virtio: Decrement avail idx on buffer detach

When detaching a buffer from a vq, the avail.idx value should be
decremented as well.

This was noticed by hot-unplugging a virtio console port and then
plugging in a new one on the same number (re-using the vqs which were
just 'disowned').  qemu reported

   'Guest moved used index from 0 to 256'

when any IO was attempted on the new port.

CC: stable@kernel.org
Reported-by: juzhang <juzhang@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 1 insertions, 0 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index cc2f73e03475..b0043fb26a4d 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -371,6 +371,7 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
                 /* detach_buf clears data, so grab it now. */
                 buf = vq->data[i];
                 detach_buf(vq, i);
+                vq->vring.avail->idx--;
                 END_USE(vq);
                 return buf;
         }