From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030827Ab2CTRXH (ORCPT ); Tue, 20 Mar 2012 13:23:07 -0400 Received: from mail-wi0-f194.google.com ([209.85.212.194]:56705 "EHLO mail-wi0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752111Ab2CTRXF (ORCPT ); Tue, 20 Mar 2012 13:23:05 -0400 Date: Tue, 20 Mar 2012 18:23:01 +0100 From: Ingo Molnar To: Kees Cook Cc: Thomas Gleixner , Serge Hallyn , linux-kernel@vger.kernel.org, Darren Hart , Peter Zijlstra , Andrew Morton , Jiri Kosina , "Eric W. Biederman" , David Howells , kernel-hardening@lists.openwall.com, spender@grsecurity.net Subject: Re: [PATCH] futex: do not leak robust list to unprivileged process Message-ID: <20120320172300.GA8888@gmail.com> References: <20120319231253.GA20893@www.outflux.net> <20120320133141.GC2918@peqn> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Kees Cook wrote: > On Tue, Mar 20, 2012 at 10:02 AM, Thomas Gleixner wrote: > > On Tue, 20 Mar 2012, Serge Hallyn wrote: > > > >> Quoting Kees Cook (keescook@chromium.org): > >> > It was possible to extract the robust list head address from a setuid > >> > process if it had used set_robust_list(), allowing an ASLR info leak. This > >> > changes the permission checks to be the same as those used for similar > >> > info that comes out of /proc. > >> > > >> > Running a setuid program that uses robust futexes would have had: > >> >   cred->euid != pcred->euid > >> >   cred->euid == pcred->uid > >> > so the old permissions check would allow it. I'm not aware of any setuid > >> > programs that use robust futexes, so this is just a preventative measure. > >> > > >> > (This patch is based on changes from grsecurity.) > >> > > >> > Signed-off-by: Kees Cook > >> > >> I like the change.  Much cleaner.  I'm not 100% sure though that > >> there are no legitimate cases of robust futexes use which would now > >> be forbidden.  (Explicitly cc:ing Ingo) > > > > get_robust_list is not necessary for robust futexes. There is no > > reference to get_robust_list in glibc. > > > > I really wonder why we have this syscall at all. > > The documentation I found yesterday while looking at this was: > http://linux.die.net/man/2/get_robust_list > > Which says "The system call is only available for debugging > purposes and is not needed for normal operations. Both system > calls are not available to application programs as functions; > they can be called using the syscall(3) function." > > Dropping the syscall entirely would certainly make it secure. > ;) The thinking was API completeness. In general it's possible for a sufficiently privileged task to figure out all the state of a task. We can query timers, fds - the robust list is such a resource as well. The information leakage was obviously not intended. Thanks, Ingo