From 839003061aa9362f23866614862b49b57bfdd6ce Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Mon, 24 Oct 2016 11:41:56 +0200 Subject: [PATCH] timers: Plug locking race vs. timer migration 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: Mike Galbraith Signed-off-by: Greg Kroah-Hartman --- 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 125407144c01..3d7588a2e97c 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -764,8 +764,15 @@ static struct tvec_base *lock_timer_base(struct timer_list *timer, __acquires(timer->base->lock) { for (;;) { - u32 tf = timer->flags; struct tvec_base *base; + 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 = per_cpu_ptr(&tvec_bases, tf & TIMER_CPUMASK);