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>

1 files changed, 7 insertions, 1 deletions

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;