xen-netback: use RING_COPY_REQUEST() throughout

commit 68a33bfd upstream.

Instead of open-coding memcpy()s and directly accessing Tx and Rx
requests, use the new RING_COPY_REQUEST() that ensures the local copy
is correct.

This is more than is strictly necessary for guest Rx requests since
only the id and gref fields are used and it is harmless if the
frontend modifies these.

This is part of XSA155.
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[bwh: Backported to 3.2:
 - s/queue/vif/g
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/net/xen-netback/netback.c

@@ -406,17 +406,17 @@ static struct netbk_rx_meta *get_next_rx_buffer(struct xenvif *vif,
 						struct netrx_pending_operations *npo)
 {
 	struct netbk_rx_meta *meta;
-	struct xen_netif_rx_request *req;
+	struct xen_netif_rx_request req;
 
-	req = RING_GET_REQUEST(&vif->rx, vif->rx.req_cons++);
+	RING_COPY_REQUEST(&vif->rx, vif->rx.req_cons++, &req);
 
 	meta = npo->meta + npo->meta_prod++;
 	meta->gso_size = 0;
 	meta->size = 0;
-	meta->id = req->id;
+	meta->id = req.id;
 
 	npo->copy_off = 0;
-	npo->copy_gref = req->gref;
+	npo->copy_gref = req.gref;
 
 	return meta;
 }
@@ -518,7 +518,7 @@ static int netbk_gop_skb(struct sk_buff *skb,
 	struct xenvif *vif = netdev_priv(skb->dev);
 	int nr_frags = skb_shinfo(skb)->nr_frags;
 	int i;
-	struct xen_netif_rx_request *req;
+	struct xen_netif_rx_request req;
 	struct netbk_rx_meta *meta;
 	unsigned char *data;
 	int head = 1;
@@ -528,14 +528,14 @@ static int netbk_gop_skb(struct sk_buff *skb,
 
 	/* Set up a GSO prefix descriptor, if necessary */
 	if (skb_shinfo(skb)->gso_size && vif->gso_prefix) {
-		req = RING_GET_REQUEST(&vif->rx, vif->rx.req_cons++);
+		RING_COPY_REQUEST(&vif->rx, vif->rx.req_cons++, &req);
 		meta = npo->meta + npo->meta_prod++;
 		meta->gso_size = skb_shinfo(skb)->gso_size;
 		meta->size = 0;
-		meta->id = req->id;
+		meta->id = req.id;
 	}
 
-	req = RING_GET_REQUEST(&vif->rx, vif->rx.req_cons++);
+	RING_COPY_REQUEST(&vif->rx, vif->rx.req_cons++, &req);
 	meta = npo->meta + npo->meta_prod++;
 
 	if (!vif->gso_prefix)
@@ -544,9 +544,9 @@ static int netbk_gop_skb(struct sk_buff *skb,
 		meta->gso_size = 0;
 
 	meta->size = 0;
-	meta->id = req->id;
+	meta->id = req.id;
 	npo->copy_off = 0;
-	npo->copy_gref = req->gref;
+	npo->copy_gref = req.gref;
 
 	data = skb->data;
 	while (data < skb_tail_pointer(skb)) {
@@ -890,7 +890,7 @@ static void netbk_tx_err(struct xenvif *vif,
 		make_tx_response(vif, txp, XEN_NETIF_RSP_ERROR);
 		if (cons == end)
 			break;
-		txp = RING_GET_REQUEST(&vif->tx, cons++);
+		RING_COPY_REQUEST(&vif->tx, cons++, txp);
 	} while (1);
 	vif->tx.req_cons = cons;
 	xen_netbk_check_rx_xenvif(vif);
@@ -957,8 +957,7 @@ static int netbk_count_requests(struct xenvif *vif,
 		if (drop_err)
 			txp = &dropped_tx;
 
-		memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + slots),
-		       sizeof(*txp));
+		RING_COPY_REQUEST(&vif->tx, cons + slots, txp);
 
 		/* If the guest submitted a frame >= 64 KiB then
 		 * first->size overflowed and following slots will
@@ -1246,8 +1245,7 @@ static int xen_netbk_get_extras(struct xenvif *vif,
 			return -EBADR;
 		}
 
-		memcpy(&extra, RING_GET_REQUEST(&vif->tx, cons),
-		       sizeof(extra));
+		RING_COPY_REQUEST(&vif->tx, cons, &extra);
 		if (unlikely(!extra.type ||
 			     extra.type >= XEN_NETIF_EXTRA_TYPE_MAX)) {
 			vif->tx.req_cons = ++cons;
@@ -1442,7 +1440,7 @@ static unsigned xen_netbk_tx_build_gops(struct xen_netbk *netbk)
 
 		idx = vif->tx.req_cons;
 		rmb(); /* Ensure that we see the request before we copy it. */
-		memcpy(&txreq, RING_GET_REQUEST(&vif->tx, idx), sizeof(txreq));
+		RING_COPY_REQUEST(&vif->tx, idx, &txreq);
 
 		/* Credit-based scheduling. */
 		if (txreq.size > vif->remaining_credit &&