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>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/net/wireguard/receive.c b/drivers/net/wireguard/receive.c
index 4a153894cee2..243ed7172dd2 100644
--- a/drivers/net/wireguard/receive.c
+++ b/drivers/net/wireguard/receive.c
@@ -56,7 +56,7 @@ 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)))
@@ -388,7 +388,7 @@ static void wg_packet_consume_data_done(struct wg_peer *peer,
 	 */
 	skb->ip_summed = CHECKSUM_UNNECESSARY;
 	skb->csum_level = ~0; /* All levels */
-	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)))