Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757736AbaDWR3F (ORCPT ); Wed, 23 Apr 2014 13:29:05 -0400 Received: from terminus.zytor.com ([198.137.202.10]:41708 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751335AbaDWR3C (ORCPT ); Wed, 23 Apr 2014 13:29:02 -0400 Message-ID: <5357F82D.8020307@zytor.com> Date: Wed, 23 Apr 2014 10:28:13 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0 MIME-Version: 1.0 To: Andrew Lutomirski CC: One Thousand Gnomes , Linus Torvalds , Borislav Petkov , "H. Peter Anvin" , Linux Kernel Mailing List , Ingo Molnar , Alexander van Heukelum , Konrad Rzeszutek Wilk , Boris Ostrovsky , Arjan van de Ven , Brian Gerst , Alexandre Julliard , Andi Kleen , Thomas Gleixner Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE* References: <20140422112312.GB15882@pd.tnic> <20140422144659.GF15882@pd.tnic> <53569467.1030809@zytor.com> <5356A3B6.5050901@zytor.com> <20140423105411.2e166dd8@alan.etchedpixels.co.uk> <5357E214.6050501@zytor.com> <5357F572.1030804@zytor.com> In-Reply-To: X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/23/2014 10:25 AM, Andrew Lutomirski wrote: > On Wed, Apr 23, 2014 at 10:16 AM, H. Peter Anvin wrote: >> On 04/23/2014 10:08 AM, Andrew Lutomirski wrote: >>> >>> The only way I can see to trigger the race is with sigreturn, but it's >>> still there. Sigh. >> >> I don't see why sigreturn needs to be involved... all you need is >> modify_ldt() on one CPU while the other is in the middle of an IRET >> return. Small window, so hard to hit, but still. > > If you set the flag as soon as anyone calls modify_ldt, before any > descriptor is installed, then I don't think this can happen. But > there's still sigreturn, and I don't think this is worth all the > complexity to save a single branch on #GP. > Who cares? Since we only need to enter the fixup path for LDT selectors, anything that is dependent on having called modify_ldt() is already redundant. In some ways that is the saving grace. SS being an LDT selector is fortunately a rare case. > I do mean intra-kernel. And yes, this has nothing to do with espfix, > but it would make write_msr_safe fail more quickly :) And, pray tell, how important is that? -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/