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=-9.0 required=3.0 tests=INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED,USER_AGENT_NEOMUTT 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 B7DC0C43387 for ; Fri, 21 Dec 2018 16:23:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9293221903 for ; Fri, 21 Dec 2018 16:23:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388712AbeLUQXz convert rfc822-to-8bit (ORCPT ); Fri, 21 Dec 2018 11:23:55 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:35027 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388202AbeLUQXy (ORCPT ); Fri, 21 Dec 2018 11:23:54 -0500 Received: from bigeasy by Galois.linutronix.de with local (Exim 4.80) (envelope-from ) id 1gaNaM-0004aJ-UV; Fri, 21 Dec 2018 17:23:39 +0100 Date: Fri, 21 Dec 2018 17:23:38 +0100 From: Sebastian Andrzej Siewior To: Greg Kroah-Hartman Cc: Borislav Petkov , Jari Ruusu , linux-kernel@vger.kernel.org, stable@vger.kernel.org, Ingo Molnar , Thomas Gleixner , Andy Lutomirski , Dave Hansen , "H. Peter Anvin" , "Jason A. Donenfeld" , kvm ML , Paolo Bonzini , Radim =?utf-8?B?S3I/bcOhPw==?= , Rik van Riel , x86-ml Subject: [PATCH v4.9 STABLE] x86/fpu: Disable bottom halves while loading FPU registers Message-ID: <20181221162338.dir7z3c76kucm6uh@linutronix.de> References: <20181204103726.750894136@linuxfoundation.org> <20181204103731.697870447@linuxfoundation.org> <5C07FC30.43601034@users.sourceforge.net> <20181205190020.GO29510@zn.tnic> <20181206105407.GH19891@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8BIT In-Reply-To: <20181206105407.GH19891@kroah.com> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- arch/x86/kernel/fpu/signal.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c index ae52ef05d0981..769831d9fd114 100644 --- a/arch/x86/kernel/fpu/signal.c +++ b/arch/x86/kernel/fpu/signal.c @@ -342,10 +342,10 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size) 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 { -- 2.20.1