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=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 2A293C433E3 for ; Fri, 24 Jul 2020 17:07:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0BEBB206D8 for ; Fri, 24 Jul 2020 17:07:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726780AbgGXRHp (ORCPT ); Fri, 24 Jul 2020 13:07:45 -0400 Received: from gecko.sbs.de ([194.138.37.40]:46604 "EHLO gecko.sbs.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726326AbgGXRHp (ORCPT ); Fri, 24 Jul 2020 13:07:45 -0400 Received: from mail2.sbs.de (mail2.sbs.de [192.129.41.66]) by gecko.sbs.de (8.15.2/8.15.2) with ESMTPS id 06OH7AVU029510 (version=TLSv1.2 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 24 Jul 2020 19:07:10 +0200 Received: from [139.22.112.247] ([139.22.112.247]) by mail2.sbs.de (8.15.2/8.15.2) with ESMTP id 06OH76bs007058; Fri, 24 Jul 2020 19:07:07 +0200 Subject: Re: [PATCH 4.9 18/22] x86/fpu: Disable bottom halves while loading FPU registers To: Greg Kroah-Hartman , linux-kernel@vger.kernel.org, Sebastian Andrzej Siewior Cc: stable@vger.kernel.org, Borislav Petkov , Ingo Molnar , Thomas Gleixner , Andy Lutomirski , Dave Hansen , "H. Peter Anvin" , "Jason A. Donenfeld" , kvm ML , Paolo Bonzini , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Rik van Riel , x86-ml , cip-dev References: <20181228113126.144310132@linuxfoundation.org> <20181228113127.414176417@linuxfoundation.org> From: Jan Kiszka Message-ID: <01857944-ce1a-c6cd-3666-1e9b6ca8cccc@siemens.com> Date: Fri, 24 Jul 2020 19:07:06 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20181228113127.414176417@linuxfoundation.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 28.12.18 12:52, Greg Kroah-Hartman wrote: > 4.9-stable review patch. If anyone has any objections, please let me know. > > ------------------ > > From: Sebastian Andrzej Siewior > > commit 68239654acafe6aad5a3c1dc7237e60accfebc03 upstream. > > The sequence > > fpu->initialized = 1; /* step A */ > preempt_disable(); /* step B */ > fpu__restore(fpu); > preempt_enable(); > > in __fpu__restore_sig() is racy in regard to a context switch. > > For 32bit frames, __fpu__restore_sig() prepares the FPU state within > fpu->state. To ensure that a context switch (switch_fpu_prepare() in > particular) does not modify fpu->state it uses fpu__drop() which sets > fpu->initialized to 0. > > After fpu->initialized is cleared, the CPU's FPU state is not saved > to fpu->state during a context switch. The new state is loaded via > fpu__restore(). It gets loaded into fpu->state from userland and > ensured it is sane. fpu->initialized is then set to 1 in order to avoid > fpu__initialize() doing anything (overwrite the new state) which is part > of fpu__restore(). > > A context switch between step A and B above would save CPU's current FPU > registers to fpu->state and overwrite the newly prepared state. This > looks like a tiny race window but the Kernel Test Robot reported this > back in 2016 while we had lazy FPU support. Borislav Petkov made the > link between that report and another patch that has been posted. Since > the removal of the lazy FPU support, this race goes unnoticed because > the warning has been removed. > > Disable bottom halves around the restore sequence to avoid the race. BH > need to be disabled because BH is allowed to run (even with preemption > disabled) and might invoke kernel_fpu_begin() by doing IPsec. > > [ bp: massage commit message a bit. ] > > Signed-off-by: Sebastian Andrzej Siewior > Signed-off-by: Borislav Petkov > Acked-by: Ingo Molnar > Acked-by: Thomas Gleixner > Cc: Andy Lutomirski > Cc: Dave Hansen > Cc: "H. Peter Anvin" > Cc: "Jason A. Donenfeld" > Cc: kvm ML > Cc: Paolo Bonzini > Cc: Radim Krčmář > Cc: Rik van Riel > Cc: stable@vger.kernel.org > Cc: x86-ml > Link: http://lkml.kernel.org/r/20181120102635.ddv3fvavxajjlfqk@linutronix.de > Link: https://lkml.kernel.org/r/20160226074940.GA28911@pd.tnic > Signed-off-by: Sebastian Andrzej Siewior > Signed-off-by: Greg Kroah-Hartman > --- > arch/x86/kernel/fpu/signal.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > --- a/arch/x86/kernel/fpu/signal.c > +++ b/arch/x86/kernel/fpu/signal.c > @@ -342,10 +342,10 @@ static int __fpu__restore_sig(void __use > sanitize_restored_xstate(tsk, &env, xfeatures, fx_only); > } > > + local_bh_disable(); > fpu->fpstate_active = 1; > - preempt_disable(); > fpu__restore(fpu); > - preempt_enable(); > + local_bh_enable(); > > return err; > } else { > > Any reason why the backport stopped back than at 4.9? I just debugged this out of a 4.4 kernel, and it is needed there as well. I'm happy to propose a backport, would just appreciate a hint if the BH protection is needed also there (my case was without BH). Thanks, Jan -- Siemens AG, Corporate Technology, CT RDA IOT SES-DE Corporate Competence Center Embedded Linux