virtio_net: fix race in RX VQ processing

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@redhat.com>
Tested-by: Jason Wang <jasowang@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[wg: Backported to 3.2]
Signed-off-by: Wolfram Gloger <wmglo@dent.med.uni-muenchen.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 6ee8410..43a6a11 100644 (file)
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -508,7 +508,7 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
 {
        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);