xen-scsiback: mark pvscsi frontend request consumed only after last read

A request in the ring buffer mustn't be read after it has been marked
as consumed. Otherwise it might already have been reused by the
frontend without violating the ring protocol.

To avoid inconsistencies in the backend only work on a private copy
of the request. This will ensure a malicious guest not being able to
bypass consistency checks of the backend by modifying an active
request.
Signed-off-by: Juergen Gross <jgross@suse.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/xen-scsiback.c

@@ -709,12 +709,11 @@ static int prepare_pending_reqs(struct vscsibk_info *info,
 static int scsiback_do_cmd_fn(struct vscsibk_info *info)
 {
 	struct vscsiif_back_ring *ring = &info->ring;
-	struct vscsiif_request *ring_req;
+	struct vscsiif_request ring_req;
 	struct vscsibk_pend *pending_req;
 	RING_IDX rc, rp;
 	int err, more_to_do;
 	uint32_t result;
-	uint8_t act;
 
 	rc = ring->req_cons;
 	rp = ring->sring->req_prod;
@@ -735,11 +734,10 @@ static int scsiback_do_cmd_fn(struct vscsibk_info *info)
 		if (!pending_req)
 			return 1;
 
-		ring_req = RING_GET_REQUEST(ring, rc);
+		ring_req = *RING_GET_REQUEST(ring, rc);
 		ring->req_cons = ++rc;
 
-		act = ring_req->act;
-		err = prepare_pending_reqs(info, ring_req, pending_req);
+		err = prepare_pending_reqs(info, &ring_req, pending_req);
 		if (err) {
 			switch (err) {
 			case -ENODEV:
@@ -755,9 +753,9 @@ static int scsiback_do_cmd_fn(struct vscsibk_info *info)
 			return 1;
 		}
 
-		switch (act) {
+		switch (ring_req.act) {
 		case VSCSIIF_ACT_SCSI_CDB:
-			if (scsiback_gnttab_data_map(ring_req, pending_req)) {
+			if (scsiback_gnttab_data_map(&ring_req, pending_req)) {
 				scsiback_fast_flush_area(pending_req);
 				scsiback_do_resp_with_sense(NULL,
 					DRIVER_ERROR << 24, 0, pending_req);
@@ -768,7 +766,7 @@ static int scsiback_do_cmd_fn(struct vscsibk_info *info)
 			break;
 		case VSCSIIF_ACT_SCSI_ABORT:
 			scsiback_device_action(pending_req, TMR_ABORT_TASK,
-				ring_req->ref_rqid);
+				ring_req.ref_rqid);
 			break;
 		case VSCSIIF_ACT_SCSI_RESET:
 			scsiback_device_action(pending_req, TMR_LUN_RESET, 0);