linux-kernel - Re: [PATCH 1/3] set_dumpable: fix the theoretical race with itself

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Date:	Mon, 18 Nov 2013 08:36:48 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"security@...nel.org" <security@...nel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Vasily Kulikov <segoon@...nwall.com>,
	Petr Matousek <pmatouse@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Alex Kelly <alex.page.kelly@...il.com>,
	Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 1/3] set_dumpable: fix the theoretical race with itself

On Sat, Nov 16, 2013 at 11:01 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> set_dumpable() updates MMF_DUMPABLE_MASK in a non-trivial way to
> ensure that get_dumpable() can't observe the intermediate state,
> but this all can't help if multiple threads call set_dumpable()
> at the same time.
>
> And in theory commit_creds()->set_dumpable(SUID_DUMP_ROOT) racing
> with sys_prctl()->set_dumpable(SUID_DUMP_DISABLE) can result in
> SUID_DUMP_USER.
>
> Change this code to update both bits atomically via cmpxchg().
>
> Note: this assumes that it is safe to mix bitops and cmpxchg. IOW,
> if, say, an architecture implements cmpxchg() using the locking
> (like arch/parisc/lib/bitops.c does), then it should use the same
> locks for set_bit/etc.
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

I can't speak to the bitops/cmpxchg, but the rest looks fine to me.

Acked-by: Kees Cook <keescook@...omium.org>

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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