Patch "timers: Plug locking race vs. timer migration" has been added to the 4.8-stable tree

Patch "timers: Plug locking race vs. timer migration" has been added to the 4.8-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    timers: Plug locking race vs. timer migration

to the 4.8-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     timers-plug-locking-race-vs.-timer-migration.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From b831275a3553c32091222ac619cfddd73a5553fb Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Mon, 24 Oct 2016 11:41:56 +0200
Subject: timers: Plug locking race vs. timer migration

From: Thomas Gleixner <tglx@linutronix.de>

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>

---
 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);


Patches currently in stable-queue which might be from tglx@linutronix.de are

queue-4.8/x86-microcode-amd-fix-more-fallout-from-config_randomize_memory-y.patch
queue-4.8/timers-lock-base-for-same-bucket-optimization.patch
queue-4.8/timers-plug-locking-race-vs.-timer-migration.patch
queue-4.8/timers-prevent-base-clock-corruption-when-forwarding.patch
queue-4.8/timers-prevent-base-clock-rewind-when-forwarding-clock.patch