Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753966AbdLGSVx (ORCPT ); Thu, 7 Dec 2017 13:21:53 -0500 Received: from mail-pf0-f173.google.com ([209.85.192.173]:45492 "EHLO mail-pf0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750973AbdLGSVv (ORCPT ); Thu, 7 Dec 2017 13:21:51 -0500 X-Google-Smtp-Source: AGs4zMY9jF0LvhBAZzULwLi4kmnpX/HJJCuYr9PDKQu36xw+eLqC53Gx4oauFV9ZyXtUsz2Ts8kKRQ== Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (1.0) Subject: Re: [PATCH] LDT improvements From: Andy Lutomirski X-Mailer: iPhone Mail (15C114) In-Reply-To: Date: Thu, 7 Dec 2017 10:21:47 -0800 Cc: Andy Lutomirski , Borislav Petkov , X86 ML , "linux-kernel@vger.kernel.org" , Brian Gerst , David Laight , Kees Cook , Peter Zijlstra Message-Id: <665F1CA8-D012-4465-B5F7-E81E088847DB@amacapital.net> References: <48fe5cf1382d6a95c7b1837415882edcc81a9781.1512631324.git.luto@kernel.org> <20171207124347.p7kdj7q4qqs3ivri@pd.tnic> To: Thomas Gleixner Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by nfs id vB7ILvGR019108 Content-Length: 1866 Lines: 47 > On Dec 7, 2017, at 9:23 AM, Thomas Gleixner wrote: > >> On Thu, 7 Dec 2017, Andy Lutomirski wrote: >> >>> On Thu, Dec 7, 2017 at 4:43 AM, Borislav Petkov wrote: >>>> On Wed, Dec 06, 2017 at 11:22:21PM -0800, Andy Lutomirski wrote: >>>> I think I like this approach. I also think it might be nice to move the >>>> whole cpu_entry_area into this new pgd range so that we can stop mucking >>>> around with the fixmap. >>> >>> Yeah, and also, I don't like the idea of sacrificing a whole PGD >>> only for the LDT crap which is optional, even. Frankly - and this >>> is just me - I'd make CONFIG_KERNEL_PAGE_TABLE_ISOLATION xor >>> CONFIG_MODIFY_LDT_SYSCALL and don't give a rat's *ss about the LDT. >> >> The PGD sacrifice doesn't bother me. Putting a writable LDT map at a >> constant address does bother me. We could probably get away with RO >> if we trapped and handled the nasty faults, but that could be very >> problematic. > > Where is the problem? You can map it RO into user space with the USER bit > cleared. The kernel knows how to access the real stuff. Blows up when the CPU tries to set the accessed bit. > >> The version here: >> >> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/pti&id=a74d1009ac72a1f04ec5bcd338a4bdbe170ab776 >> >> actually seems to work. > > The approach I've taken is to create a VMA and map it into user space with > the USER bit cleared. A little bit more effort code wise, but that avoids > all the page table muck and keeps it straight attached to the process. > > Will post once in a bit. I don't love mucking with user address space. I'm also quite nervous about putting it in our near anything that could pass an access_ok check, since we're totally screwed if the bad guys can figure out how to write to it. > > Thanks, > > tglx >