Patch "ptr_ring: add barriers" has been added to the 4.14-stable tree

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

ptr_ring: add barriers

to the 4.14-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:
ptr_ring-add-barriers.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Michael S. Tsirkin" <[email protected]>
Date: Tue, 5 Dec 2017 21:29:37 +0200
Subject: ptr_ring: add barriers

From: "Michael S. Tsirkin" <[email protected]>

[ Upstream commit a8ceb5dbfde1092b466936bca0ff3be127ecf38e ]

Users of ptr_ring expect that it's safe to give the data structure a pointer and have it be available to consumers, but that actually requires an smb_wmb or a stronger barrier.

In absence of such barriers and on architectures that reorder writes, consumer might read an un=initialized value from an skb pointer stored in the skb array. This was observed causing crashes.

To fix, add memory barriers. The barrier we use is a wmb, the assumption being that producers do not need to read the value so we do not need to order these reads.

---
 include/linux/ptr_ring.h |    9 +++++++++
 1 file changed, 9 insertions(+)

@@ -101,12 +101,18 @@ static inline bool ptr_ring_full_bh(stru
 
 /* Note: callers invoking this in a loop must use a compiler barrier,
  * for example cpu_relax(). Callers must hold producer_lock.
+ * Callers are responsible for making sure pointer that is being queued
+ * points to a valid data.
  */
 static inline int __ptr_ring_produce(struct ptr_ring *r, void *ptr)
 {
 	if (unlikely(!r->size) || r->queue[r->producer])
 		return -ENOSPC;
 
+	/* Make sure the pointer we are storing points to a valid data. */
+	/* Pairs with smp_read_barrier_depends in __ptr_ring_consume. */
+	smp_wmb();
+
 	r->queue[r->producer++] = ptr;
 	if (unlikely(r->producer >= r->size))
 		r->producer = 0;

@@ -275,6 +281,9 @@ static inline void *__ptr_ring_consume(s
 	if (ptr)
 		__ptr_ring_discard_one(r);
 
+	/* Make sure anyone accessing data through the pointer is up to date. */
+	/* Pairs with smp_wmb in __ptr_ring_produce. */
+	smp_read_barrier_depends();
 	return ptr;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/ptr_ring-add-barriers.patch