[PATCH net 08/11] ipvs: fix MTU check for GSO packets in tunnel mode

Pablo Neira Ayuso
2026-04-16T13:15:13Z

From: Yingnan Zhang <342144303@qq.com>

Currently, IPVS skips MTU checks for GSO packets by excluding them with
the !skb_is_gso(skb) condition. This creates problems when IPVS tunnel
mode encapsulates GSO packets with IPIP headers.

The issue manifests in two ways:

1. MTU violation after encapsulation:
When a GSO packet passes through IPVS tunnel mode, the original MTU
check is bypassed. After adding the IPIP tunnel header, the packet
size may exceed the outgoing interface MTU, leading to unexpected
fragmentation at the IP layer.

2. Fragmentation with problematic IP IDs:
When net.ipv4.vs.pmtu_disc=1 and a GSO packet with multiple segments
is fragmented after encapsulation, each segment gets a sequentially
incremented IP ID (0, 1, 2, ...). This happens because:

a) The GSO packet bypasses MTU check and gets encapsulated
b) At __ip_finish_output, the oversized GSO packet is split into
separate SKBs (one per segment), with IP IDs incrementing
c) Each SKB is then fragmented again based on the actual MTU

This sequential IP ID allocation differs from the expected behavior
and can cause issues with fragment reassembly and packet tracking.

Fix this by properly validating GSO packets using
skb_gso_validate_network_len(). This function correctly validates
whether the GSO segments will fit within the MTU after segmentation. If
validation fails, send an ICMP Fragmentation Needed message to enable
proper PMTU discovery.

Fixes: 4cdd34084d53 ("netfilter: nf_conntrack_ipv6: improve fragmentation handling")
Signed-off-by: Yingnan Zhang <342144303@qq.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
net/netfilter/ipvs/ip_vs_xmit.c | 19 +++++++++++++++----
1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/net/netfilter/ipvs/ip_vs_xmit.c b/net/netfilter/ipvs/ip_vs_xmit.c
index 3601eb86d025..7c570f48ade2 100644
--- a/net/netfilter/ipvs/ip_vs_xmit.c
+++ b/net/netfilter/ipvs/ip_vs_xmit.c
@@ -102,6 +102,18 @@ __ip_vs_dst_check(struct ip_vs_dest *dest)
return dest_dst;
}

+/* Based on ip_exceeds_mtu(). */
+static bool ip_vs_exceeds_mtu(const struct sk_buff *skb, unsigned int mtu)
+{
+ if (skb->len <= mtu)
+ return false;
+
+ if (skb_is_gso(skb) && skb_gso_validate_network_len(skb, mtu))
+ return false;
+
+ return true;
+}
+
static inline bool
__mtu_check_toobig_v6(const struct sk_buff *skb, u32 mtu)
{
@@ -111,10 +123,9 @@ __mtu_check_toobig_v6(const struct sk_buff *skb, u32 mtu)
*/
if (IP6CB(skb)->frag_max_size > mtu)
return true; /* largest fragment violate MTU */
- }
- else if (skb->len > mtu && !skb_is_gso(skb)) {
+ } else if (ip_vs_exceeds_mtu(skb, mtu))
return true; /* Packet size violate MTU size */
- }
+
return false;
}

@@ -232,7 +243,7 @@ static inline bool ensure_mtu_is_adequate(struct netns_ipvs *ipvs, int skb_af,
return true;

if (unlikely(ip_hdr(skb)->frag_off & htons(IP_DF) &&
- skb->len > mtu && !skb_is_gso(skb) &&
+ ip_vs_exceeds_mtu(skb, mtu) &&
!ip_vs_iph_icmp(ipvsh))) {
icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
htonl(mtu));
--
2.47.3