Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752410AbdLSUeF (ORCPT ); Tue, 19 Dec 2017 15:34:05 -0500 Received: from out3-smtp.messagingengine.com ([66.111.4.27]:40163 "EHLO out3-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751024AbdLSUeB (ORCPT ); Tue, 19 Dec 2017 15:34:01 -0500 X-ME-Sender: Date: Wed, 20 Dec 2017 07:33:57 +1100 From: "Tobin C. Harding" To: Matthew Wilcox Cc: Dmitry Vyukov , Kees Cook , Tetsuo Handa , Linux-MM , syzbot , David Windsor , keun-o.park@darkmatter.ae, Laura Abbott , LKML , Mark Rutland , Ingo Molnar , syzkaller-bugs@googlegroups.com, Will Deacon Subject: Re: BUG: bad usercopy in memdup_user Message-ID: <20171219203357.GT19604@eros> References: <001a113e9ca8a3affd05609d7ccf@google.com> <6a50d160-56d0-29f9-cfed-6c9202140b43@I-love.SAKURA.ne.jp> <20171219083746.GR19604@eros> <20171219132246.GD13680@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171219132246.GD13680@bombadil.infradead.org> X-Mailer: Mutt 1.5.24 (2015-08-30) User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1765 Lines: 42 On Tue, Dec 19, 2017 at 05:22:46AM -0800, Matthew Wilcox wrote: > On Tue, Dec 19, 2017 at 07:37:46PM +1100, Tobin C. Harding wrote: > > On Tue, Dec 19, 2017 at 09:12:58AM +0100, Dmitry Vyukov wrote: > > > On Tue, Dec 19, 2017 at 1:57 AM, Kees Cook wrote: > > > > On Mon, Dec 18, 2017 at 6:22 AM, Tetsuo Handa > > > >> This BUG is reporting > > > >> > > > >> [ 26.089789] usercopy: kernel memory overwrite attempt detected to 0000000022a5b430 (kmalloc-1024) (1024 bytes) > > > >> > > > >> line. But isn't 0000000022a5b430 strange for kmalloc(1024, GFP_KERNEL)ed kernel address? > > > > > > > > The address is hashed (see the %p threads for 4.15). > > > > > > > > > +Tobin, is there a way to disable hashing entirely? The only > > > designation of syzbot is providing crash reports to kernel developers > > > with as much info as possible. It's fine for it to leak whatever. > > > > We have new specifier %px to print addresses in hex if leaking info is > > not a worry. > > Could we have a way to know that the printed address is hashed and not just > a pointer getting completely scrogged? Perhaps prefix it with ... a hash! > So this line would look like: > > [ 26.089789] usercopy: kernel memory overwrite attempt detected to #0000000022a5b430 (kmalloc-1024) (1024 bytes) This poses the risk of breaking userland tools that parse the address. The zeroing of the first 32 bits was a design compromise to keep the address format while making _kind of_ explicit that some funny business was going on. > Or does that miss the point of hashing the address, so the attacker > thinks its a real address? No subterfuge intended. Bonus points Wily, I had to go to 'The New Hackers Dictionary' to look up 'scrogged' :) thanks, Tobin.