Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763301AbdLSOIS (ORCPT ); Tue, 19 Dec 2017 09:08:18 -0500 Received: from www262.sakura.ne.jp ([202.181.97.72]:44066 "EHLO www262.sakura.ne.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751011AbdLSOIR (ORCPT ); Tue, 19 Dec 2017 09:08:17 -0500 To: dvyukov@google.com, willy@infradead.org Cc: me@tobin.cc, keescook@chromium.org, linux-mm@kvack.org, bot+719398b443fd30155f92f2a888e749026c62b427@syzkaller.appspotmail.com, dave@nullcore.net, keun-o.park@darkmatter.ae, labbott@redhat.com, linux-kernel@vger.kernel.org, mark.rutland@arm.com, mingo@kernel.org, syzkaller-bugs@googlegroups.com, will.deacon@arm.com Subject: Re: BUG: bad usercopy in memdup_user From: Tetsuo Handa References: <20171219083746.GR19604@eros> <20171219132246.GD13680@bombadil.infradead.org> In-Reply-To: Message-Id: <201712192308.HJJ05711.SHQFVFLOMFOOJt@I-love.SAKURA.ne.jp> X-Mailer: Winbiff [Version 2.51 PL2] X-Accept-Language: ja,en,zh Date: Tue, 19 Dec 2017 23:08:14 +0900 Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1589 Lines: 34 Dmitry Vyukov wrote: > On Tue, Dec 19, 2017 at 2:22 PM, Matthew Wilcox wrote: > >> > >> 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) > > > > Or does that miss the point of hashing the address, so the attacker > > thinks its a real address? > > If we do something with this, I would suggest that we just disable > hashing. Any of the concerns that lead to hashed pointers are not > applicable in this context, moreover they are harmful, cause confusion > and make it harder to debug these bugs. That perfectly can be an > opt-in CONFIG_DEBUG_INSECURE_BLA_BLA_BLA. > Why not a kernel command line option? Hashing by default.