[PATCH 2/6] xen-netfront: copy response out of shared buffer before accessing it

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

Make local copy of the response, otherwise backend might modify it while
frontend is already processing it - leading to time of check / time of
use issue.

This is complementary to XSA155.

Cc: stable@vger.kernel.org
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
---
 drivers/net/xen-netfront.c | 51 +++++++++++++++++++--------------------
 1 file changed, 25 insertions(+), 26 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 4dd0668..dc99763 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -387,13 +387,13 @@ static void xennet_tx_buf_gc(struct netfront_queue *queue)
 		rmb(); /* Ensure we see responses up to 'rp'. */
 
 		for (cons = queue->tx.rsp_cons; cons != prod; cons++) {
-			struct xen_netif_tx_response *txrsp;
+			struct xen_netif_tx_response txrsp;
 
-			txrsp = RING_GET_RESPONSE(&queue->tx, cons);
-			if (txrsp->status == XEN_NETIF_RSP_NULL)
+			RING_COPY_RESPONSE(&queue->tx, cons, &txrsp);
+			if (txrsp.status == XEN_NETIF_RSP_NULL)
 				continue;
 
-			id  = txrsp->id;
+			id  = txrsp.id;
 			skb = queue->tx_skbs[id].skb;
 			if (unlikely(gnttab_query_foreign_access(
 				queue->grant_tx_ref[id]) != 0)) {
@@ -741,7 +741,7 @@ static int xennet_get_extras(struct netfront_queue *queue,
 			     RING_IDX rp)
 
 {
-	struct xen_netif_extra_info *extra;
+	struct xen_netif_extra_info extra;
 	struct device *dev = &queue->info->netdev->dev;
 	RING_IDX cons = queue->rx.rsp_cons;
 	int err = 0;
@@ -757,24 +757,23 @@ static int xennet_get_extras(struct netfront_queue *queue,
 			break;
 		}
 
-		extra = (struct xen_netif_extra_info *)
-			RING_GET_RESPONSE(&queue->rx, ++cons);
+		RING_COPY_RESPONSE(&queue->rx, ++cons, &extra);
 
-		if (unlikely(!extra->type ||
-			     extra->type >= XEN_NETIF_EXTRA_TYPE_MAX)) {
+		if (unlikely(!extra.type ||
+			     extra.type >= XEN_NETIF_EXTRA_TYPE_MAX)) {
 			if (net_ratelimit())
 				dev_warn(dev, "Invalid extra type: %d\n",
-					extra->type);
+					extra.type);
 			err = -EINVAL;
 		} else {
-			memcpy(&extras[extra->type - 1], extra,
-			       sizeof(*extra));
+			memcpy(&extras[extra.type - 1], &extra,
+			       sizeof(extra));
 		}
 
 		skb = xennet_get_rx_skb(queue, cons);
 		ref = xennet_get_rx_ref(queue, cons);
 		xennet_move_rx_slot(queue, skb, ref);
-	} while (extra->flags & XEN_NETIF_EXTRA_FLAG_MORE);
+	} while (extra.flags & XEN_NETIF_EXTRA_FLAG_MORE);
 
 	queue->rx.rsp_cons = cons;
 	return err;
@@ -784,28 +783,28 @@ static int xennet_get_responses(struct netfront_queue *queue,
 				struct netfront_rx_info *rinfo, RING_IDX rp,
 				struct sk_buff_head *list)
 {
-	struct xen_netif_rx_response *rx = &rinfo->rx;
+	struct xen_netif_rx_response rx = rinfo->rx;
 	struct xen_netif_extra_info *extras = rinfo->extras;
 	struct device *dev = &queue->info->netdev->dev;
 	RING_IDX cons = queue->rx.rsp_cons;
 	struct sk_buff *skb = xennet_get_rx_skb(queue, cons);
 	grant_ref_t ref = xennet_get_rx_ref(queue, cons);
-	int max = MAX_SKB_FRAGS + (rx->status <= RX_COPY_THRESHOLD);
+	int max = MAX_SKB_FRAGS + (rx.status <= RX_COPY_THRESHOLD);
 	int slots = 1;
 	int err = 0;
 	unsigned long ret;
 
-	if (rx->flags & XEN_NETRXF_extra_info) {
+	if (rx.flags & XEN_NETRXF_extra_info) {
 		err = xennet_get_extras(queue, extras, rp);
 		cons = queue->rx.rsp_cons;
 	}
 
 	for (;;) {
-		if (unlikely(rx->status < 0 ||
-			     rx->offset + rx->status > XEN_PAGE_SIZE)) {
+		if (unlikely(rx.status < 0 ||
+			     rx.offset + rx.status > XEN_PAGE_SIZE)) {
 			if (net_ratelimit())
 				dev_warn(dev, "rx->offset: %u, size: %d\n",
-					 rx->offset, rx->status);
+					 rx.offset, rx.status);
 			xennet_move_rx_slot(queue, skb, ref);
 			err = -EINVAL;
 			goto next;
@@ -819,7 +818,7 @@ static int xennet_get_responses(struct netfront_queue *queue,
 		if (ref == GRANT_INVALID_REF) {
 			if (net_ratelimit())
 				dev_warn(dev, "Bad rx response id %d.\n",
-					 rx->id);
+					 rx.id);
 			err = -EINVAL;
 			goto next;
 		}
@@ -832,7 +831,7 @@ static int xennet_get_responses(struct netfront_queue *queue,
 		__skb_queue_tail(list, skb);
 
 next:
-		if (!(rx->flags & XEN_NETRXF_more_data))
+		if (!(rx.flags & XEN_NETRXF_more_data))
 			break;
 
 		if (cons + slots == rp) {
@@ -842,7 +841,7 @@ static int xennet_get_responses(struct netfront_queue *queue,
 			break;
 		}
 
-		rx = RING_GET_RESPONSE(&queue->rx, cons + slots);
+		RING_COPY_RESPONSE(&queue->rx, cons + slots, &rx);
 		skb = xennet_get_rx_skb(queue, cons + slots);
 		ref = xennet_get_rx_ref(queue, cons + slots);
 		slots++;
@@ -898,9 +897,9 @@ static RING_IDX xennet_fill_frags(struct netfront_queue *queue,
 	struct sk_buff *nskb;
 
 	while ((nskb = __skb_dequeue(list))) {
-		struct xen_netif_rx_response *rx =
-			RING_GET_RESPONSE(&queue->rx, ++cons);
+		struct xen_netif_rx_response rx;
 		skb_frag_t *nfrag = &skb_shinfo(nskb)->frags[0];
+		RING_COPY_RESPONSE(&queue->rx, ++cons, &rx);
 
 		if (shinfo->nr_frags == MAX_SKB_FRAGS) {
 			unsigned int pull_to = NETFRONT_SKB_CB(skb)->pull_to;
@@ -911,7 +910,7 @@ static RING_IDX xennet_fill_frags(struct netfront_queue *queue,
 		BUG_ON(shinfo->nr_frags >= MAX_SKB_FRAGS);
 
 		skb_add_rx_frag(skb, shinfo->nr_frags, skb_frag_page(nfrag),
-				rx->offset, rx->status, PAGE_SIZE);
+				rx.offset, rx.status, PAGE_SIZE);
 
 		skb_shinfo(nskb)->nr_frags = 0;
 		kfree_skb(nskb);
@@ -1007,7 +1006,7 @@ static int xennet_poll(struct napi_struct *napi, int budget)
 	i = queue->rx.rsp_cons;
 	work_done = 0;
 	while ((i != rp) && (work_done < budget)) {
-		memcpy(rx, RING_GET_RESPONSE(&queue->rx, i), sizeof(*rx));
+		RING_COPY_RESPONSE(&queue->rx, i, rx);
 		memset(extras, 0, sizeof(rinfo.extras));
 
 		err = xennet_get_responses(queue, &rinfo, rp, &tmpq);
-- 
git-series 0.9.1