Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752481AbdLHJ6q (ORCPT ); Fri, 8 Dec 2017 04:58:46 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:55415 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753720AbdLHJzl (ORCPT ); Fri, 8 Dec 2017 04:55:41 -0500 Date: Fri, 8 Dec 2017 10:55:36 +0100 (CET) From: Thomas Gleixner To: Ingo Molnar cc: Andy Lutomirski , Andy Lutomirski , Borislav Petkov , X86 ML , "linux-kernel@vger.kernel.org" , Brian Gerst , David Laight , Kees Cook , Peter Zijlstra Subject: Re: [PATCH] LDT improvements In-Reply-To: <20171208094400.wqnezwukq5yx4mgq@gmail.com> Message-ID: References: <48fe5cf1382d6a95c7b1837415882edcc81a9781.1512631324.git.luto@kernel.org> <20171207124347.p7kdj7q4qqs3ivri@pd.tnic> <665F1CA8-D012-4465-B5F7-E81E088847DB@amacapital.net> <20171208073454.dicyefwncsihq7sm@gmail.com> <20171208094400.wqnezwukq5yx4mgq@gmail.com> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) 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: 2226 Lines: 52 On Fri, 8 Dec 2017, Ingo Molnar wrote: > * Thomas Gleixner wrote: > > > On Fri, 8 Dec 2017, Ingo Molnar wrote: > > > * Andy Lutomirski wrote: > > > > 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. > > > > > > Hm, robustness of the LDT address wrt. access_ok() is a valid concern. > > > > > > Can we have vmas with high addresses, in the vmalloc space for example? > > > IIRC the GPU code has precedents in that area. > > > > > > Since this is x86-64, limitation of the vmalloc() space is not an issue. > > > > > > I like Thomas's solution: > > > > > > - have the LDT in a regular mmap space vma (hence per process ASLR randomized), > > > but with the system bit set. > > > > > > - That would be an advantage even for non-PTI kernels, because mmap() is probably > > > more randomized than kmalloc(). > > > > Randomization is pointless as long as you can get the LDT address in user > > space, i.e. w/o UMIP. > > But with UMIP unprivileged user-space won't be able to get the linear address of > the LDT. Now it's written out in /proc/self/maps. We can expose it nameless like other VMAs, but then it's 128k sized so it can be figured out. But when it's RO then it's not really a problem, even the kernel can't write to it. > > > - It would also be a cleaner approach all around, and would avoid the fixmap > > > complications and the scheduler muckery. > > > > The error code of such an access is always 0x03. So I added a special > > handler, which checks whether the address is in the LDT map range and > > verifies that the access bit in the descriptor is 0. If that's the case it > > sets it and returns. If not, the thing dies. That works. > > Are SMP races possible? For example two threads both triggering the accessed bit > fault, but only one of them succeeding in setting it. The other thread should not > die in this case, right? Right. I'm trying to figure out whether there is a way to reliably detect that write access bit mode. Thanks, tglx