From patchwork Sat Apr 13 00:56:03 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Fan Wu X-Patchwork-Id: 13628531 Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 8BA921CABD; Sat, 13 Apr 2024 00:56:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=13.77.154.182 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712969777; cv=none; b=dBR1zgKiqM5l/u8zwxOZ7Skjko4q1Ahs9V/Emd+FMXSGRhGZBV12YMzhUJHm80HIaWBn4wyRjKLsAVVumZGgatENN3xrmkGVniNPJBFnbWxJ4CVftPseDKKnp4QWcuAuzYgLGMNiiyLX/QcEQJmx/cOiIoDxBU+hOJcpjYDHE94= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712969777; c=relaxed/simple; bh=jxuWpKyq56yak3at44V13gEBY0Gm0IwhhB3FgTjdyBA=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References; b=psA1A/I2OQWd+B0eQlmn04btQXuwsEya3ico9iUuIlWAC00jVvrvbERuxMW35b+awMvrDfHIcfcKLOhoIC/f4H9Hk7IPNDoBf9oOyHovuwkmeZHN/F/N5nxEZhfGrRwlfi1BDburdbBp4x5zBLJDSQ71vHpup2alR72ZV6qyb8g= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com; spf=pass smtp.mailfrom=linux.microsoft.com; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b=WM0Vwxdb; arc=none smtp.client-ip=13.77.154.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b="WM0Vwxdb" Received: by linux.microsoft.com (Postfix, from userid 1052) id 9E46C20FC5F8; Fri, 12 Apr 2024 17:56:06 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 9E46C20FC5F8 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1712969766; bh=fkpM7b6dPDRUAmzBIhSMYaVqv4GOML18VZzApD5bST8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=WM0VwxdbKcqu1mOOvJSnqJGMKfKk6/IqJep2pZWvH1FT5sU5jki2FFei3ESWvUz6X QJOFYvezQCMfGnKpJM6fd9zxAasFQYApTv/ToD819gczPq1KLRF6G3aNTKY6ryiTf3 z1d+qeNDP0u26dBG0MT5429S7TI0QowPj34IZ9P8= From: Fan Wu To: corbet@lwn.net, zohar@linux.ibm.com, jmorris@namei.org, serge@hallyn.com, tytso@mit.edu, ebiggers@kernel.org, axboe@kernel.dk, agk@redhat.com, snitzer@kernel.org, eparis@redhat.com, paul@paul-moore.com Cc: linux-doc@vger.kernel.org, linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, fsverity@lists.linux.dev, linux-block@vger.kernel.org, dm-devel@lists.linux.dev, audit@vger.kernel.org, linux-kernel@vger.kernel.org, Deven Bowers , Fan Wu Subject: [PATCH v17 20/21] Documentation: add ipe documentation Date: Fri, 12 Apr 2024 17:56:03 -0700 Message-Id: <1712969764-31039-21-git-send-email-wufan@linux.microsoft.com> X-Mailer: git-send-email 1.8.3.1 In-Reply-To: <1712969764-31039-1-git-send-email-wufan@linux.microsoft.com> References: <1712969764-31039-1-git-send-email-wufan@linux.microsoft.com> Precedence: bulk X-Mailing-List: fsverity@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: From: Deven Bowers Add IPE's admin and developer documentation to the kernel tree. Co-developed-by: Fan Wu Signed-off-by: Deven Bowers Signed-off-by: Fan Wu Reviewed-by: Bagas Sanjaya --- v2: + No Changes v3: + Add Acked-by + Fixup code block syntax + Fix a minor grammatical issue. v4: + Update documentation with the results of other code changes. v5: + No changes v6: + No changes v7: + Add additional developer-level documentation + Update admin-guide docs to reflect changes. + Drop Acked-by due to significant changes + Added section about audit events in admin-guide v8: + Correct terminology from "audit event" to "audit record" + Add associated documentation with the correct "audit event" terminology. + Add some context to the historical motivation for IPE and design philosophy. + Add some content about the securityfs layout in the policies directory. + Various spelling and grammatical corrections. v9: + Correct spelling of "pitfalls" + Update the docs w.r.t the new parser and new audit formats v10: + Refine user docs per upstream suggestions + Update audit events part v11: + No changes v12: + Update audit formats + Update initramfs related docs + Add test suite link v13: + No changes v14: + No changes v15: + Update boot_verified part + Fix format issues + Add IPE doc link to fsverity.rst v16: + Explicitly mention fsverity builtin signature v17: + Rewrite many parts of Documentation/admin-guide/LSM/ipe.rst + Fix incorrect path name of policyfs interfaces --- Documentation/admin-guide/LSM/index.rst | 1 + Documentation/admin-guide/LSM/ipe.rst | 797 ++++++++++++++++++ .../admin-guide/kernel-parameters.txt | 12 + Documentation/filesystems/fsverity.rst | 5 +- Documentation/security/index.rst | 1 + Documentation/security/ipe.rst | 444 ++++++++++ 6 files changed, 1259 insertions(+), 1 deletion(-) create mode 100644 Documentation/admin-guide/LSM/ipe.rst create mode 100644 Documentation/security/ipe.rst 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 `_ +- 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 `_. + +.. [#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: + Determine whether IPE starts in permissive (0) or + enforce (1) mode. The default is enforce. + + ipe.success_audit= + [IPE] + Format: + 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 `_ that +can test both user interfaces and enforcement functionalities.