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From: Jann Horn <jannh@google.com>
Date: Wed, 24 Jan 2024 21:51:13 +0100
Message-ID: <CAG48ez3v=dWVNaLwQi_f0j5X2+g5e9ubuaZoEkivsCTVK5u24Q@mail.gmail.com>
Subject: Re: [PATCH] exec: Check __FMODE_EXEC instead of in_execve for LSMs
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>, Josh Triplett <josh@joshtriplett.org>, 
	Kevin Locke <kevin@kevinlocke.name>, John Johansen <john.johansen@canonical.com>, 
	Paul Moore <paul@paul-moore.com>, James Morris <jmorris@namei.org>, 
	"Serge E. Hallyn" <serge@hallyn.com>, Kentaro Takeda <takedakn@nttdata.co.jp>, 
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>, 
	Alexander Viro <viro@zeniv.linux.org.uk>, Christian Brauner <brauner@kernel.org>, Jan Kara <jack@suse.cz>, 
	Eric Biederman <ebiederm@xmission.com>, Andrew Morton <akpm@linux-foundation.org>, 
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, 
	apparmor@lists.ubuntu.com, linux-security-module@vger.kernel.org, 
	linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
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On Wed, Jan 24, 2024 at 9:47=E2=80=AFPM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Wed, 24 Jan 2024 at 12:15, Kees Cook <keescook@chromium.org> wrote:
> >
> > Hmpf, and frustratingly Ubuntu (and Debian) still builds with
> > CONFIG_USELIB, even though it was reported[2] to them almost 4 years ag=
o.
>
> 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...