[28/84] virtio_net: fix race in RX VQ processing

[Ben Hutchings]

3.2.50-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Michael S. Tsirkin <mst@xxxxxxxxxx>

commit cbdadbbf0c790f79350a8f36029208944c5487d0 upstream.

virtio net called virtqueue_enable_cq on RX path after napi_complete, so
with NAPI_STATE_SCHED clear - outside the implicit napi lock.
This violates the requirement to synchronize virtqueue_enable_cq wrt
virtqueue_add_buf.  In particular, used event can move backwards,
causing us to lose interrupts.
In a debug build, this can trigger panic within START_USE.

Jason Wang reports that he can trigger the races artificially,
by adding udelay() in virtqueue_enable_cb() after virtio_mb().

However, we must call napi_complete to clear NAPI_STATE_SCHED before
polling the virtqueue for used buffers, otherwise napi_schedule_prep in
a callback will fail, causing us to lose RX events.

To fix, call virtqueue_enable_cb_prepare with NAPI_STATE_SCHED
set (under napi lock), later call virtqueue_poll with
NAPI_STATE_SCHED clear (outside the lock).

Reported-by: Jason Wang <jasowang@xxxxxxxxxx>
Tested-by: Jason Wang <jasowang@xxxxxxxxxx>
Acked-by: Jason Wang <jasowang@xxxxxxxxxx>
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
[wg: Backported to 3.2]
Signed-off-by: Wolfram Gloger <wmglo@xxxxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
diff -upr linux-3.2.49/drivers/net/virtio_net.c linux-3.2.49wg/drivers/net/virtio_net.c
--- linux-3.2.49/drivers/net/virtio_net.c	2012-01-05 00:55:44.000000000 +0100
+++ linux-3.2.49wg/drivers/net/virtio_net.c	2013-07-27 13:57:33.000000000 +0200
@@ -508,7 +508,7 @@ static int virtnet_poll(struct napi_stru
 {
 	struct virtnet_info *vi = container_of(napi, struct virtnet_info, napi);
 	void *buf;
-	unsigned int len, received = 0;
+	unsigned int r, len, received = 0;
 
 again:
 	while (received < budget &&
@@ -525,8 +525,9 @@ again:
 
 	/* Out of packets? */
 	if (received < budget) {
+		r = virtqueue_enable_cb_prepare(vi->rvq);
 		napi_complete(napi);
-		if (unlikely(!virtqueue_enable_cb(vi->rvq)) &&
+		if (unlikely(virtqueue_poll(vi->rvq, r)) &&
 		    napi_schedule_prep(napi)) {
 			virtqueue_disable_cb(vi->rvq);
 			__napi_schedule(napi);

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