FAILED: patch "[PATCH] timers: Plug locking race vs. timer migration" failed to apply to 4.4-stable tree

FAILED: patch "[PATCH] timers: Plug locking race vs. timer migration" failed to apply to 4.4-stable tree

[gregkh]

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>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: [PATCH] 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>

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 2d47980a1bc4..0d4b91c5a374 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);