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

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Message-ID: <202401241206.031E2C75B@keescook>
Date: Wed, 24 Jan 2024 12:15:05 -0800
From: Kees Cook <keescook@...omium.org>
To: Jann Horn <jannh@...gle.com>
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 08:58:55PM +0100, Jann Horn wrote:
> 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.

Hrm, that's true.

We've been trying to remove uselib for at least 10 years[1]. :(

> 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.

Hmpf, and frustratingly Ubuntu (and Debian) still builds with
CONFIG_USELIB, even though it was reported[2] to them almost 4 years ago.

-Kees

[1] https://lore.kernel.org/lkml/20140221181103.GA5773@jtriplet-mobl1/
[2] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1879454

-- 
Kees Cook