Re: [PATCH v7 0/5] IMA: restrict the accepted digest algorithms for the security.ima xattr

[Mimi Zohar <zohar@linux.ibm.com>]

[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")


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