Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753945AbcKILGv (ORCPT ); Wed, 9 Nov 2016 06:06:51 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:54326 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933466AbcKILGp (ORCPT ); Wed, 9 Nov 2016 06:06:45 -0500 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Linus Torvalds , Thomas Gleixner , Andrew Morton , Peter Zijlstra Subject: [PATCH 4.8 044/138] timers: Plug locking race vs. timer migration Date: Wed, 9 Nov 2016 11:45:27 +0100 Message-Id: <20161109102846.689435979@linuxfoundation.org> X-Mailer: git-send-email 2.10.2 In-Reply-To: <20161109102844.808685475@linuxfoundation.org> References: <20161109102844.808685475@linuxfoundation.org> User-Agent: quilt/0.64 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1503 Lines: 47 4.8-stable review patch. If anyone has any objections, please let me know. ------------------ From: Thomas Gleixner commit b831275a3553c32091222ac619cfddd73a5553fb upstream. Linus noticed that lock_timer_base() lacks a READ_ONCE() for accessing the timer flags. As a consequence the compiler is allowed to reload the flags between the initial check for TIMER_MIGRATION and the following timer base computation and the spin lock of the base. While this has not been observed (yet), we need to make sure that it never happens. Fixes: 0eeda71bc30d ("timer: Replace timer base by a cpu index") Reported-by: Linus Torvalds Signed-off-by: Thomas Gleixner Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1610241711220.4983@nanos Cc: Andrew Morton Cc: Peter Zijlstra Signed-off-by: Greg Kroah-Hartman --- kernel/time/timer.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -937,7 +937,14 @@ static struct timer_base *lock_timer_bas { for (;;) { struct timer_base *base; - u32 tf = timer->flags; + u32 tf; + + /* + * We need to use READ_ONCE() here, otherwise the compiler + * might re-read @tf between the check for TIMER_MIGRATING + * and spin_lock(). + */ + tf = READ_ONCE(timer->flags); if (!(tf & TIMER_MIGRATING)) { base = get_timer_base(tf);