| Message ID | 20201111092302.1589-1-roberto.sassu@huawei.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | evm: Improve usability of portable signatures | expand |
Hi Roberto, On Wed, 2020-11-11 at 10:22 +0100, Roberto Sassu wrote: > EVM portable signatures are particularly suitable for the protection of > metadata of immutable files where metadata is signed by a software vendor. > They can be used for example in conjunction with an IMA policy that > appraises only executed and memory mapped files. The existing "appraise_tcb" builtin policy verify all root owned files. Defining a new builtin policy to verify only executed and memory mmapped files would make a nice addition and would probably simplify testing. > > However, some usability issues are still unsolved, especially when EVM is > used without loading an HMAC key. This patch set attempts to fix the open > issues. We need regression tests for each of these changes. To prevent affecting the running system, the appraise policy rules could be limited to a loopback mounted filesystem. > > Patch 1 allows EVM to be used without loading an HMAC key. Patch 2 avoids > appraisal verification of public keys (they are already verified by the key > subsystem). Loading the EVM key(s) occurs early, either the builtin x509 EVM key or during the initramfs, makes testing difficult. Based on security/evm/evm, different tests could be defined for when only x509 keys, only HMAC key, or both EVM key types are loaded. > > Patches 3-5 allow metadata verification to be turned off when no HMAC key > is loaded and to use this mode in a safe way (by ensuring that IMA > revalidates metadata when there is a change). > > Patches 6-8 make portable signatures more usable if metadata verification > is not turned off, by ignoring the INTEGRITY_NOLABEL error when no HMAC key > is loaded, by accepting any metadata modification until signature > verification succeeds (useful when xattrs/attrs are copied sequentially > from a source) and by allowing operations that don't change metadata. > > Patch 9 makes it possible to use portable signatures when the IMA policy > requires file signatures and patch 10 shows portable signatures in the > measurement list when the ima-sig template is selected. ima-evm-utils needs to be updated to support EVM portable & immutable signatures. > > Lastly, patch 11 avoids undesired removal of security.ima when a file is > not selected by the IMA policy. thanks, Mimi
EVM portable signatures are particularly suitable for the protection of metadata of immutable files where metadata is signed by a software vendor. They can be used for example in conjunction with an IMA policy that appraises only executed and memory mapped files. However, some usability issues are still unsolved, especially when EVM is used without loading an HMAC key. This patch set attempts to fix the open issues. Patch 1 allows EVM to be used without loading an HMAC key. Patch 2 avoids appraisal verification of public keys (they are already verified by the key subsystem). Patches 3-5 allow metadata verification to be turned off when no HMAC key is loaded and to use this mode in a safe way (by ensuring that IMA revalidates metadata when there is a change). Patches 6-8 make portable signatures more usable if metadata verification is not turned off, by ignoring the INTEGRITY_NOLABEL error when no HMAC key is loaded, by accepting any metadata modification until signature verification succeeds (useful when xattrs/attrs are copied sequentially from a source) and by allowing operations that don't change metadata. Patch 9 makes it possible to use portable signatures when the IMA policy requires file signatures and patch 10 shows portable signatures in the measurement list when the ima-sig template is selected. Lastly, patch 11 avoids undesired removal of security.ima when a file is not selected by the IMA policy. Roberto Sassu (11): evm: Execute evm_inode_init_security() only when an HMAC key is loaded evm: Load EVM key in ima_load_x509() to avoid appraisal evm: Refuse EVM_ALLOW_METADATA_WRITES only if an HMAC key is loaded ima: Move ima_reset_appraise_flags() call to post hooks evm: Introduce evm_status_revalidate() evm: Ignore INTEGRITY_NOLABEL if no HMAC key is loaded evm: Allow xattr/attr operations for portable signatures evm: Allow setxattr() and setattr() for unmodified metadata ima: Allow imasig requirement to be satisfied by EVM portable signatures ima: Introduce template field evmsig and write to field sig as fallback ima: Don't remove security.ima if file must not be appraised Documentation/ABI/testing/evm | 5 +- Documentation/security/IMA-templates.rst | 4 +- fs/xattr.c | 2 + include/linux/evm.h | 6 + include/linux/ima.h | 19 ++- include/linux/integrity.h | 1 + security/integrity/evm/evm_main.c | 178 ++++++++++++++++++++-- security/integrity/evm/evm_secfs.c | 2 +- security/integrity/iint.c | 2 + security/integrity/ima/ima_appraise.c | 52 +++++-- security/integrity/ima/ima_init.c | 4 + security/integrity/ima/ima_template.c | 2 + security/integrity/ima/ima_template_lib.c | 32 +++- security/integrity/ima/ima_template_lib.h | 2 + security/security.c | 4 +- 15 files changed, 275 insertions(+), 40 deletions(-)