xen-scsiback: safely copy requests

commit be69746e upstream.

The copy of the ring request was lacking a following barrier(),
potentially allowing the compiler to optimize the copy away.

Use RING_COPY_REQUEST() to ensure the request is copied to local
memory.

This is part of XSA155.
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/xen/xen-scsiback.c

@@ -733,7 +733,7 @@ static int scsiback_do_cmd_fn(struct vscsibk_info *info)
 		if (!pending_req)
 			return 1;
 
-		ring_req = *RING_GET_REQUEST(ring, rc);
+		RING_COPY_REQUEST(ring, rc, &ring_req);
 		ring->req_cons = ++rc;
 
 		err = prepare_pending_reqs(info, &ring_req, pending_req);