Re: [PATCH 0/2] support to read and tune appraise mode in runtime

[Tianjia Zhang]

On 2020/4/14 5:55, Mimi Zohar wrote:
> On Thu, 2020-04-09 at 11:39 +0800, Tianjia Zhang wrote:
> > Support the read and write operations of ima_appraise by adding a
> > securifyfs file 'appraise_mode'.
> >
> > In order to tune appraise mode in runtime, writing a PKCS#7 signature
> > corresponding the signed content is required. The content should be off,
> > enforce, log or fix. Given a simple way to archive this:
> >
> > $ echo -n off > mode
> > $ openssl smime -sign -nocerts -noattr -binary \
> >      -in mode -inkey <system_trusted_key> \
> >      -signer <cert> -outform der -out mode.p7s
> > $ sudo cat mode.p7s \
> >      > /sys/kernel/security/ima/appraise_mode
> >
> > Note that the signing key must be a trust key located in
> > system trusted keyring. So even the root privilege cannot
> > simply disable the enforcement.
>
> There are major problems with disabling IMA appraisal. ÂThis patch set
> proposes disabling IMA appraisal without even providing the motivation
> for such support.
>
> A lot of effort went into preventing custom IMA policies from
> disabling appraising the kexec or kernel module signatures. ÂIn
> addition, the "lockdown" patch set was upstreamed permitting IMA
> signature verification. ÂThis patch set would break both of these
> features.
>
> IMA relies on its own keyring for verifying file signatures, not the
> builtin or secondary trusted kernel keyrings.
>
> Two methods already exist - xattr and appended signatures - for
> verifying file signatures. ÂThis patch set assumes creating and
> signing a file, which is then written to a securityfs file. ÂLike for
> loading a custom IMA policy, instead of cat'ing the file, write the
> pathname to the securityfs file.
>
> If you must define a new IMA method for verifying file signatures,
> then it needs to be generic and added to ima_appraise_measurement().
>  Â(Refer to the new IMA appended signature support.)
>
> Mimi
>
> > Tianjia Zhang (2):
> >    ima: support to read appraise mode
> >    ima: support to tune appraise mode in runtime
> >
> >   security/integrity/ima/ima_fs.c | 134 +++++++++++++++++++++++++++++++-
> >   1 file changed, 133 insertions(+), 1 deletion(-)

Thanks for your suggestion, the way to close the appraise mode here is 
indeed a bit rude, I will reconsider again according to your suggestions.

In addition, [PATCH 1/2] ima: support to read appraise mode, by the way, 
see if this patch is acceptable.

Thanks and best,
Tianjia