From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnd Bergmann Subject: Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core) Date: Tue, 21 Jun 2016 21:47:44 +0200 Message-ID: <6248429.A4IJrfgOW3@wuerfel> References: <13212319.WrhLzgRA6Z@wuerfel> Reply-To: kernel-hardening@lists.openwall.com Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Return-path: List-Post: List-Help: List-Unsubscribe: List-Subscribe: In-Reply-To: To: Kees Cook Cc: Andy Lutomirski , "x86@kernel.org" , LKML , linux-arch , Borislav Petkov , Nadav Amit , Brian Gerst , "kernel-hardening@lists.openwall.com" , Linus Torvalds , Josh Poimboeuf , Jann Horn , Heiko Carstens List-Id: linux-arch.vger.kernel.org On Tuesday, June 21, 2016 10:16:21 AM CEST Kees Cook wrote: > On Tue, Jun 21, 2016 at 2:24 AM, Arnd Bergmann wrote: > > On Monday, June 20, 2016 4:43:30 PM CEST Andy Lutomirski wrote: > >> > >> On my laptop, this adds about 1.5=B5s of overhead to task creation, > >> which seems to be mainly caused by vmalloc inefficiently allocating > >> individual pages even when a higher-order page is available on the > >> freelist. > > > > Would it help to have a fixed virtual address for the stack instead > > and map the current stack to that during a task switch, similar to > > how we handle fixmap pages? > > > > That would of course trade the allocation overhead for a task switch > > overhead, which may be better or worse. It would also give "current" > > a constant address, which may give a small performance advantage > > but may also introduce a new attack vector unless we randomize it > > again. >=20 > Right: we don't want a fixed address. That makes attacks WAY easier. Do we care about making the address more random then? When I look at /proc/vmallocinfo, I see that allocations are all using consecutive addresses, so if you can figure out the virtual address of the stack for one process that would give you a good chance of guessing the address for the next pid. Arnd From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mout.kundenserver.de ([217.72.192.75]:62056 "EHLO mout.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750701AbcFUTru convert rfc822-to-8bit (ORCPT ); Tue, 21 Jun 2016 15:47:50 -0400 From: Arnd Bergmann Subject: Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core) Date: Tue, 21 Jun 2016 21:47:44 +0200 Message-ID: <6248429.A4IJrfgOW3@wuerfel> In-Reply-To: References: <13212319.WrhLzgRA6Z@wuerfel> MIME-Version: 1.0 Content-Transfer-Encoding: 8BIT Content-Type: text/plain; charset="iso-8859-1" Sender: linux-arch-owner@vger.kernel.org List-ID: To: Kees Cook Cc: Andy Lutomirski , "x86@kernel.org" , LKML , linux-arch , Borislav Petkov , Nadav Amit , Brian Gerst , "kernel-hardening@lists.openwall.com" , Linus Torvalds , Josh Poimboeuf , Jann Horn , Heiko Carstens Message-ID: <20160621194744.lHRYBl42wzOvDg44PxY2pO6sMV6EUNWdHX8ARF4RJc8@z> On Tuesday, June 21, 2016 10:16:21 AM CEST Kees Cook wrote: > On Tue, Jun 21, 2016 at 2:24 AM, Arnd Bergmann wrote: > > On Monday, June 20, 2016 4:43:30 PM CEST Andy Lutomirski wrote: > >> > >> On my laptop, this adds about 1.5µs of overhead to task creation, > >> which seems to be mainly caused by vmalloc inefficiently allocating > >> individual pages even when a higher-order page is available on the > >> freelist. > > > > Would it help to have a fixed virtual address for the stack instead > > and map the current stack to that during a task switch, similar to > > how we handle fixmap pages? > > > > That would of course trade the allocation overhead for a task switch > > overhead, which may be better or worse. It would also give "current" > > a constant address, which may give a small performance advantage > > but may also introduce a new attack vector unless we randomize it > > again. > > Right: we don't want a fixed address. That makes attacks WAY easier. Do we care about making the address more random then? When I look at /proc/vmallocinfo, I see that allocations are all using consecutive addresses, so if you can figure out the virtual address of the stack for one process that would give you a good chance of guessing the address for the next pid. Arnd