linux-kernel - Re: Credentials not fully initialized before bprm

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Message-ID: <Z_f-uBGhBq9CYmaw@lei>
Date: Thu, 10 Apr 2025 17:24:08 +0000
From: sergeh@...nel.org
To: Roberto Sassu <roberto.sassu@...weicloud.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Kees Cook <kees@...nel.org>,
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org,
	linux-integrity@...r.kernel.org, zohar@...ux.ibm.com
Subject: Re: Credentials not fully initialized before bprm_check LSM hook

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?

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()?

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

> 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
>