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 <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: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 7ea22b408bfb1d903b71254668f4bc2e31cf6bf1..86fd301d51556353890c9055de79d20f0ee2236a 100644 (file)
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -937,7 +937,14 @@ 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);