Patch "xen/evtchn: fix ring resize when binding new events" has been added to the 4.5-stable tree

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

xen/evtchn: fix ring resize when binding new events

to the 4.5-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:
xen-evtchn-fix-ring-resize-when-binding-new-events.patch
and it can be found in the queue-4.5 subdirectory.

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

From: Jan Beulich <[email protected]>
Date: Wed, 4 May 2016 07:02:36 -0600
Subject: xen/evtchn: fix ring resize when binding new events

From: Jan Beulich <[email protected]>

commit 27e0e6385377c4dc68a4ddaf1a35a2dfa951f3c5 upstream.

The copying of ring data was wrong for two cases: For a full ring nothing got copied at all (as in that case the canonicalized producer and consumer indexes are identical). And in case one or both of the canonicalized (after the resize) indexes would point into the second half of the buffer, the copied data ended up in the wrong (free) part of the new buffer. In both cases uninitialized data would get passed back to the caller.

Fix this by simply copying the old ring contents twice: Once to the low half of the new buffer, and a second time to the high half.

This addresses the inability to boot a HVM guest with 64 or more vCPUs. This regression was caused by 8620015499101090 (xen/evtchn:
dynamically grow pending event channel ring).

---
 drivers/xen/evtchn.c |   20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

@@ -316,7 +316,6 @@ static int evtchn_resize_ring(struct per
 {
 	unsigned int new_size;
 	evtchn_port_t *new_ring, *old_ring;
-	unsigned int p, c;
 
 	/*
 	 * Ensure the ring is large enough to capture all possible

@@ -346,20 +345,17 @@ static int evtchn_resize_ring(struct per
 	/*
 	 * Copy the old ring contents to the new ring.
 	 *
-	 * If the ring contents crosses the end of the current ring,
-	 * it needs to be copied in two chunks.
+	 * To take care of wrapping, a full ring, and the new index
+	 * pointing into the second half, simply copy the old contents
+	 * twice.
 	 *
 	 * +---------+    +------------------+
-	 * |34567  12| -> |       1234567    |
-	 * +-----p-c-+    +------------------+
+	 * |34567  12| -> |34567  1234567  12|
+	 * +-----p-c-+    +-------c------p---+
 	 */
-	p = evtchn_ring_offset(u, u->ring_prod);
-	c = evtchn_ring_offset(u, u->ring_cons);
-	if (p < c) {
-		memcpy(new_ring + c, u->ring + c, (u->ring_size - c) * sizeof(*u->ring));
-		memcpy(new_ring + u->ring_size, u->ring, p * sizeof(*u->ring));
-	} else
-		memcpy(new_ring + c, u->ring + c, (p - c) * sizeof(*u->ring));
+	memcpy(new_ring, old_ring, u->ring_size * sizeof(*u->ring));
+	memcpy(new_ring + u->ring_size, old_ring,
+	       u->ring_size * sizeof(*u->ring));
 
 	u->ring = new_ring;
 	u->ring_size = new_size;

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

queue-4.5/xen-evtchn-fix-ring-resize-when-binding-new-events.patch