[PATCH net 2/3] ovpn: fix possible use-after-free in ovpn_net_xmit

[Antonio Quartulli <antonio@openvpn.net>]

From: Ralf Lici <ralf@mandelbit.com>

When building the skb_list in ovpn_net_xmit, skb_share_check will free
the original skb if it is shared. The current implementation continues
to use the stale skb pointer for subsequent operations:
- peer lookup,
- skb_dst_drop (even though all segments produced by skb_gso_segment
  will have a dst attached),
- ovpn_peer_stats_increment_tx.

Fix this by moving the peer lookup and skb_dst_drop before segmentation
so that the original skb is still valid when used. Return early if all
segments fail skb_share_check and the list ends up empty.
Also switch ovpn_peer_stats_increment_tx to use skb_list.next; the next
patch fixes the stats logic.

Fixes: 08857b5ec5d9 ("ovpn: implement basic TX path (UDP)")
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
---
 drivers/net/ovpn/io.c | 52 ++++++++++++++++++++++++++-----------------
 1 file changed, 31 insertions(+), 21 deletions(-)

diff --git a/drivers/net/ovpn/io.c b/drivers/net/ovpn/io.c
index 3e9e7f8444b3..f70c58b10599 100644
--- a/drivers/net/ovpn/io.c
+++ b/drivers/net/ovpn/io.c
@@ -365,7 +365,27 @@ netdev_tx_t ovpn_net_xmit(struct sk_buff *skb, struct net_device *dev)
 	/* verify IP header size in network packet */
 	proto = ovpn_ip_check_protocol(skb);
 	if (unlikely(!proto || skb->protocol != proto))
-		goto drop;
+		goto drop_no_peer;
+
+	/* retrieve peer serving the destination IP of this packet */
+	peer = ovpn_peer_get_by_dst(ovpn, skb);
+	if (unlikely(!peer)) {
+		switch (skb->protocol) {
+		case htons(ETH_P_IP):
+			net_dbg_ratelimited("%s: no peer to send data to dst=%pI4\n",
+					    netdev_name(ovpn->dev),
+					    &ip_hdr(skb)->daddr);
+			break;
+		case htons(ETH_P_IPV6):
+			net_dbg_ratelimited("%s: no peer to send data to dst=%pI6c\n",
+					    netdev_name(ovpn->dev),
+					    &ipv6_hdr(skb)->daddr);
+			break;
+		}
+		goto drop_no_peer;
+	}
+	/* dst was needed for peer selection - it can now be dropped */
+	skb_dst_drop(skb);
 
 	if (skb_is_gso(skb)) {
 		segments = skb_gso_segment(skb, 0);
@@ -396,34 +416,24 @@ netdev_tx_t ovpn_net_xmit(struct sk_buff *skb, struct net_device *dev)
 
 		__skb_queue_tail(&skb_list, curr);
 	}
-	skb_list.prev->next = NULL;
 
-	/* retrieve peer serving the destination IP of this packet */
-	peer = ovpn_peer_get_by_dst(ovpn, skb);
-	if (unlikely(!peer)) {
-		switch (skb->protocol) {
-		case htons(ETH_P_IP):
-			net_dbg_ratelimited("%s: no peer to send data to dst=%pI4\n",
-					    netdev_name(ovpn->dev),
-					    &ip_hdr(skb)->daddr);
-			break;
-		case htons(ETH_P_IPV6):
-			net_dbg_ratelimited("%s: no peer to send data to dst=%pI6c\n",
-					    netdev_name(ovpn->dev),
-					    &ipv6_hdr(skb)->daddr);
-			break;
-		}
-		goto drop;
+	/* no segments survived: don't jump to 'drop' because we already
+	 * incremented the counter for each failure in the loop
+	 */
+	if (unlikely(skb_queue_empty(&skb_list))) {
+		ovpn_peer_put(peer);
+		return NETDEV_TX_OK;
 	}
-	/* dst was needed for peer selection - it can now be dropped */
-	skb_dst_drop(skb);
+	skb_list.prev->next = NULL;
 
-	ovpn_peer_stats_increment_tx(&peer->vpn_stats, skb->len);
+	ovpn_peer_stats_increment_tx(&peer->vpn_stats, skb_list.next->len);
 	ovpn_send(ovpn, skb_list.next, peer);
 
 	return NETDEV_TX_OK;
 
 drop:
+	ovpn_peer_put(peer);
+drop_no_peer:
 	dev_dstats_tx_dropped(ovpn->dev);
 	skb_tx_error(skb);
 	kfree_skb_list(skb);
-- 
2.52.0