[PATCH] fou: reject FOU_ENCAP_DIRECT with protocol 0 to prevent sk_buff leak

[Hui Peng <benquike@gmail.com>]

In fou_udp_recv(), returning -fou->protocol to udp_queue_rcv_one_skb()
triggers IP protocol resubmission when fou->protocol > 0, whereas returning
0 tells the UDP tunnel layer that the skb was consumed without freeing it.
When a FOU_ENCAP_DIRECT socket is configured with FOU_ATTR_IPPROTO == 0,
every received packet returns 0 from fou_udp_recv() and leaks the sk_buff.

Reject FOU_ENCAP_DIRECT configurations with protocol 0 in parse_nl_config()
and drop packets if !fou->protocol in fou_udp_recv().

Fixes: 08d323234d10 ("net: fou: rename the source for linking")
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
diff --git a/net/ipv4/fou_core.c b/net/ipv4/fou_core.c
index 5e867f1b5c1d..3fc087c808bc 100644
--- a/net/ipv4/fou_core.c
+++ b/net/ipv4/fou_core.c
@@ -77,6 +77,9 @@ static int fou_udp_recv(struct sock *sk, struct sk_buff *skb)
 	if (!fou)
 		return 1;
 
+	if (unlikely(!fou->protocol))
+		goto drop;
+
 	if (fou_recv_pull(skb, fou, sizeof(struct udphdr)))
 		goto drop;
 
@@ -696,6 +699,9 @@ static int parse_nl_config(struct genl_info *info,
 	if (info->attrs[FOU_ATTR_TYPE])
 		cfg->type = nla_get_u8(info->attrs[FOU_ATTR_TYPE]);
 
+	if (cfg->type == FOU_ENCAP_DIRECT && !cfg->protocol)
+		return -EINVAL;
+
 	if (info->attrs[FOU_ATTR_REMCSUM_NOPARTIAL])
 		cfg->flags |= FOU_F_REMCSUM_NOPARTIAL;