xen-netback: don't produce zero-size SKB frags

While frontends may submit zero-size requests (wasting a precious slot),
core networking code as of at least 3ece782 ("sock: skb_copy_ubufs
support for compound pages") can't deal with SKBs when they have all
zero-size fragments. Respond to empty requests right when populating
fragments; all further processing is fragment based and hence won't
encounter these empty requests anymore.

In a way this should have been that way from the beginning: When no data
is to be transferred for a particular request, there's not even a point
in validating the respective grant ref. That's no different from e.g.
passing NULL into memcpy() when at the same time the size is 0.

This is XSA-448 / CVE-2023-46838.

Cc: stable@vger.kernel.org
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>

Author: jbeulich

drivers/net/xen-netback/netback.c

@@ -463,12 +463,25 @@ static void xenvif_get_requests(struct xenvif_queue *queue,
 	}
 
 	for (shinfo->nr_frags = 0; nr_slots > 0 && shinfo->nr_frags < MAX_SKB_FRAGS;
-	     shinfo->nr_frags++, gop++, nr_slots--) {
+	     nr_slots--) {
+		if (unlikely(!txp->size)) {
+			unsigned long flags;
+
+			spin_lock_irqsave(&queue->response_lock, flags);
+			make_tx_response(queue, txp, 0, XEN_NETIF_RSP_OKAY);
+			push_tx_responses(queue);
+			spin_unlock_irqrestore(&queue->response_lock, flags);
+			++txp;
+			continue;
+		}
+
 		index = pending_index(queue->pending_cons++);
 		pending_idx = queue->pending_ring[index];
 		xenvif_tx_create_map_op(queue, pending_idx, txp,
 				        txp == first ? extra_count : 0, gop);
 		frag_set_pending_idx(&frags[shinfo->nr_frags], pending_idx);
+		++shinfo->nr_frags;
+		++gop;
 
 		if (txp == first)
 			txp = txfrags;
@@ -481,20 +494,39 @@ static void xenvif_get_requests(struct xenvif_queue *queue,
 		shinfo = skb_shinfo(nskb);
 		frags = shinfo->frags;
 
-		for (shinfo->nr_frags = 0; shinfo->nr_frags < nr_slots;
-		     shinfo->nr_frags++, txp++, gop++) {
+		for (shinfo->nr_frags = 0; shinfo->nr_frags < nr_slots; ++txp) {
+			if (unlikely(!txp->size)) {
+				unsigned long flags;
+
+				spin_lock_irqsave(&queue->response_lock, flags);
+				make_tx_response(queue, txp, 0,
+						 XEN_NETIF_RSP_OKAY);
+				push_tx_responses(queue);
+				spin_unlock_irqrestore(&queue->response_lock,
+						       flags);
+				continue;
+			}
+
 			index = pending_index(queue->pending_cons++);
 			pending_idx = queue->pending_ring[index];
 			xenvif_tx_create_map_op(queue, pending_idx, txp, 0,
 						gop);
 			frag_set_pending_idx(&frags[shinfo->nr_frags],
 					     pending_idx);
+			++shinfo->nr_frags;
+			++gop;
 		}
 
-		skb_shinfo(skb)->frag_list = nskb;
-	} else if (nskb) {
+		if (shinfo->nr_frags) {
+			skb_shinfo(skb)->frag_list = nskb;
+			nskb = NULL;
+		}
+	}
+
+	if (nskb) {
 		/* A frag_list skb was allocated but it is no longer needed
-		 * because enough slots were converted to copy ops above.
+		 * because enough slots were converted to copy ops above or some
+		 * were empty.
 		 */
 		kfree_skb(nskb);
 	}