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

Roberto Sassu
2020-05-29T07:33:38Z

> 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.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

> thanks,
>
> Takashi
>
> >
> > Thanks
> >
> > Roberto
> >
> > HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
> > Managing Director: Li Peng, Li Jian, Shi Yanli
> >
> > > thanks,
> > >
> > > Takashi
> > >
> > > --- a/security/integrity/ima/ima_template_lib.c
> > > +++ b/security/integrity/ima/ima_template_lib.c
> > > @@ -13,7 +13,8 @@
> > >
> > > static bool ima_template_hash_algo_allowed(u8 algo)
> > > {
> > > - if (algo == HASH_ALGO_SHA1 || algo == HASH_ALGO_MD5)
> > > + if (algo == HASH_ALGO_SHA1 || algo == HASH_ALGO_SHA256 ||
> > > + algo == HASH_ALGO_MD5)
> > > return true;
> > >
> > > return false;
> >