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

[Tianjia Zhang]

On 2020/4/14 21:41, Mimi Zohar wrote:
> On Tue, 2020-04-14 at 11:36 +0800, Tianjia Zhang wrote:
> > 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.
>
> My comments were not meant as suggestions, but as an explanation as to
> how IMA works. ÂMore details follow.
>
> IMA is based on policy. ÂThat decision was made a long time ago. ÂIt
> allowed distros to configure IMA, allowing customers to experiment
> with it. ÂYou have one opportunity to totally change the boot time
> policy rules, by loading a custom policy. ÂAfter that, rules may only
> be added.
>
> There is no valid reason for "turning off" the policy once it has been
> enabled. ÂIt breaks existing expectations.
>
> Mimi

Thank you very much for your explanation. I'm sorry I may not have 
stated clearly. I didn't have to change the working mode of IMA. I was 
convinced by you to give up the idea of "turn off" the appraise. 
However, it should be possible to support appraise mode reading. Right?