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

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Date: Wed, 24 Jan 2024 21:51:13 +0100
From: Jann Horn <jannh@...gle.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Kees Cook <keescook@...omium.org>, Josh Triplett <josh@...htriplett.org>, 
	Kevin Locke <kevin@...inlocke.name>, 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 9:47 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Wed, 24 Jan 2024 at 12:15, Kees Cook <keescook@...omium.org> wrote:
> >
> > Hmpf, and frustratingly Ubuntu (and Debian) still builds with
> > CONFIG_USELIB, even though it was reported[2] to them almost 4 years ago.
>
> Well, we could just remove the __FMODE_EXEC from uselib.
>
> It's kind of wrong anyway.
>
> Unlike a real execve(), where the target executable actually takes
> control and you can't actually control it (except with ptrace, of
> course), 'uselib()' really is just a wrapper around a special mmap.
>
> And you can see it in the "acc_mode" flags: uselib already requires
> MAY_READ for that reason. So you cannot uselib() a non-readable file,
> unlike execve().
>
> So I think just removing __FMODE_EXEC would just do the
> RightThing(tm), and changes nothing for any sane situation.

Sounds like a good idea. That makes this codepath behave more as if
userspace had done the same steps manually...