From foo@baz Mon Nov 29 07:12:46 PM CET 2021
From: Juergen Gross <jgross@suse.com>
Date: Mon, 29 Nov 2021 15:57:03 +0100
Subject: xen/netfront: read response from backend only once

From: Juergen Gross <jgross@suse.com>

commit 8446066bf8c1f9f7b7412c43fbea0fb87464d75b upstream.

In order to avoid problems in case the backend is modifying a response
on the ring page while the frontend has already seen it, just read the
response into a local buffer in one go and then operate on that buffer
only.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/net/xen-netfront.c |   38 +++++++++++++++++++-------------------
 1 file changed, 19 insertions(+), 19 deletions(-)

--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -387,13 +387,13 @@ static void xennet_tx_buf_gc(struct netf
 		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)) {
@@ -736,7 +736,7 @@ static int xennet_get_extras(struct netf
 			     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;
@@ -752,24 +752,22 @@ static int xennet_get_extras(struct netf
 			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));
+			extras[extra.type - 1] = 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;
@@ -779,7 +777,7 @@ static int xennet_get_responses(struct n
 				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, rx_local;
 	struct xen_netif_extra_info *extras = rinfo->extras;
 	struct device *dev = &queue->info->netdev->dev;
 	RING_IDX cons = queue->rx.rsp_cons;
@@ -837,7 +835,8 @@ next:
 			break;
 		}
 
-		rx = RING_GET_RESPONSE(&queue->rx, cons + slots);
+		RING_COPY_RESPONSE(&queue->rx, cons + slots, &rx_local);
+		rx = &rx_local;
 		skb = xennet_get_rx_skb(queue, cons + slots);
 		ref = xennet_get_rx_ref(queue, cons + slots);
 		slots++;
@@ -892,10 +891,11 @@ static int xennet_fill_frags(struct netf
 	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 (skb_shinfo(skb)->nr_frags == MAX_SKB_FRAGS) {
 			unsigned int pull_to = NETFRONT_SKB_CB(skb)->pull_to;
 
@@ -910,7 +910,7 @@ static int xennet_fill_frags(struct netf
 
 		skb_add_rx_frag(skb, skb_shinfo(skb)->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);
@@ -1008,7 +1008,7 @@ static int xennet_poll(struct napi_struc
 	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);