From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932153AbbGYSqw (ORCPT ); Sat, 25 Jul 2015 14:46:52 -0400 Received: from wtarreau.pck.nerim.net ([62.212.114.60]:11704 "EHLO 1wt.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755020AbbGYSqv (ORCPT ); Sat, 25 Jul 2015 14:46:51 -0400 Date: Sat, 25 Jul 2015 20:45:45 +0200 From: Willy Tarreau To: Andy Lutomirski Cc: Andy Lutomirski , Peter Zijlstra , Steven Rostedt , "security@kernel.org" , X86 ML , Borislav Petkov , Sasha Levin , "linux-kernel@vger.kernel.org" , Konrad Rzeszutek Wilk , Boris Ostrovsky , Andrew Cooper , Jan Beulich , xen-devel , Kees Cook Subject: Re: [PATCH 4/3] x86/ldt: allow to disable modify_ldt at runtime Message-ID: <20150725184545.GB17740@1wt.eu> References: <7286d77aa81abc38dc40362e2439861427064f6f.1437802102.git.luto@kernel.org> <20150725062343.GA3902@1wt.eu> <20150725075052.GA3918@1wt.eu> <20150725130340.GA17257@1wt.eu> <20150725163356.GD17659@1wt.eu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.3i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jul 25, 2015 at 10:42:14AM -0700, Andy Lutomirski wrote: > On Sat, Jul 25, 2015 at 9:33 AM, Willy Tarreau wrote: > > On Sat, Jul 25, 2015 at 09:08:39AM -0700, Andy Lutomirski wrote: > >> There's one thing that I think is incomplete here. Currently, espfix > >> triggers if SS points to the LDT. It's possible for SS to point to > >> the LDT even with modify_ldt disabled, and there's a decent amount of > >> attack surface there. > >> > >> Can we improve this? Two ideas: > >> > >> 1. In the asm, patch out or otherwise disable espfix if that sysctl > >> has never been set. (Ick.) > >> > >> 2. When modify_ldt is runtime-disabled (or compile-time disabled, > >> perhaps), disallow setting the LDT bit in SS in the handful of places > >> that would allow it (ptrace and sigreturn off the top of my head). We > >> don't need to worry about (regs->ss & 4) being set on kernel entry > >> because we'll never be in user mode with that bit set if the LDT is > >> disabled, but that bit could still be set using kernel APIs. (In > >> fact, my sigreturn test does exactly that.) > >> > >> Hmm. With synchronous LDT, we could plausibly check at runtime in the > >> espfix code, too. We used to use LAR to do this, but hpa removed it > >> when he realized that it was racy. It shouldn't be racy any more, > >> because, with my patches applied, the LDT never changes while > >> interrupts are off. > > > > I understand it's not complete but I'm a bit bothered with conflating > > this sysctl with other setting methods, because if the purpose of the > > sysctl is to disable the syscall, it should do that only. I'd rather > > document that it's less complete than the Kconfig method and continue > > to recommend using your option whenever possible (eg: all my kernels > > will use it just as I've already disabled X86_16BIT everywhere). > > > > Agreed. We can certainly tighten up the espfix code later. > > > Also one benefit of having both options is that it will mechanically > > make LDT a much less interesting target for future attacks, since it > > will significantly reduce the likeliness of success, hence the motivation > > for writing exploits that only work in conferences. > > > > Patch looks fine to me. OK thanks. Willy