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 <[email protected]>
Signed-off-by: Anton Danilov <[email protected]>
Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Willem de Bruijn <[email protected]>
---
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
