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=-10.4 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 14219C48BE5 for ; Wed, 16 Jun 2021 19:01:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EC6916008E for ; Wed, 16 Jun 2021 19:01:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232233AbhFPTD6 (ORCPT ); Wed, 16 Jun 2021 15:03:58 -0400 Received: from mga17.intel.com ([192.55.52.151]:47239 "EHLO mga17.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230269AbhFPTD5 (ORCPT ); Wed, 16 Jun 2021 15:03:57 -0400 IronPort-SDR: Ipi4O299gnVO502oU/DDbBOTXI0a0JNhi8vPDoovo3nhsp4e0BsiO53EvAGtG7GtbdqHtuWXI1 w7V09T4EZ1ig== X-IronPort-AV: E=McAfee;i="6200,9189,10016"; a="186617486" X-IronPort-AV: E=Sophos;i="5.83,278,1616482800"; d="scan'208";a="186617486" Received: from fmsmga007.fm.intel.com ([10.253.24.52]) by fmsmga107.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 16 Jun 2021 12:01:51 -0700 IronPort-SDR: zE1p7mnJuFwdfqGsTbqRwBSQ+JGAchFnL+LPhz5WrmDf5gk4YoC9vSlJuIjvx3jdahsYq4Bitw TimzQEpC9tgQ== X-IronPort-AV: E=Sophos;i="5.83,278,1616482800"; d="scan'208";a="415846309" Received: from jamarin-mobl1.amr.corp.intel.com (HELO [10.209.105.29]) ([10.209.105.29]) by fmsmga007-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 16 Jun 2021 12:01:50 -0700 Subject: Re: [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of dynamic user state To: "Bae, Chang Seok" , Andy Lutomirski Cc: 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> 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: <36D0486A-D955-4C32-941A-A2A4985A450C@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 6/16/21 11:47 AM, Bae, Chang Seok wrote: > On Jun 16, 2021, at 11:12, Andy Lutomirski wrote: >> On 6/16/21 9:27 AM, Dave Hansen wrote: >>> On 5/23/21 12:32 PM, Chang S. Bae wrote: >>>> @@ -571,6 +612,8 @@ static inline void switch_fpu_finish(struct fpu *new_fpu) >>>> >>>> set_thread_flag(TIF_NEED_FPU_LOAD); >>>> >>>> + xfd_switch(old_fpu, new_fpu); >>> This seems fragile. >>> >>> XSAVE* will decline to write out state for feature i when XFD[i]=1 and >>> will instead write out the init state. That means that, at this point, >>> we switch XFD and yet leave state for feature i in place. >>> >>> That means this *MUST* come before any copy_fpregs_to_fpstate() occurs. >>> >>> Could we please add some FPU_WARN_ON() checks to ensure that no XSAVE* >>> is ever performed with XINUSE=1 *and* XFD armed? > So, xfd_switch() is after copy_fpregs_to_fpstate(): > > __switch_to() > --> switch_fpu_prepare() --> copy_fpregs_to_fpstate() > --> switch_fpu_finish() --> xfd_switch() > > I don't see someone intentionally flip this order. I'm more worried about how it might interact with KVM, or how future code might try to optimize the whole thing. Either way: the point stands: XSAVE with XINIT[i]=0 and XFD[i]=1 kills puppies. I ideally want a warning if the puppy slaughter ever happens. If that's not feasible, I'll also settle for a big fat comment about it. But, something is needed. The XSAVE code is littered with assumptions that demonstrate a lack of imagination about how future hacking might kill puppies. > Reading XINUSE via XGETBV is cheap but not free. I don't know spending a > hundred cycles for this WARN is big deal but this is one of the most > performance-critical paths. Is XGETBV(1) really a hundred cycles? That seems absurdly high for a non-serializing register read. If you're really worried, let's put it under X86_FEATURE_DEBUG_FPU or something.