[PATCH] 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 <redacted>
Signed-off-by: Amit Shah <redacted>
---
 drivers/virtio/virtio_ring.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index cc2f73e..b0043fb 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;
 	}

-- 
1.7.4