linux-hardening - Re: [PATCH] exec: Check __FMODE_EXEC instead of in

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Date: Wed, 24 Jan 2024 20:58:55 +0100
From: Jann Horn <jannh@...gle.com>
To: Kees Cook <keescook@...omium.org>
Cc: Josh Triplett <josh@...htriplett.org>, Kevin Locke <kevin@...inlocke.name>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, John Johansen <john.johansen@...onical.com>, 
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>, 
	"Serge E. Hallyn" <serge@...lyn.com>, Kentaro Takeda <takedakn@...data.co.jp>, 
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Eric Biederman <ebiederm@...ssion.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	apparmor@...ts.ubuntu.com, linux-security-module@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] exec: Check __FMODE_EXEC instead of in_execve for LSMs

On Wed, Jan 24, 2024 at 8:22 PM Kees Cook <keescook@...omium.org> wrote:
> After commit 978ffcbf00d8 ("execve: open the executable file before
> doing anything else"), current->in_execve was no longer in sync with the
> open(). This broke AppArmor and TOMOYO which depend on this flag to
> distinguish "open" operations from being "exec" operations.
>
> Instead of moving around in_execve, switch to using __FMODE_EXEC, which
> is where the "is this an exec?" intent is stored. Note that TOMOYO still
> uses in_execve around cred handling.

I think this is wrong. When CONFIG_USELIB is enabled, the uselib()
syscall will open a file with __FMODE_EXEC but without going through
execve(). From what I can tell, there are no bprm hooks on this path.

I don't know if it _matters_ much, given that it'll only let you
read/execute stuff from files with valid ELF headers, but still.