wireguard: queueing: account for skb->protocol==0

We carry out checks to the effect of:

  if (skb->protocol != wg_examine_packet_protocol(skb))
    goto err;

By having wg_skb_examine_untrusted_ip_hdr return 0 on failure, this
means that the check above still passes in the case where skb->protocol
is zero, which is possible to hit with AF_PACKET:

  struct sockaddr_pkt saddr = { .spkt_device = "wg0" };
  unsigned char buffer[5] = { 0 };
  sendto(socket(AF_PACKET, SOCK_PACKET, /* skb->protocol = */ 0),
         buffer, sizeof(buffer), 0, (const struct sockaddr *)&saddr, sizeof(saddr));

Additional checks mean that this isn't actually a problem in the code
base, but I could imagine it becoming a problem later if the function is
used more liberally.

I would prefer to fix this by having wg_examine_packet_protocol return a
32-bit ~0 value on failure, which will never match any value of
skb->protocol, which would simply change the generated code from a mov
to a movzx. However, sparse complains, and adding __force casts doesn't
seem like a good idea, so instead we just add a simple helper function
to check for the zero return value. Since wg_examine_packet_protocol
itself gets inlined, this winds up not adding an additional branch to
the generated code, since the 0 return value already happens in a
mergable branch.

Reported-by: Fabian Freyer <fabianfreyer@radicallyopensecurity.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

3 files changed, 10 insertions, 4 deletions

diff --git a/src/device.c b/src/device.c
index 73c892a..5ea039b 100644
--- a/src/device.c
+++ b/src/device.c
@@ -130,7 +130,7 @@ static netdev_tx_t wg_xmit(struct sk_buff *skb, struct net_device *dev)
 	u32 mtu;
 	int ret;
 
-	if (unlikely(wg_skb_examine_untrusted_ip_hdr(skb) != skb->protocol)) {
+	if (unlikely(!wg_check_packet_protocol(skb))) {
 		ret = -EPROTONOSUPPORT;
 		net_dbg_ratelimited("%s: Invalid IP packet\n", dev->name);
 		goto err;
diff --git a/src/queueing.h b/src/queueing.h
index e49a464..256c6be 100644
--- a/src/queueing.h
+++ b/src/queueing.h
@@ -66,7 +66,7 @@ struct packet_cb {
 #define PACKET_PEER(skb) (PACKET_CB(skb)->keypair->entry.peer)
 
 /* Returns either the correct skb->protocol value, or 0 if invalid. */
-static inline __be16 wg_skb_examine_untrusted_ip_hdr(struct sk_buff *skb)
+static inline __be16 wg_examine_packet_protocol(struct sk_buff *skb)
 {
 	if (skb_network_header(skb) >= skb->head &&
 	    (skb_network_header(skb) + sizeof(struct iphdr)) <=
@@ -81,6 +81,12 @@ static inline __be16 wg_skb_examine_untrusted_ip_hdr(struct sk_buff *skb)
 	return 0;
 }
 
+static inline bool wg_check_packet_protocol(struct sk_buff *skb)
+{
+	__be16 real_protocol = wg_examine_packet_protocol(skb);
+	return real_protocol && skb->protocol == real_protocol;
+}
+
 static inline void wg_reset_packet(struct sk_buff *skb)
 {
 	const int pfmemalloc = skb->pfmemalloc;
diff --git a/src/receive.c b/src/receive.c
index a94fcd7..dde4109 100644
--- a/src/receive.c
+++ b/src/receive.c
@@ -57,7 +57,7 @@ static int prepare_skb_header(struct sk_buff *skb, struct wg_device *wg)
 	size_t data_offset, data_len, header_len;
 	struct udphdr *udp;
 
-	if (unlikely(wg_skb_examine_untrusted_ip_hdr(skb) != skb->protocol ||
+	if (unlikely(!wg_check_packet_protocol(skb) ||
 		     skb_transport_header(skb) < skb->head ||
 		     (skb_transport_header(skb) + sizeof(struct udphdr)) >
 			     skb_tail_pointer(skb)))
@@ -392,7 +392,7 @@ static void wg_packet_consume_data_done(struct wg_peer *peer,
 #ifndef COMPAT_CANNOT_USE_CSUM_LEVEL
 	skb->csum_level = ~0; /* All levels */
 #endif
-	skb->protocol = wg_skb_examine_untrusted_ip_hdr(skb);
+	skb->protocol = wg_examine_packet_protocol(skb);
 	if (skb->protocol == htons(ETH_P_IP)) {
 		len = ntohs(ip_hdr(skb)->tot_len);
 		if (unlikely(len < sizeof(struct iphdr)))