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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 D32E2C43441 for ; Mon, 26 Nov 2018 03:07:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 97C462086B for ; Mon, 26 Nov 2018 03:07:11 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 97C462086B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com 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 S1726164AbeKZN7u (ORCPT ); Mon, 26 Nov 2018 08:59:50 -0500 Received: from mga05.intel.com ([192.55.52.43]:24027 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726079AbeKZN7u (ORCPT ); Mon, 26 Nov 2018 08:59:50 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga004.jf.intel.com ([10.7.209.38]) by fmsmga105.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 25 Nov 2018 19:07:08 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.56,280,1539673200"; d="scan'208";a="252530192" Received: from tassilo.jf.intel.com (HELO tassilo.localdomain) ([10.7.201.126]) by orsmga004.jf.intel.com with ESMTP; 25 Nov 2018 19:07:09 -0800 Received: by tassilo.localdomain (Postfix, from userid 1000) id F41F6309C88; Sun, 25 Nov 2018 19:07:08 -0800 (PST) Date: Sun, 25 Nov 2018 19:07:08 -0800 From: Andi Kleen To: Thomas Gleixner 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() Message-ID: <20181126030708.GP13936@tassilo.jf.intel.com> References: <20181125183328.318175777@linutronix.de> <20181125185005.466447057@linutronix.de> <20181125205330.GO13936@tassilo.jf.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. -Andi