FAILED: patch "[PATCH] vhost/vsock: fix incorrect used length reported to the guest" failed to apply to 4.9-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 49d8c5ffad07ca014cfae72a1b9b8c52b6ad9cb8 Mon Sep 17 00:00:00 2001
From: Stefano Garzarella <sgarzare@redhat.com>
Date: Mon, 22 Nov 2021 17:35:24 +0100
Subject: [PATCH] vhost/vsock: fix incorrect used length reported to the guest

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 <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20211122163525.294024-2-sgarzare@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 938aefbc75ec..4e3b95af7ee4 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -554,7 +554,7 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work *work)
 			virtio_transport_free_pkt(pkt);
 
 		len += sizeof(pkt->hdr);
-		vhost_add_used(vq, head, len);
+		vhost_add_used(vq, head, 0);
 		total_len += len;
 		added = true;
 	} while(likely(!vhost_exceeds_weight(vq, ++pkts, total_len)));