[PATCH 3.19 040/175] xen-scsiback: mark pvscsi frontend request consumed only after last read

[Greg Kroah-Hartman]

3.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Juergen Gross <jgross@xxxxxxxx>

commit facb5732b0bb59ebbc11b5d5abc249e677ddbeb6 upstream.

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@xxxxxxxx>
Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/xen/xen-scsiback.c |   14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

--- a/drivers/xen/xen-scsiback.c
+++ b/drivers/xen/xen-scsiback.c
@@ -708,12 +708,11 @@ static int prepare_pending_reqs(struct v
 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;
@@ -734,11 +733,10 @@ static int scsiback_do_cmd_fn(struct vsc
 		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:
@@ -754,9 +752,9 @@ static int scsiback_do_cmd_fn(struct vsc
 			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);
@@ -767,7 +765,7 @@ static int scsiback_do_cmd_fn(struct vsc
 			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);


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