[tip:timers/urgent] timers: Plug locking race vs. timer migration

[tip-bot for Thomas Gleixner <tipbot@zytor.com>]

Commit-ID:  b831275a3553c32091222ac619cfddd73a5553fb
Gitweb:     http://git.kernel.org/tip/b831275a3553c32091222ac619cfddd73a5553fb
Author:     Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Mon, 24 Oct 2016 11:41:56 +0200
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 25 Oct 2016 16:27:39 +0200

timers: Plug locking race vs. timer migration

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 <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1610241711220.4983@nanos
Cc: stable@vger.kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
---
 kernel/time/timer.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 2d47980..0d4b91c 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -943,7 +943,14 @@ static struct timer_base *lock_timer_base(struct timer_list *timer,
 {
 	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);