[PATCH net-next] fou: Prevent unbounded recursion in GUE error handler

[Stefano Brivio]

Handling exceptions for direct UDP encapsulation in GUE (that is,
UDP-in-UDP) leads to unbounded recursion in the GUE exception handler,
syzbot reported.

While draft-ietf-intarea-gue-06 doesn't explicitly forbid direct
encapsulation of UDP in GUE, it probably doesn't make sense to set up GUE
this way, and it's currently not even possible to configure this.

Skip exception handling if the GUE proto/ctype field is set to the UDP
protocol number. Should we need to handle exceptions for UDP-in-GUE one
day, we might need to either explicitly set a bound for recursion, or
implement a special iterative handling for these cases.

Reported-and-tested-by: syzbot+43f6755d1c2e62743468@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: b8a51b38e4d4 ("fou, fou6: ICMP error handlers for FoU and GUE")
Signed-off-by: Stefano Brivio <sbrivio@xxxxxxxxxx>
---
 net/ipv4/fou.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/ipv4/fou.c b/net/ipv4/fou.c
index 0d0ad19ecb87..0c9f171fb085 100644
--- a/net/ipv4/fou.c
+++ b/net/ipv4/fou.c
@@ -1061,6 +1061,13 @@ static int gue_err(struct sk_buff *skb, u32 info)
 	if (validate_gue_flags(guehdr, optlen))
 		return -EINVAL;
 
+	/* Handling exceptions for direct UDP encapsulation in GUE would lead to
+	 * recursion. Besides, this kind of encapsulation can't even be
+	 * configured currently. Discard this.
+	 */
+	if (guehdr->proto_ctype == IPPROTO_UDP)
+		return -EOPNOTSUPP;
+
 	skb_set_transport_header(skb, -(int)sizeof(struct icmphdr));
 	ret = gue_err_proto_handler(guehdr->proto_ctype, skb, info);
 
-- 
2.19.2