Re: Credentials not fully initialized before bprm_check LSM hook

[Roberto Sassu <roberto.sassu@huaweicloud.com>]

On Thu, 2025-04-10 at 17:24 +0000, sergeh@kernel.org wrote:
> On Thu, Apr 10, 2025 at 01:47:07PM +0200, Roberto Sassu wrote:
> > Hi everyone
> > 
> > recently I discovered a problem in the implementation of our IMA
> > bprm_check hook, in particular when the policy is matched against the
> > bprm credentials (to be committed later during execve().
> > 
> > Before commit 56305aa9b6fab ("exec: Compute file based creds only
> > once"), bprm_fill_uid() was called in prepare_binprm() and filled the
> > euid/egid before calling security_bprm_check(), which in turns calls
> > IMA.
> > 
> > After that commit, bprm_fill_uid() was moved to begin_new_exec(), which
> > is when the last interpreter is found.
> > 
> > The consequence is that IMA still sees the not yet ready credentials
> > and an IMA rule like:
> > 
> > measure func=CREDS_CHECK euid=0
> 
> "IMA still sees" at which point exactly?

IMA sees the credentials in bprm->cred prepared with
prepare_bprm_creds(), where the euid/egid are taken from the current
process.

> Do I understand right that the problem is that ima's version of
> security_bprm_creds_for_exec() needs to run after
> bprm_creds_from_file()?

IMA's version of security_bprm_check(). security_bprm_creds_for_exec()
is for checking scripts executed by the interpreters with execveat()
and the AT_EXECVE_CHECK flag.

Uhm, it would not be technically a problem to move the IMA hook later,
but it would miss the intermediate binary search steps, which are
visible with security_bprm_check().

> Given that Eric's commit message said that no bprm handlers use
> the uid, it seems it should be safe to just move that?

Well, we just found one :)

Thanks

Roberto

> > will not be matched for sudo-like applications.
> > 
> > It does work however with SELinux, because it computes the transition
> > before IMA in the bprm_creds_for_exec hook.
> > 
> > Since IMA needs to be involved for each execution in the chain of
> > interpreters, we cannot move to the bprm_creds_from_file hook.
> > 
> > How do we solve this problem? The commit mentioned that it is an
> > optimization, so probably would not be too hard to partially revert it
> > (and keeping what is good).
> > 
> > Thanks
> > 
> > Roberto
> >