[Cc'ing linux-audit]

Hi Simon,

On Wed, 2021-08-11 at 11:40 +0000, THOBY Simon wrote:

Other than the two questions on " IMA: add a policy option to restrict
xattr hash algorithms on appraisal" patch, the patch set is looking
good.

thanks,

Mimi

Here is also a short description of the new audit messages, but I can
send it in a followup mail if that is not the proper place:

When writing the xattr with an algorithm not built in the kernel (here the
kernel was built with CONFIG_CRYPTO_MD5 unset), e.g. with
"evmctl ima_hash -a md5 /usr/bin/strace":
	audit(1628066120.418:121): pid=1344 uid=0 auid=0 ses=1 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 op=set_data cause=unavailable-hash-algorithm comm="evmctl" name="/usr/bin/strace" dev="dm-0" ino=2632657 res=0 errno=0

With the same command and the policy rule
"appraise func=SETXATTR_CHECK appraise_algos=sha256,sha384,sha512", we get:
	audit(1628066210.141:127): pid=1362 uid=0 auid=0 ses=1 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 op=set_data cause=denied-hash-algorithm comm="evmctl" name="/usr/bin/strace" dev="dm-0" ino=2632657 res=0 errno=0

Note that the cause is now 'denied-hash-algorithm' instead of
'unavailable-hash-algorithm'. We get that audit message for any algorithm
outside of sha256/384/512 (including algorithms not compiled in the kernel
like MD5). In a sense, 'denied-hash-algorithm' takes predecence over
'unavailable-hash-algorithm'.

When appraising files, e.g. trying to execute a file whose xattr was hashed
with sha1 while the policy rule
"appraise func=BPRM_CHECK fowner=0 appraise_algos=sha256" is enabled:
	audit(1628066349.230:130): pid=1369 uid=0 auid=0 ses=1 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 op=collect_data cause=denied-hash-algorithm comm="bash" name="/usr/bin/strace" dev="dm-0" ino=2632657 res=0 errno=0


This series is based on the following repo/branch:
 repo: https://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity.git
 branch: next-integrity-testing
 commit e37be5343ae2b9419aea1442b07e5d2428b437b4 ("Merge branch 'ima-buffer-measurement-changes-v4' into next-integrity")