When a UDP encapsulation socket (e.g., FOU) receives a multicast
packet, __udp4_lib_mcast_deliver() and __udp6_lib_mcast_deliver()
call consume_skb() when udp_queue_rcv_skb() returns a positive value.
A positive return value from udp_queue_rcv_skb() indicates that the
encap_rcv handler (e.g., fou_udp_recv) has consumed the UDP header
and wants the packet to be resubmitted to the IP protocol handler
for further processing (e.g., as a GRE packet).

The unicast paths handle this correctly by propagating the return
value up to ip_protocol_deliver_rcu() / ip6_protocol_deliver_rcu()
for resubmission. However, the multicast paths destroy the packet
via consume_skb() instead of resubmitting it, causing silent packet
loss.

This affects any UDP encapsulation (FOU, GUE) combined with multicast
destination addresses.

Fix this by returning the value from udp_queue_rcv_skb() when it is
positive, matching the behavior of the corresponding unicast paths.
Note the sign difference between IPv4 and IPv6:

  - IPv4: udp_unicast_rcv_skb() returns -ret, and
    ip_protocol_deliver_rcu() resubmits when ret < 0
    (using -ret as the protocol number).
  - IPv6: udp6_unicast_rcv_skb() returns ret, and
    ip6_protocol_deliver_rcu() resubmits when ret > 0
    (using ret as the nexthdr).

Both mcast paths now follow the same convention as their respective
unicast paths.

Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Anton Danilov <redacted>
Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Willem de Bruijn <willemb@google.com>
---
 net/ipv4/udp.c | 6 ++++--
 net/ipv6/udp.c | 6 ++++--
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 59248a59358c..d3ddcbfc8477 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c

@@ -2476,6 +2476,7 @@ static int __udp4_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 	struct udp_hslot *hslot;
 	struct sk_buff *nskb;
 	bool use_hash2;
+	int ret;
 
 	hash2_any = 0;
 	hash2 = 0;

@@ -2520,8 +2521,9 @@ static int __udp4_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 	}
 
 	if (first) {
-		if (udp_queue_rcv_skb(first, skb) > 0)
-			consume_skb(skb);
+		ret = udp_queue_rcv_skb(first, skb);
+		if (ret > 0)
+			return -ret;
 	} else {
 		kfree_skb(skb);
 		__UDP_INC_STATS(net, UDP_MIB_IGNOREDMULTI);

diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 392e18b97045..0910cc171776 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c

@@ -949,6 +949,7 @@ static int __udp6_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 	struct udp_hslot *hslot;
 	struct sk_buff *nskb;
 	bool use_hash2;
+	int ret;
 
 	hash2_any = 0;
 	hash2 = 0;

@@ -998,8 +999,9 @@ static int __udp6_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 	}
 
 	if (first) {
-		if (udpv6_queue_rcv_skb(first, skb) > 0)
-			consume_skb(skb);
+		ret = udpv6_queue_rcv_skb(first, skb);
+		if (ret > 0)
+			return ret;
 	} else {
 		kfree_skb(skb);
 		__UDP6_INC_STATS(net, UDP_MIB_IGNOREDMULTI);

-- 
2.47.3