RE: [PATCH v3 0/6] evm: Prepare for moving to the LSM infrastructure

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> From: Roberto Sassu
> Sent: Tuesday, April 27, 2021 1:37 PM
> This patch set depends on:
> roberto.sassu@xxxxxxxxxx/
> roberto.sassu@xxxxxxxxxx/
> One of the challenges that must be tackled to move IMA and EVM to the
> infrastructure is to ensure that EVM is capable to correctly handle
> multiple stacked LSMs providing an xattr at file creation. At the moment,
> there are few issues that would prevent a correct integration. This patch
> set aims at solving them.


anything I should do more for this patch set?

The first two patches are bug fixes and should be ready for merge.

For the rest, this patch set would be still useful even if IMA/EVM are
not moved to the LSM infrastructure. Its main purpose is to make it
possible to use multiple LSMs providing an implementation for the
inode_init_security hook.

Although this wouldn't be needed by LSMs already in the kernel,
as there can be only one major LSM, it would be useful for people
developing new LSMs.



Managing Director: Li Peng, Li Jian, Shi Yanli

> From the LSM infrastructure side, the LSM stacking feature added the
> possibility of registering multiple implementations of the security hooks,
> that are called sequentially whenever someone calls the corresponding
> security hook. However, security_inode_init_security() and
> security_old_inode_init_security() are currently limited to support one
> xattr provided by LSM and one by EVM.
> In addition, using the call_int_hook() macro causes some issues. According
> to the documentation in include/linux/lsm_hooks.h, it is a legitimate case
> that an LSM returns -EOPNOTSUPP when it does not want to provide an
> xattr.
> However, the loop defined in the macro would stop calling subsequent LSMs
> if that happens. In the case of security_old_inode_init_security(), using
> the macro would also cause a memory leak due to replacing the *value
> pointer, if multiple LSMs provide an xattr.
> From EVM side, the first operation to be done is to change the definition
> of evm_inode_init_security() to be compatible with the security hook
> definition. Unfortunately, the current definition does not provide enough
> information for EVM, as it must have visibility of all xattrs provided by
> LSMs to correctly calculate the HMAC. This patch set changes the security
> hook definition by replacing the name, value and len triple with the xattr
> array allocated by security_inode_init_security().
> Secondly, EVM must know how many elements are in the xattr array. EVM
> can
> rely on the fact that the xattr array must be terminated with an element
> with name field set to NULL, but can also benefit from the enhancements
> that have been included in this version of the patch set.
> Casey suggested to use the reservation mechanism currently implemented
> for
> other security blobs, for xattrs. In this way,
> security_inode_init_security() can know after LSM initialization how many
> slots for xattrs should be allocated, and LSMs know the offset in the
> array from where they can start writing xattrs.
> One of the problem was that LSMs can decide at run-time, although they
> reserved a slot, to not use it (for example because they were not
> initialized). Given that the initxattrs() method implemented by filesystems
> expect that the array is continuous, they would miss the slots after the
> one not being initialized. security_inode_init_security() should have been
> modified to compact the array.
> Instead, the preferred solution was to introduce the base slot as a
> parameter, in addition to the xattr array, containing the up to date
> information about the slots used by previous LSMs. The correctness of the
> update of the slot is ensured by both the LSMs, if they use the new helper
> lsm_find_xattr_slot(), and by security_inode_init_security() which checks
> the slot each time after an LSM executes the inode_init_security hook.
> This patch set has been tested by introducing several instances of a
> TestLSM (some providing an xattr, some not, one with a wrong
> implementation
> to see how the LSM infrastructure handles it). The patch is not included
> in this set but it is available here:
> 269fff7e30053f9
> The test, added to ima-evm-utils, is available here:
> devel-v7/tests/evm_multiple_lsms.test
> The test takes a UML kernel built by Travis and launches it several times,
> each time with a different combination of LSMs. After boot, it first checks
> that there is an xattr for each LSM providing it, and then calculates the
> HMAC in user space and compares it with the HMAC calculated by EVM in
> kernel space.
> A test report can be obtained here:
> SELinux Test Suite result (diff 5.11.14-200.fc33.x86_64 5.12.0-rc8+):
> -Files=70, Tests=1099, 82 wallclock secs ( 0.35 usr  0.09 sys +  7.39 cusr 10.14
> csys = 17.97 CPU)
> +Files=70, Tests=1108, 85 wallclock secs ( 0.34 usr  0.10 sys +  7.25 cusr 11.39
> csys = 19.08 CPU)
>  Result: FAIL
> -Failed 2/70 test programs. 5/1099 subtests failed.
> +Failed 2/70 test programs. 5/1108 subtests failed.
> Smack Test Suite result:
> smack_set_ambient 1 TPASS: Test "smack_set_ambient" success.
> smack_set_current 1 TPASS: Test "smack_set_current" success.
> smack_set_doi 1 TPASS: Test "smack_set_doi" success.
> smack_set_netlabel 1 TPASS: Test "smack_set_netlabel" success.
> smack_set_socket_labels    1  TPASS  :  Test smack_set_socket_labels
> success.
> smack_set_cipso 1 TPASS: Test "smack_set_cipso" success.
> smack_set_direct 1 TPASS: Test "smack_set_direct" success.
> smack_set_load 1 TPASS: Test "smack_set_load" success.
> smack_set_onlycap 1 TFAIL: The smack label reported for "/smack/onlycap"
> Lastly, running the test on reiserfs to check
> security_old_inode_init_security(), some issues have been discovered: a
> free of xattr->name which is not correct after commit 9548906b2bb7 ('xattr:
> Constify ->name member of "struct xattr"'), missing calls to
> reiserfs_security_free() and a misalignment with
> security_inode_init_security() (the old version expects the full xattr name
> with the security. prefix, the new version just the suffix). The last issue
> has not been fixed yet.
> Changelog
> v2:
> - rewrite selinux_old_inode_init_security() to use
>   security_inode_init_security()
> - add lbs_xattr field to lsm_blob_sizes structure, to give the ability to
>   LSMs to reserve slots in the xattr array (suggested by Casey)
> - add new parameter base_slot to inode_init_security hook definition
> v1:
> - add calls to reiserfs_security_free() and initialize sec->value to NULL
>   (suggested by Tetsuo and Mimi)
> - change definition of inode_init_security hook, replace the name, value
>   and len triple with the xattr array (suggested by Casey)
> - introduce lsm_find_xattr_slot() helper for LSMs to find an unused slot in
>   the passed xattr array
> Roberto Sassu (6):
>   reiserfs: Add missing calls to reiserfs_security_free()
>   security: Rewrite security_old_inode_init_security()
>   security: Pass xattrs allocated by LSMs to the inode_init_security
>     hook
>   security: Support multiple LSMs implementing the inode_init_security
>     hook
>   evm: Align evm_inode_init_security() definition with LSM
>     infrastructure
>   evm: Support multiple LSMs providing an xattr
>  fs/reiserfs/namei.c                 |   4 +
>  fs/reiserfs/xattr_security.c        |   1 +
>  include/linux/evm.h                 |  19 +++--
>  include/linux/lsm_hook_defs.h       |   4 +-
>  include/linux/lsm_hooks.h           |  22 +++++-
>  security/integrity/evm/evm.h        |   2 +
>  security/integrity/evm/evm_crypto.c |   9 ++-
>  security/integrity/evm/evm_main.c   |  30 +++++--
>  security/security.c                 | 116 +++++++++++++++++++++++-----
>  security/selinux/hooks.c            |  18 +++--
>  security/smack/smack_lsm.c          |  27 ++++---
>  11 files changed, 194 insertions(+), 58 deletions(-)
> --
> 2.25.1

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