Re: Oops at boot with linux-next kernel with IMA boot options

Takashi Iwai
2020-05-29T07:45:45Z

On Fri, 29 May 2020 09:33:34 +0200,
Roberto Sassu wrote:
>
> > From: Takashi Iwai [mailto:tiwai@suse.de]
> > On Thu, 28 May 2020 19:36:55 +0200,
> > Roberto Sassu wrote:
> > >
> > > > From: linux-integrity-owner@vger.kernel.org [mailto:linux-integrity-
> > > > owner@vger.kernel.org] On Behalf Of Takashi Iwai
> > > > On Thu, 28 May 2020 17:35:16 +0200,
> > > > Takashi Iwai wrote:
> > > > >
> > > > > Hi Roberto,
> > > > >
> > > > > it seems that the recent changes in IMA in linux-next caused a
> > > > > regression: namely it triggers an Oops when booting with the options
> > > > > ima_policy=tcb ima_template_fmt='d-ng|n-ng|d|ng'
> > > >
> > > > And further experiment revealed that passing only
> > ima_template_fmt=d
> > > > is enough for triggering the bug. Other formats don't matter.
> > > >
> > > > (snip)
> > > > > It's a KVM instance without any TPM stuff, just passed the options
> > > > > above. I could trigger the same bug on a bare metal, too.
> > > > >
> > > > > Then I performed bisection and it spotted the commit:
> > > > > 6f1a1d103b48b1533a9c804e7a069e2c8e937ce7
> > > > > ima: Switch to ima_hash_algo for boot aggregate
> > > > >
> > > > > Actually reverting this commit fixed the Oops again.
> > > >
> > > > So, looking at the fact above (triggered by "d") and this bisection
> > > > result, it seems that the issue is specific to ima_eventdigest_init().
> > > > The difference from others is that this has a check by
> > > > ima_template_hash_algo_allowed(), and currently the check allows only
> > > > SHA1 and MD5, while now SHA256 is assigned as default. So I tested
> > > > adding SHA256 there like below, and it seems working.
> > > >
> > > > Hopefully I'm heading to a right direction...
> > >
> > > Hi Takashi
> > >
> > > boot_aggregate is the only entry for which there is no file descriptor.
> > > The file descriptor is used to recalculate the digest if it is not SHA1
> > > or MD5. For boot_aggregate, we should use instead
> > > ima_calc_boot_aggregate(). I will provide a patch.
> > >
> > > I see that the .file member of event_data in
> > > ima_add_boot_aggregate() is not initialized. Can you please try
> > > to set .file to NULL?
> >
> > Tested and it didn't help. The field was already zero-initialized via
> > C99-style initialization, I believe.
>
> Found the issue.
>
> ima_evendigest_init() returns an error and after that IMA is not
> initialized. Unfortunately, ima_must_appraise() does not check
> ima_policy_flag, so the kernel crashes when ima_match_policy()
> tries to evaluate the policy which is not loaded (ima_rules = NULL).
>
> if you add at the beginning of ima_must_appraise()
>
> if (!ima_policy_flag)
> return 0;
>
> the kernel should not crash.

Confirmed. The patch below fixed the Oops.
When you cook up a proper patch with that change, feel free to put my
tested-by tag
Reported-and-tested-by: Takashi Iwai <tiwai@suse.de>

Thanks!

Takashi

--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -53,6 +53,9 @@ int ima_must_appraise(struct inode *inode, int mask, enum ima_hooks func)
if (!ima_appraise)
return 0;

+ if (!ima_policy_flag)
+ return 0;
+
security_task_getsecid(current, &secid);
return ima_match_policy(inode, current_cred(), secid, func, mask,
IMA_APPRAISE | IMA_HASH, NULL, NULL, NULL);