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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 0EC5CC11F67 for ; Tue, 29 Jun 2021 18:51:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DD02D61DC0 for ; Tue, 29 Jun 2021 18:51:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235251AbhF2Sx2 (ORCPT ); Tue, 29 Jun 2021 14:53:28 -0400 Received: from mga02.intel.com ([134.134.136.20]:48494 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232315AbhF2Sx1 (ORCPT ); Tue, 29 Jun 2021 14:53:27 -0400 X-IronPort-AV: E=McAfee;i="6200,9189,10030"; a="195367488" X-IronPort-AV: E=Sophos;i="5.83,309,1616482800"; d="scan'208";a="195367488" Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by orsmga101.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Jun 2021 11:50:59 -0700 X-IronPort-AV: E=Sophos;i="5.83,309,1616482800"; d="scan'208";a="643820237" Received: from bzhang2-mobl1.amr.corp.intel.com (HELO [10.255.231.86]) ([10.255.231.86]) by fmsmga005-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 Jun 2021 11:50:59 -0700 Subject: Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of dynamic user state To: "Bae, Chang Seok" Cc: "Lutomirski, Andy" , Borislav Petkov , Thomas Gleixner , Ingo Molnar , X86 ML , "Brown, Len" , "Liu, Jing2" , "Shankar, Ravi V" , LKML References: <20210523193259.26200-1-chang.seok.bae@intel.com> <20210523193259.26200-15-chang.seok.bae@intel.com> <872cb0a2-3659-2e6c-52a8-33f1a2f0a2cd@kernel.org> <36D0486A-D955-4C32-941A-A2A4985A450C@intel.com> <48e86785-838d-f5d4-c93c-3232b8ffd915@intel.com> <3AB6DECB-2A53-4EC2-84A6-0CACE44CFC1C@intel.com> <6754330a-bbbb-aa29-7800-f2d16216ad8c@intel.com> <406DB587-F598-484F-A128-990E2DB6EC78@intel.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <406DB587-F598-484F-A128-990E2DB6EC78@intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/29/21 11:35 AM, Bae, Chang Seok wrote: > if (likely(use_xsave())) { > + /* > + * MSR IA32_XFD write follows after this XSAVE(S). So if a > + * state component is in use, XFD should not be armed for > + * current. But, for potential changes in the future, > + * cross-check XINUSE and XFD values. If a XINUSE state > + * is XFD-armed, the following XSAVE(S) does not save the > + * state. > + * > + * Reference the shadow XFD value instead of reading the > + * MSR. > + */ > + if (xfd_capable() && boot_cpu_has(X86_FEATURE_XGETBV1)) { > + u64 current_xfd = (fpu->state_mask & xfd_capable()) ^ xfd_capable(); > + > + WARN_ON_FPU(xgetbv(1) & current_xfd); > + } The code looks fine. But, as usual, I hate the comment. Maybe: /* * If XFD is armed for an xfeature, XSAVE* will not save * its state. Ensure XFD is clear for all features that * are in use before XSAVE*. */ BTW, the ->state_mask calculation is a little confusing to me. I understand that fpu->state_mask shouldn't have any bits set that are unset in xgetbv(1). This code seems to be asking the question: Are any dynamic features in their init state *and* XFD-armed? Is it actually important to make sure that they are dynamic features? Is there *any* case where a feature (dynamic or not) can have XFD armed and be out of its init state?