From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761942AbYDKT1r (ORCPT ); Fri, 11 Apr 2008 15:27:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1761692AbYDKT1X (ORCPT ); Fri, 11 Apr 2008 15:27:23 -0400 Received: from gprs189-60.eurotel.cz ([160.218.189.60]:4297 "EHLO spitz.ucw.cz" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1761653AbYDKT1W (ORCPT ); Fri, 11 Apr 2008 15:27:22 -0400 Date: Fri, 11 Apr 2008 21:26:56 +0200 From: Pavel Machek To: Jan Kara Cc: Jiri Kosina , Michal Hocko , Meelis Roos , Linux Kernel list , linux-fsdevel@vger.kernel.org Subject: Re: file offset corruption on 32-bit machines? Message-ID: <20080411192656.GB4492@ucw.cz> References: <200804101555.58643.mhocko@suse.cz> <20080410142700.GC6725@atrey.karlin.mff.cuni.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080410142700.GC6725@atrey.karlin.mff.cuni.cz> User-Agent: Mutt/1.5.9i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 2008-04-10 16:27:00, Jan Kara wrote: > > On Thu, 10 Apr 2008, Michal Hocko wrote: > > > > > > Jeff Robertson analyzes the behaviour of different operating systems' > > > > 64-bit file offset implementation and concludes that on 32-bit > > > > machines, Linux and Solaris lack any locking to keep the two 32-bit > > > > halves in sync and this could cause rare file offset corruption. > > > > http://jeffr-tech.livejournal.com/21014.html > > > AFAICS, this race is theoretically possible, but it is very hard (almost > > > impossible) to trigger with a sane file usage pattern. Note that you > > > have to access shared struct file (same file descriptor) in different > > > threads which should be synchronized by caller anyway (*). > > > > ... but not in cases the caller is an intentionally evil code, right? :) > Yes. > > > > I also don't see any security implications from this race, but maybe > > > someone with more knowlage about fs can see (f_pos is used at many > > > places in the kernel code). > > > > The f_pos races are in fact exploitable, we've already been there. See > > for example http://www.isec.pl/vulnerabilities/isec-0016-procleaks.txt > Well, this race is more subtle - the window is just one instruction > wide (stores to f_pos from CPU2 must come between the store of lower and > upper 32-bits of f_pos on CPU1). And the only result is that f_pos has > 32-bits from one file pointer and 32-bits from the other one. So I can > hardly imagine this would be exploitable... Don't we have rlimit on max file size? I'd guess this could work around it? Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html