735.161:64): 707974686F6E3300746573742F6D61696E2E7079002D6E00 + +This event indicates that IPE made an access control decision; the IPE +specific record (1420) is always emitted in conjunction with a +``AUDITSYSCALL`` record. + +Determining whether IPE is in permissive or enforced mode can be derived +from ``success`` property and exit code of the ``AUDITSYSCALL`` record. + + +Field descriptions: + ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| Field | Value Type | Optional? | Description of Value | ++===========+============+===========+=================================================================================+ +| ipe_op | string | No | The IPE operation name associated with the log | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| ipe_hook | string | No | The name of the LSM hook that triggered the IPE event | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| enforcing | integer | No | The current IPE enforcing state 1 is in enforcing mode, 0 is in permissive mode | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| pid | integer | No | The pid of the process that triggered the IPE event. | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| comm | string | No | The command line program name of the process that triggered the IPE event | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| path | string | Yes | The absolute path to the evaluated file | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| ino | integer | Yes | The inode number of the evaluated file | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| dev | string | Yes | The device name of the evaluated file, e.g. vda | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ +| rule | string | No | The matched policy rule | ++-----------+------------+-----------+---------------------------------------------------------------------------------+ + +1421 AUDIT_IPE_CONFIG_CHANGE +^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Event Example:: + + type=1421 audit(1653425583.136:54): old_active_pol_name="Allow_All" old_active_pol_version=0.0.0 old_policy_digest=sha256:E3B0C44298FC1C149AFBF4C8996FB92427AE41E4649B934CA495991B7852B855 new_active_pol_name="boot_verified" new_active_pol_version=0.0.0 new_policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD8EED7B8F4DB auid=4294967295 ses=4294967295 lsm=ipe res=1 + type=1300 audit(1653425583.136:54): SYSCALL arch=c000003e syscall=1 success=yes exit=2 a0=3 a1=5596fcae1fb0 a2=2 a3=2 items=0 ppid=184 pid=229 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=4294967295 comm="python3" exe="/usr/bin/python3.10" key=(null) + type=1327 audit(1653425583.136:54): PROCTITLE proctitle=707974686F6E3300746573742F6D61696E2E7079002D66002E2 + +This event indicates that IPE switched the active poliy from one to another +along with the version and the hash digest of the two policies. +Note IPE can only have one policy active at a time, all access decision +evaluation is based on the current active policy. +The normal procedure to deploy a new policy is loading the policy to deploy +into the kernel first, then switch the active policy to it. + +This record will always be emitted in conjunction with a ``AUDITSYSCALL`` record for the ``write`` syscall. + +Field descriptions: + ++------------------------+------------+-----------+---------------------------------------------------+ +| Field | Value Type | Optional? | Description of Value | ++========================+============+===========+===================================================+ +| old_active_pol_name | string | No | The name of previous active policy | ++------------------------+------------+-----------+---------------------------------------------------+ +| old_active_pol_version | string | No | The version of previous active policy | ++------------------------+------------+-----------+---------------------------------------------------+ +| old_policy_digest | string | No | The hash of previous active policy | ++------------------------+------------+-----------+---------------------------------------------------+ +| new_active_pol_name | string | No | The name of current active policy | ++------------------------+------------+-----------+---------------------------------------------------+ +| new_active_pol_version | string | No | The version of current active policy | ++------------------------+------------+-----------+---------------------------------------------------+ +| new_policy_digest | string | No | The hash of current active policy | ++------------------------+------------+-----------+---------------------------------------------------+ +| auid | integer | No | The login user ID | ++------------------------+------------+-----------+---------------------------------------------------+ +| ses | integer | No | The login session ID | ++------------------------+------------+-----------+---------------------------------------------------+ +| lsm | string | No | The lsm name associated with the event | ++------------------------+------------+-----------+---------------------------------------------------+ +| res | integer | No | The result of the audited operation(success/fail) | ++------------------------+------------+-----------+---------------------------------------------------+ + +1422 AUDIT_IPE_POLICY_LOAD +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Event Example:: + + type=1422 audit(1653425529.927:53): policy_name="boot_verified" policy_version=0.0.0 policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD8EED7B8F4DB auid=4294967295 ses=4294967295 lsm=ipe res=1 + type=1300 audit(1653425529.927:53): arch=c000003e syscall=1 success=yes exit=2567 a0=3 a1=5596fcae1fb0 a2=a07 a3=2 items=0 ppid=184 pid=229 auid=4294967295 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0 ses=4294967295 comm="python3" exe="/usr/bin/python3.10" key=(null) + type=1327 audit(1653425529.927:53): PROCTITLE proctitle=707974686F6E3300746573742F6D61696E2E7079002D66002E2E + +This record indicates a new policy has been loaded into the kernel with the policy name, policy version and policy hash. + +This record will always be emitted in conjunction with a ``AUDITSYSCALL`` record for the ``write`` syscall. + +Field descriptions: + ++----------------+------------+-----------+---------------------------------------------------+ +| Field | Value Type | Optional? | Description of Value | ++================+============+===========+===================================================+ +| policy_name | string | No | The policy_name | ++----------------+------------+-----------+---------------------------------------------------+ +| policy_version | string | No | The policy_version | ++----------------+------------+-----------+---------------------------------------------------+ +| policy_digest | string | No | The policy hash | ++----------------+------------+-----------+---------------------------------------------------+ +| auid | integer | No | The login user ID | ++----------------+------------+-----------+---------------------------------------------------+ +| ses | integer | No | The login session ID | ++----------------+------------+-----------+---------------------------------------------------+ +| lsm | string | No | The lsm name associated with the event | ++----------------+------------+-----------+---------------------------------------------------+ +| res | integer | No | The result of the audited operation(success/fail) | ++----------------+------------+-----------+---------------------------------------------------+ + + +1404 AUDIT_MAC_STATUS +^^^^^^^^^^^^^^^^^^^^^ + +Event Examples:: + + type=1404 audit(1653425689.008:55): enforcing=0 old_enforcing=1 auid=4294967295 ses=4294967295 enabled=1 old-enabled=1 lsm=ipe res=1 + type=1300 audit(1653425689.008:55): arch=c000003e syscall=1 success=yes exit=2 a0=1 a1=55c1065e5c60 a2=2 a3=0 items=0 ppid=405 pid=441 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=) + type=1327 audit(1653425689.008:55): proctitle="-bash" + + type=1404 audit(1653425689.008:55): enforcing=1 old_enforcing=0 auid=4294967295 ses=4294967295 enabled=1 old-enabled=1 lsm=ipe res=1 + type=1300 audit(1653425689.008:55): arch=c000003e syscall=1 success=yes exit=2 a0=1 a1=55c1065e5c60 a2=2 a3=0 items=0 ppid=405 pid=441 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=) + type=1327 audit(1653425689.008:55): proctitle="-bash" + +This record will always be emitted in conjunction with a ``AUDITSYSCALL`` record for the ``write`` syscall. + +Field descriptions: + ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| Field | Value Type | Optional? | Description of Value | ++===============+============+===========+=================================================================================================+ +| enforcing | integer | No | The enforcing state IPE is being switched to, 1 is in enforcing mode, 0 is in permissive mode | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| old_enforcing | integer | No | The enforcing state IPE is being switched from, 1 is in enforcing mode, 0 is in permissive mode | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| auid | integer | No | The login user ID | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| ses | integer | No | The login session ID | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| enabled | integer | No | The new TTY audit enabled setting | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| old-enabled | integer | No | The old TTY audit enabled setting | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| lsm | string | No | The lsm name associated with the event | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ +| res | integer | No | The result of the audited operation(success/fail) | ++---------------+------------+-----------+-------------------------------------------------------------------------------------------------+ + + +Success Auditing +^^^^^^^^^^^^^^^^ + +IPE supports success auditing. When enabled, all events that pass IPE +policy and are not blocked will emit an audit event. This is disabled by +default, and can be enabled via the kernel command line +``ipe.success_audit=(0|1)`` or +``/sys/kernel/security/ipe/success_audit`` securityfs file. + +This is *very* noisy, as IPE will check every userspace binary on the +system, but is useful for debugging policies. + +.. NOTE:: + + If a traditional MAC system is enabled (SELinux, apparmor, smack, etcetera), + all writes to ipe's securityfs nodes require ``CAP_MAC_ADMIN``. + +Properties +---------- + +As explained above, IPE properties are ``key=value`` pairs expressed in IPE +policy. Two properties are built-into the policy parser: 'op' and 'action'. +The other properties are used to restrict immutable security properties +about the files being evaluated. Currently those properties are: +'``boot_verified``', '``dmverity_signature``', '``dmverity_roothash``', +'``fsverity_signature``', '``fsverity_digest``'. A description of all +properties supported by IPE are listed below: + +op +~~ + +Indicates the operation for a rule to apply to. Must be in every rule, +as the first token. IPE supports the following operations: + + ``EXECUTE`` + + Pertains to any file attempting to be executed, or loaded as an + executable. + + ``FIRMWARE``: + + Pertains to firmware being loaded via the firmware_class interface. + This covers both the preallocated buffer and the firmware file + itself. + + ``KMODULE``: + + Pertains to loading kernel modules via ``modprobe`` or ``insmod``. + + ``KEXEC_IMAGE``: + + Pertains to kernel images loading via ``kexec``. + + ``KEXEC_INITRAMFS`` + + Pertains to initrd images loading via ``kexec --initrd``. + + ``POLICY``: + + Controls loading policies via reading a kernel-space initiated read. + + An example of such is loading IMA policies by writing the path + to the policy file to ``$securityfs/ima/policy`` + + ``X509_CERT``: + + Controls loading IMA certificates through the Kconfigs, + ``CONFIG_IMA_X509_PATH`` and ``CONFIG_EVM_X509_PATH``. + +action +~~~~~~ + + Determines what IPE should do when a rule matches. Must be in every + rule, as the final clause. Can be one of: + + ``ALLOW``: + + If the rule matches, explicitly allow access to the resource to proceed + without executing any more rules. + + ``DENY``: + + If the rule matches, explicitly prohibit access to the resource to + proceed without executing any more rules. + +boot_verified +~~~~~~~~~~~~~ + + This property can be utilized for authorization of files from initramfs. + The format of this property is:: + + boot_verified=(TRUE|FALSE) + + + .. WARNING:: + + This property will trust files from initramfs(rootfs). It should + only be used during early booting stage. Before mounting the real + rootfs on top of the initramfs, initramfs script will recursively + remove all files and directories on the initramfs. This is typically + implemented by using switch_root(8) [#switch_root]_. Therefore the + initramfs will be empty and not accessible after the real + rootfs takes over. It is advised to switch to a different policy + that doesn't rely on the property after this point. + This ensures that the trust policies remain relevant and effective + throughout the system's operation. + +dmverity_roothash +~~~~~~~~~~~~~~~~~ + + This property can be utilized for authorization or revocation of + specific dm-verity volumes, identified via its root hash. It has a + dependency on the DM_VERITY module. This property is controlled by + the ``IPE_PROP_DM_VERITY`` config option, it will be automatically + selected when ``IPE_SECURITY`` , ``DM_VERITY`` and + ``DM_VERITY_VERIFY_ROOTHASH_SIG`` are all enabled. + The format of this property is:: + + dmverity_roothash=DigestName:HexadecimalString + + The supported DigestNames for dmverity_roothash are [#dmveritydigests]_ [#securedigest]_ : + + + blake2b-512 + + blake2s-256 + + sha1 + + sha256 + + sha384 + + sha512 + + sha3-224 + + sha3-256 + + sha3-384 + + sha3-512 + + md4 + + md5 + + sm3 + + rmd160 + +dmverity_signature +~~~~~~~~~~~~~~~~~~ + + This property can be utilized for authorization of all dm-verity + volumes that have a signed roothash that validated by a keyring + specified by dm-verity's configuration, either the system trusted + keyring, or the secondary keyring. It depends on + ``DM_VERITY_VERIFY_ROOTHASH_SIG`` config option and is controlled by + the ``IPE_PROP_DM_VERITY`` config option, it will be automatically + selected when ``IPE_SECURITY``, ``DM_VERITY`` and + ``DM_VERITY_VERIFY_ROOTHASH_SIG`` are all enabled. + The format of this property is:: + + dmverity_signature=(TRUE|FALSE) + +fsverity_digest +~~~~~~~~~~~~~~~ + + This property can be utilized for authorization or revocation of + specific fsverity enabled file, identified via its fsverity digest. + It depends on ``FS_VERITY`` config option and is controlled by + ``CONFIG_IPE_PROP_FS_VERITY``. The format of this property is:: + + fsverity_digest=DigestName:HexadecimalString + + The supported DigestNames for fsverity_roothash are [#fsveritydigest]_ [#securedigest]_ : + + + sha256 + + sha512 + +fsverity_signature +~~~~~~~~~~~~~~~~~~ + + This property is used to authorize all fs-verity enabled files that have + been verified by fs-verity's built-in signature mechanism. The signature + verification relies on a key stored within the ".fs-verity" keyring. It + depends on ``CONFIG_FS_VERITY_BUILTIN_SIGNATURES`` and it is controlled by + the Kconfig ``CONFIG_IPE_PROP_FS_VERITY``. The format of this + property is:: + + fsverity_signature=(TRUE|FALSE) + +Policy Examples +--------------- + +Allow all +~~~~~~~~~ + +:: + + policy_name=Allow_All policy_version=0.0.0 + DEFAULT action=ALLOW + +Allow only initramfs +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + policy_name=Allow_All_Initramfs policy_version=0.0.0 + DEFAULT action=DENY + + op=EXECUTE boot_verified=TRUE action=ALLOW + +Allow any signed dm-verity volume and the initramfs +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + policy_name=AllowSignedAndInitramfs policy_version=0.0.0 + DEFAULT action=DENY + + op=EXECUTE boot_verified=TRUE action=ALLOW + op=EXECUTE dmverity_signature=TRUE action=ALLOW + +Prohibit execution from a specific dm-verity volume +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + policy_name=AllowSignedAndInitramfs policy_version=0.0.0 + DEFAULT action=DENY + + op=EXECUTE dmverity_roothash=sha256:cd2c5bae7c6c579edaae4353049d58eb5f2e8be0244bf05345bc8e5ed257baff action=DENY + + op=EXECUTE boot_verified=TRUE action=ALLOW + op=EXECUTE dmverity_signature=TRUE action=ALLOW + +Allow only a specific dm-verity volume +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + policy_name=AllowSignedAndInitramfs policy_version=0.0.0 + DEFAULT action=DENY + + op=EXECUTE dmverity_roothash=sha256:401fcec5944823ae12f62726e8184407a5fa9599783f030dec146938 action=ALLOW + +Allow any signed fs-verity file +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + policy_name=AllowSignedFSVerity policy_version=0.0.0 + DEFAULT action=DENY + + op=EXECUTE fsverity_signature=TRUE action=ALLOW + +Prohibit execution of a specific fs-verity file +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +:: + + policy_name=ProhibitSpecificFSVF policy_version=0.0.0 + DEFAULT action=DENY + + op=EXECUTE fsverity_digest=sha256:fd88f2b8824e197f850bf4c5109bea5cf0ee38104f710843bb72da796ba5af9e action=DENY + op=EXECUTE boot_verified=TRUE action=ALLOW + op=EXECUTE dmverity_signature=TRUE action=ALLOW + +Additional Information +---------------------- + +- `Github Repository <https://github.com/microsoft/ipe>`_ +- Documentation/security/ipe.rst + +FAQ +--- + +Q: + What's the difference between other LSMs which provide a measure of + trust-based access control? + +A: + + In general, there's two other LSMs that can provide similar functionality: + IMA, and Loadpin. + + IMA and IPE are functionally very similar. The significant difference between + the two is the policy. [#devdoc]_ + + Loadpin and IPE differ fairly dramatically, as Loadpin only covers the IPE's + kernel read operations, whereas IPE is capable of controlling execution + on top of kernel read. The trust model is also different; Loadpin roots its + trust in the initial super-block, whereas trust in IPE is stemmed from kernel + itself (via ``SYSTEM_TRUSTED_KEYS``). + +----------- + +.. [#diglim] https://lore.kernel.org/bpf/4d6932e96d774227b42721d9f645ba51@huawei.com/T/ + +.. [#interpreters] There is `some interest in solving this issue <https://lore.kernel.org/lkml/20220321161557.495388-1-mic@digikod.net/>`_. + +.. [#devdoc] Please see Documentation/security/ipe.rst for more on this topic. + +.. [#switch_root] https://man7.org/linux/man-pages/man8/switch_root.8.html + +.. [#fsveritydigest] These hash algorithms are based on values accepted by fsverity-utils; + IPE does not impose any restrictions on the digest algorithm itself; + thus, this list may be out of date. + +.. [#dmveritydigests] These hash algorithms are based on values accepted by dm-verity, + specifically ``crypto_alloc_ahash`` in ``verity_ctr``; ``veritysetup`` + does support more algorithms than the list above. IPE does not impose + any restrictions on the digest algorithm itself; thus, this list + may be out of date. + +.. [#securedigest] Please ensure you are using cryptographically secure hash functions; + just because something is *supported* does not mean it is *secure*. diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 70046a019d42..7b7a24a59747 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -2321,6 +2321,18 @@ ipcmni_extend [KNL,EARLY] Extend the maximum number of unique System V IPC identifiers from 32,768 to 16,777,216. + ipe.enforce= [IPE] + Format: <bool> + Determine whether IPE starts in permissive (0) or + enforce (1) mode. The default is enforce. + + ipe.success_audit= + [IPE] + Format: <bool> + Start IPE with success auditing enabled, emitting + an audit event when a binary is allowed. The default + is 0. + irqaffinity= [SMP] Set the default irq affinity mask The argument is a cpu list, as described above. diff --git a/Documentation/filesystems/fsverity.rst b/Documentation/filesystems/fsverity.rst index 362b7a5dc300..46ab280e1b13 100644 --- a/Documentation/filesystems/fsverity.rst +++ b/Documentation/filesystems/fsverity.rst @@ -92,7 +92,9 @@ authenticating fs-verity file hashes include: "IPE policy" specifically allows for the authorization of fs-verity files using properties ``fsverity_digest`` for identifying files by their verity digest, and ``fsverity_signature`` to authorize - files with a verified fs-verity's built-in signature. + files with a verified fs-verity's built-in signature. For + details on configuring IPE policies and understanding its operational + modes, please refer to Documentation/admin-guide/LSM/ipe.rst. - Trusted userspace code in combination with `Built-in signature verification`_. This approach should be used only with great care. @@ -508,6 +510,7 @@ be carefully considered before using them: files with a verified fs-verity builtin signature to perform certain operations, such as execution. Note that IPE doesn't require fs.verity.require_signatures=1. + Please refer to Documentation/admin-guide/LSM/ipe.rst for more details. - A file's builtin signature can only be set at the same time that fs-verity is being enabled on the file. Changing or deleting the diff --git a/Documentation/security/index.rst b/Documentation/security/index.rst index 59f8fc106cb0..3e0a7114a862 100644 --- a/Documentation/security/index.rst +++ b/Documentation/security/index.rst @@ -19,3 +19,4 @@ Security Documentation digsig landlock secrets/index + ipe diff --git a/Documentation/security/ipe.rst b/Documentation/security/ipe.rst new file mode 100644 index 000000000000..674827982a72 --- /dev/null +++ b/Documentation/security/ipe.rst @@ -0,0 +1,444 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Integrity Policy Enforcement (IPE) - Kernel Documentation +========================================================= + +.. NOTE:: + + This is documentation targeted at developers, instead of administrators. + If you're looking for documentation on the usage of IPE, please see + Documentation/admin-guide/LSM/ipe.rst + +Historical Motivation +--------------------- + +The original issue that prompted IPE's implementation was the creation +of a locked-down system. This system would be born-secure, and have +strong integrity guarantees over both the executable code, and specific +*data files* on the system, that were critical to its function. These +specific data files would not be readable unless they passed integrity +policy. A mandatory access control system would be present, and +as a result, xattrs would have to be protected. This lead to a selection +of what would provide the integrity claims. At the time, there were two +main mechanisms considered that could guarantee integrity for the system +with these requirements: + + 1. IMA + EVM Signatures + 2. DM-Verity + +Both options were carefully considered, however the choice to use DM-Verity +over IMA+EVM as the *integrity mechanism* in the original use case of IPE +was due to three main reasons: + + 1. Protection of additional attack vectors: + + * With IMA+EVM, without an encryption solution, the system is vulnerable + to offline attack against the aforementioned specific data files. + + Unlike executables, read operations (like those on the protected data + files), cannot be enforced to be globally integrity verified. This means + there must be some form of selector to determine whether a read should + enforce the integrity policy, or it should not. + + At the time, this was done with mandatory access control labels. An IMA + policy would indicate what labels required integrity verification, which + presented an issue: EVM would protect the label, but if an attacker could + modify filesystem offline, the attacker could wipe all the xattrs - + including the SELinux labels that would be used to determine whether the + file should be subject to integrity policy. + + With DM-Verity, as the xattrs are saved as part of the Merkel tree, if + offline mount occurs against the filesystem protected by dm-verity, the + checksum no longer matches and the file fails to be read. + + * As userspace binaries are paged in Linux, dm-verity also offers the + additional protection against a hostile block device. In such an attack, + the block device reports the appropriate content for the IMA hash + initially, passing the required integrity check. Then, on the page fault + that accesses the real data, will report the attacker's payload. Since + dm-verity will check the data when the page fault occurs (and the disk + access), this attack is mitigated. + + 2. Performance: + + * dm-verity provides integrity verification on demand as blocks are + read versus requiring the entire file being read into memory for + validation. + + 3. Simplicity of signing: + + * No need for two signatures (IMA, then EVM): one signature covers + an entire block device. + * Signatures can be stored externally to the filesystem metadata. + * The signature supports an x.509-based signing infrastructure. + +The next step was to choose a *policy* to enforce the integrity mechanism. +The minimum requirements for the policy were: + + 1. The policy itself must be integrity verified (preventing trivial + attack against it). + 2. The policy itself must be resistant to rollback attacks. + 3. The policy enforcement must have a permissive-like mode. + 4. The policy must be able to be updated, in its entirety, without + a reboot. + 5. Policy updates must be atomic. + 6. The policy must support *revocations* of previously authored + components. + 7. The policy must be auditable, at any point-of-time. + +IMA, as the only integrity policy mechanism at the time, was +considered against these list of requirements, and did not fulfill +all of the minimum requirements. Extending IMA to cover these +requirements was considered, but ultimately discarded for a +two reasons: + + 1. Regression risk; many of these changes would result in + dramatic code changes to IMA, which is already present in the + kernel, and therefore might impact users. + + 2. IMA was used in the system for measurement and attestation; + separation of measurement policy from local integrity policy + enforcement was considered favorable. + +Due to these reasons, it was decided that a new LSM should be created, +whose responsibility would be only the local integrity policy enforcement. + +Role and Scope +-------------- + +IPE, as its name implies, is fundamentally an integrity policy enforcement +solution; IPE does not mandate how integrity is provided, but instead +leaves that decision to the system administrator to set the security bar, +via the mechanisms that they select that suit their individual needs. +There are several different integrity solutions that provide a different +level of security guarantees; and IPE allows sysadmins to express policy for +theoretically all of them. + +IPE does not have an inherent mechanism to ensure integrity on its own. +Instead, there are more effective layers available for building systems that +can guarantee integrity. It's important to note that the mechanism for proving +integrity is independent of the policy for enforcing that integrity claim. + +Therefore, IPE was designed around: + + 1. Easy integrations with integrity providers. + 2. Ease of use for platform administrators/sysadmins. + +Design Rationale: +----------------- + +IPE was designed after evaluating existing integrity policy solutions +in other operating systems and environments. In this survey of other +implementations, there were a few pitfalls identified: + + 1. Policies were not readable by humans, usually requiring a binary + intermediary format. + 2. A single, non-customizable action was implicitly taken as a default. + 3. Debugging the policy required manual steps to determine what rule was violated. + 4. Authoring a policy required an in-depth knowledge of the larger system, + or operating system. + +IPE attempts to avoid all of these pitfalls. + +Policy +~~~~~~ + +Plain Text +^^^^^^^^^^ + +IPE's policy is plain-text. This introduces slightly larger policy files than +other LSMs, but solves two major problems that occurs with some integrity policy +solutions on other platforms. + +The first issue is one of code maintenance and duplication. To author policies, +the policy has to be some form of string representation (be it structured, +through XML, JSON, YAML, etcetera), to allow the policy author to understand +what is being written. In a hypothetical binary policy design, a serializer +is necessary to write the policy from the human readable form, to the binary +form, and a deserializer is needed to interpret the binary form into a data +structure in the kernel. + +Eventually, another deserializer will be needed to transform the binary from +back into the human-readable form with as much information preserved. This is because a +user of this access control system will have to keep a lookup table of a checksum +and the original file itself to try to understand what policies have been deployed +on this system and what policies have not. For a single user, this may be alright, +as old policies can be discarded almost immediately after the update takes hold. +For users that manage computer fleets in the thousands, if not hundreds of thousands, +with multiple different operating systems, and multiple different operational needs, +this quickly becomes an issue, as stale policies from years ago may be present, +quickly resulting in the need to recover the policy or fund extensive infrastructure +to track what each policy contains. + +With now three separate serializer/deserializers, maintenance becomes costly. If the +policy avoids the binary format, there is only one required serializer: from the +human-readable form to the data structure in kernel, saving on code maintenance, +and retaining operability. + +The second issue with a binary format is one of transparency. As IPE controls +access based on the trust of the system's resources, it's policy must also be +trusted to be changed. This is done through signatures, resulting in needing +signing as a process. Signing, as a process, is typically done with a +high security bar, as anything signed can be used to attack integrity +enforcement systems. It is also important that, when signing something, that +the signer is aware of what they are signing. A binary policy can cause +obfuscation of that fact; what signers see is an opaque binary blob. A +plain-text policy, on the other hand, the signers see the actual policy +submitted for signing. + +Boot Policy +~~~~~~~~~~~ + +IPE, if configured appropriately, is able to enforce a policy as soon as a +kernel is booted and usermode starts. That implies some level of storage +of the policy to apply the minute usermode starts. Generally, that storage +can be handled in one of three ways: + + 1. The policy file(s) live on disk and the kernel loads the policy prior + to an code path that would result in an enforcement decision. + 2. The policy file(s) are passed by the bootloader to the kernel, who + parses the policy. + 3. There is a policy file that is compiled into the kernel that is + parsed and enforced on initialization. + +The first option has problems: the kernel reading files from userspace +is typically discouraged and very uncommon in the kernel. + +The second option also has problems: Linux supports a variety of bootloaders +across its entire ecosystem - every bootloader would have to support this +new methodology or there must be an independent source. It would likely +result in more drastic changes to the kernel startup than necessary. + +The third option is the best but it's important to be aware that the policy +will take disk space against the kernel it's compiled in. It's important to +keep this policy generalized enough that userspace can load a new, more +complicated policy, but restrictive enough that it will not overauthorize +and cause security issues. + +The initramfs provides a way that this bootup path can be established. The +kernel starts with a minimal policy, that trusts the initramfs only. Inside +the initramfs, when the real rootfs is mounted, but not yet transferred to, +it deploys and activates a policy that trusts the new root filesystem. +This prevents overauthorization at any step, and keeps the kernel policy +to a minimal size. + +Startup +^^^^^^^ + +Not every system, however starts with an initramfs, so the startup policy +compiled into the kernel will need some flexibility to express how trust +is established for the next phase of the bootup. To this end, if we just +make the compiled-in policy a full IPE policy, it allows system builders +to express the first stage bootup requirements appropriately. + +Updatable, Rebootless Policy +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +As requirements change over time (vulnerabilities are found in previously +trusted applications, keys roll, etcetera). Updating a kernel to change the +meet those security goals is not always a suitable option, as updates are not +always risk-free, and blocking a security update leaves systems vulnerable. +This means IPE requires a policy that can be completely updated (allowing +revocations of existing policy) from a source external to the kernel (allowing +policies to be updated without updating the kernel). + +Additionally, since the kernel is stateless between invocations, and reading +policy files off the disk from kernel space is a bad idea(tm), then the +policy updates have to be done rebootlessly. + +To allow an update from an external source, it could be potentially malicious, +so this policy needs to have a way to be identified as trusted. This is +done via a signature chained to a trust source in the kernel. Arbitrarily, +this is the ``SYSTEM_TRUSTED_KEYRING``, a keyring that is initially +populated at kernel compile-time, as this matches the expectation that the +author of the compiled-in policy described above is the same entity that can +deploy policy updates. + +Anti-Rollback / Anti-Replay +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Over time, vulnerabilities are found and trusted resources may not be +trusted anymore. IPE's policy has no exception to this. There can be +instances where a mistaken policy author deploys an insecure policy, +before correcting it with a secure policy. + +Assuming that as soon as the insecure policy is signed, and an attacker +acquires the insecure policy, IPE needs a way to prevent rollback +from the secure policy update to the insecure policy update. + +Initially, IPE's policy can have a policy_version that states the +minimum required version across all policies that can be active on +the system. This will prevent rollback while the system is live. + +.. WARNING:: + + However, since the kernel is stateless across boots, this policy + version will be reset to 0.0.0 on the next boot. System builders + need to be aware of this, and ensure the new secure policies are + deployed ASAP after a boot to ensure that the window of + opportunity is minimal for an attacker to deploy the insecure policy. + +Implicit Actions: +~~~~~~~~~~~~~~~~~ + +The issue of implicit actions only becomes visible when you consider +a mixed level of security bars across multiple operations in a system. +For example, consider a system that has strong integrity guarantees +over both the executable code, and specific *data files* on the system, +that were critical to its function. In this system, three types of policies +are possible: + + 1. A policy in which failure to match any rules in the policy results + in the action being denied. + 2. A policy in which failure to match any rules in the policy results + in the action being allowed. + 3. A policy in which the action taken when no rules are matched is + specified by the policy author. + +The first option could make a policy like this:: + + op=EXECUTE integrity_verified=YES action=ALLOW + +In the example system, this works well for the executables, as all +executables should have integrity guarantees, without exception. The +issue becomes with the second requirement about specific data files. +This would result in a policy like this (assuming each line is +evaluated in order):: + + op=EXECUTE integrity_verified=YES action=ALLOW + + op=READ integrity_verified=NO label=critical_t action=DENY + op=READ action=ALLOW + +This is somewhat clear if you read the docs, understand the policy +is executed in order and that the default is a denial; however, the +last line effectively changes that default to an ALLOW. This is +required, because in a realistic system, there are some unverified +reads (imagine appending to a log file). + +The second option, matching no rules results in an allow, is clearer +for the specific data files:: + + op=READ integrity_verified=NO label=critical_t action=DENY + +And, like the first option, falls short with the opposite scenario, +effectively needing to override the default:: + + op=EXECUTE integrity_verified=YES action=ALLOW + op=EXECUTE action=DENY + + op=READ integrity_verified=NO label=critical_t action=DENY + +This leaves the third option. Instead of making users be clever +and override the default with an empty rule, force the end-user +to consider what the appropriate default should be for their +scenario and explicitly state it:: + + DEFAULT op=EXECUTE action=DENY + op=EXECUTE integrity_verified=YES action=ALLOW + + DEFAULT op=READ action=ALLOW + op=READ integrity_verified=NO label=critical_t action=DENY + +Policy Debugging: +~~~~~~~~~~~~~~~~~ + +When developing a policy, it is useful to know what line of the policy +is being violated to reduce debugging costs; narrowing the scope of the +investigation to the exact line that resulted in the action. Some integrity +policy systems do not provide this information, instead providing the +information that was used in the evaluation. This then requires a correlation +with the policy to evaluate what went wrong. + +Instead, IPE just emits the rule that was matched. This limits the scope +of the investigation to the exact policy line (in the case of a specific +rule), or the section (in the case of a DEFAULT). This decreases iteration +and investigation times when policy failures are observed while evaluating +policies. + +IPE's policy engine is also designed in a way that it makes it obvious to +a human of how to investigate a policy failure. Each line is evaluated in +the sequence that is written, so the algorithm is very simple to follow +for humans to recreate the steps and could have caused the failure. In other +surveyed systems, optimizations occur (sorting rules, for instance) when loading +the policy. In those systems, it requires multiple steps to debug, and the +algorithm may not always be clear to the end-user without reading the code first. + +Simplified Policy: +~~~~~~~~~~~~~~~~~~ + +Finally, IPE's policy is designed for sysadmins, not kernel developers. Instead +of covering individual LSM hooks (or syscalls), IPE covers operations. This means +instead of sysadmins needing to know that the syscalls ``mmap``, ``mprotect``, +``execve``, and ``uselib`` must have rules protecting them, they must simple know +that they want to restrict code execution. This limits the amount of bypasses that +could occur due to a lack of knowledge of the underlying system; whereas the +maintainers of IPE, being kernel developers can make the correct choice to determine +whether something maps to these operations, and under what conditions. + +Implementation Notes +-------------------- + +Anonymous Memory +~~~~~~~~~~~~~~~~ + +Anonymous memory isn't treated any differently from any other access in IPE. +When anonymous memory is mapped with ``+X``, it still comes into the ``file_mmap`` +or ``file_mprotect`` hook, but with a ``NULL`` file object. This is submitted to +the evaluation, like any other file, however, all current trust mechanisms will +return false as there is nothing to evaluate. This means anonymous memory +execution is subject to whatever the ``DEFAULT`` is for ``EXECUTE``. + +.. WARNING:: + + This also occurs with the ``kernel_load_data`` hook, which is used by signed + and compressed kernel modules. Using signed and compressed kernel modules with + IPE will always result in the ``DEFAULT`` action for ``KMODULE``. + +Securityfs Interface +~~~~~~~~~~~~~~~~~~~~ + +The per-policy securityfs tree is somewhat unique. For example, for +a standard securityfs policy tree:: + + MyPolicy + |- active + |- delete + |- name + |- pkcs7 + |- policy + |- update + |- version + +The policy is stored in the ``->i_private`` data of the MyPolicy inode. + +Tests +----- + +IPE has KUnit Tests for the policy parser. Recommended kunitconfig:: + + CONFIG_KUNIT=y + CONFIG_SECURITY=y + CONFIG_SECURITYFS=y + CONFIG_PKCS7_MESSAGE_PARSER=y + CONFIG_SYSTEM_DATA_VERIFICATION=y + CONFIG_FS_VERITY=y + CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y + CONFIG_BLOCK=y + CONFIG_MD=y + CONFIG_BLK_DEV_DM=y + CONFIG_DM_VERITY=y + CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG=y + CONFIG_NET=y + CONFIG_AUDIT=y + CONFIG_AUDITSYSCALL=y + CONFIG_BLK_DEV_INITRD=y + + CONFIG_SECURITY_IPE=y + CONFIG_IPE_PROP_DM_VERITY=y + CONFIG_IPE_PROP_FS_VERITY=y + CONFIG_SECURITY_IPE_KUNIT_TEST=y + +In addition, IPE has a python based integration +`test suite <https://github.com/microsoft/ipe/tree/test-suite>`_ that +can test both user interfaces and enforcement functionalities.