[PATCH 4.9 24/62] vhost/vsock: fix incorrect used length reported to the guest

From: Stefano Garzarella <sgarzare@redhat.com>

commit 49d8c5ffad07ca014cfae72a1b9b8c52b6ad9cb8 upstream.

The "used length" reported by calling vhost_add_used() must be the
number of bytes written by the device (using "in" buffers).

In vhost_vsock_handle_tx_kick() the device only reads the guest
buffers (they are all "out" buffers), without writing anything,
so we must pass 0 as "used length" to comply virtio spec.

Fixes: 433fc58e6bf2 ("VSOCK: Introduce vhost_vsock.ko")
Cc: stable@vger.kernel.org
Reported-by: Halil Pasic <pasic@linux.ibm.com>
Suggested-by: Jason Wang <redacted>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20211122163525.294024-2-sgarzare@redhat.com (local)
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/vhost/vsock.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c

@@ -406,7 +406,7 @@ static void vhost_vsock_handle_tx_kick(s
 		else
 			virtio_transport_free_pkt(pkt);
 
-		vhost_add_used(vq, head, sizeof(pkt->hdr) + len);
+		vhost_add_used(vq, head, 0);
 		added = true;
 	}