xen/netback: Fix buffer overrun triggered by unusual packet

stable inclusion
from stable-v4.19.290
commit 11e6919ae028b5de1fc48007354ea07069561b31
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I7RQ63
CVE: CVE-2023-34319

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=11e6919ae028b5de1fc48007354ea07069561b31

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

commit 534fc31d09b706a16d83533e16b5dc855caf7576 upstream.

It is possible that a guest can send a packet that contains a head + 18
slots and yet has a len <= XEN_NETBACK_TX_COPY_LEN. This causes nr_slots
to underflow in xenvif_get_requests() which then causes the subsequent
loop's termination condition to be wrong, causing a buffer overrun of
queue->tx_map_ops.

Rework the code to account for the extra frag_overflow slots.

This is CVE-2023-34319 / XSA-432.

Fixes: ad7f402a ("xen/netback: Ensure protocol headers don't fall in the non-linear area")
Signed-off-by: NRoss Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: NPaul Durrant <paul@xen.org>
Reviewed-by: NWei Liu <wei.liu@kernel.org>
Signed-off-by: NJuergen Gross <jgross@suse.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Conflicts:
	drivers/net/xen-netback/netback.c
Signed-off-by: NZhengchao Shao <shaozhengchao@huawei.com>

drivers/net/xen-netback/netback.c

@@ -384,7 +384,7 @@ static void xenvif_get_requests(struct xenvif_queue *queue,
 	struct gnttab_map_grant_ref *gop = queue->tx_map_ops + *map_ops;
 	struct xen_netif_tx_request *txp = first;
 
-	nr_slots = shinfo->nr_frags + 1;
+	nr_slots = shinfo->nr_frags + frag_overflow + 1;
 
 	copy_count(skb) = 0;
 
@@ -439,8 +439,8 @@ static void xenvif_get_requests(struct xenvif_queue *queue,
 		}
 	}
 
-	for (shinfo->nr_frags = 0; shinfo->nr_frags < nr_slots;
-	     shinfo->nr_frags++, gop++) {
+	for (shinfo->nr_frags = 0; nr_slots > 0 && shinfo->nr_frags < MAX_SKB_FRAGS;
+	     shinfo->nr_frags++, gop++, nr_slots--) {
 		index = pending_index(queue->pending_cons++);
 		pending_idx = queue->pending_ring[index];
 		xenvif_tx_create_map_op(queue, pending_idx, txp,
@@ -453,12 +453,12 @@ static void xenvif_get_requests(struct xenvif_queue *queue,
 			txp++;
 	}
 
-	if (frag_overflow) {
+	if (nr_slots > 0) {
 
 		shinfo = skb_shinfo(nskb);
 		frags = shinfo->frags;
 
-		for (shinfo->nr_frags = 0; shinfo->nr_frags < frag_overflow;
+		for (shinfo->nr_frags = 0; shinfo->nr_frags < nr_slots;
 		     shinfo->nr_frags++, txp++, gop++) {
 			index = pending_index(queue->pending_cons++);
 			pending_idx = queue->pending_ring[index];
@@ -469,6 +469,11 @@ static void xenvif_get_requests(struct xenvif_queue *queue,
 		}
 
 		skb_shinfo(skb)->frag_list = nskb;
+	} else if (nskb) {
+		/* A frag_list skb was allocated but it is no longer needed
+		 * because enough slots were converted to copy ops above.
+		 */
+		kfree_skb(nskb);
 	}
 
 	(*copy_ops) = cop - queue->tx_copy_ops;