From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9F529C43441 for ; Mon, 26 Nov 2018 06:51:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5FD7620672 for ; Mon, 26 Nov 2018 06:51:04 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5FD7620672 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linutronix.de Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726252AbeKZRoJ (ORCPT ); Mon, 26 Nov 2018 12:44:09 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:53592 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726150AbeKZRoJ (ORCPT ); Mon, 26 Nov 2018 12:44:09 -0500 Received: from p4fea46ac.dip0.t-ipconnect.de ([79.234.70.172] helo=nanos) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1gRAjP-0001Rj-8x; Mon, 26 Nov 2018 07:50:55 +0100 Date: Mon, 26 Nov 2018 07:50:54 +0100 (CET) From: Thomas Gleixner To: Andi Kleen cc: LKML , x86@kernel.org, Peter Zijlstra , Andy Lutomirski , Linus Torvalds , Jiri Kosina , Tom Lendacky , Josh Poimboeuf , Andrea Arcangeli , David Woodhouse , Tim Chen , Dave Hansen , Casey Schaufler , Asit Mallick , Arjan van de Ven , Jon Masters , Waiman Long , Greg KH , Dave Stewart , Kees Cook Subject: Re: [patch V2 21/28] x86/speculation: Prepare for conditional IBPB in switch_mm() In-Reply-To: <20181126030708.GP13936@tassilo.jf.intel.com> Message-ID: References: <20181125183328.318175777@linutronix.de> <20181125185005.466447057@linutronix.de> <20181125205330.GO13936@tassilo.jf.intel.com> <20181126030708.GP13936@tassilo.jf.intel.com> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 25 Nov 2018, Andi Kleen wrote: > On Sun, Nov 25, 2018 at 11:20:50PM +0100, Thomas Gleixner wrote: > > On Sun, 25 Nov 2018, Andi Kleen wrote: > > > > > > The current check whether two tasks belong to the same context is using the > > > > tasks context id. While correct, it's simpler to use the mm pointer because > > > > it allows to mangle the TIF_SPEC_IB bit into it. The context id based > > > > mechanism requires extra storage, which creates worse code. > > > > > > [We tried similar in some really early versions, but it was replaced > > > with the context id later.] > > > > > > One issue with using the pointer is that the pointer can be reused > > > when the original mm_struct is freed, and then gets reallocated > > > immediately to an attacker. Then the attacker may avoid the IBPB. > > > > > > Given it's probably hard to generate any reasonable leak bandwidth with > > > such a complex scenario, but it still seemed better to close the hole. > > > > Sorry, but that's really a purely academic exercise. > > Ok fair enough. I guess it's acceptable if you add a comment explaining it. Will do. Thanks, tglx