4.8-stable review patch. If anyone has any objections, please let me know.
------------------ From: Thomas Gleixner <[email protected]> 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 <[email protected]> Signed-off-by: Thomas Gleixner <[email protected]> Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1610241711220.4983@nanos Cc: Andrew Morton <[email protected]> Cc: Peter Zijlstra <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> --- 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);
