Re: [PATCH net v2] ipvs: fix PMTU for GUE/GRE tunnel ICMP errors

[Julian Anastasov <ja@ssi.bg>]

	Hello,

On Thu, 2 Jul 2026, Yizhou Zhao wrote:

> When an ICMP Fragmentation Needed error is received for a tunneled IPVS
> connection, ip_vs_in_icmp() recomputes the MTU that the original packet
> can use by subtracting the tunnel overhead from the reported next-hop
> MTU.
> 
> The current code always subtracts sizeof(struct iphdr), which is only
> the IPIP overhead. For GUE and GRE tunnels, ipvs_udp_decap() and
> ipvs_gre_decap() already compute the additional tunnel header length,
> but that value is scoped to the decapsulation block and is lost before
> the ICMP_FRAG_NEEDED handling. As a result, the ICMP error sent back to
> the client advertises an MTU that is too large, so PMTUD can fail to
> converge for GUE/GRE-tunneled real servers.
> 
> With a reported next-hop MTU of 1400, a GUE tunnel currently returns
> 1380 to the client. The correct value is 1368:
> 
>   1400 - sizeof(struct iphdr) - sizeof(struct udphdr) -
>   sizeof(struct guehdr)
> 
> Hoist the tunnel header length into the main ip_vs_in_icmp() scope and
> subtract sizeof(struct iphdr) + ulen in the Fragmentation Needed path.
> The IPIP path keeps ulen as 0, so its existing 1400 - 20 = 1380 result
> is unchanged.
> 
> Fixes: 508f744c0de3 ("ipvs: strip udp tunnel headers from icmp errors")
> Cc: stable@vger.kernel.org
> Reported-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
> Reported-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
> Reported-by: Ao Wang <wangao@seu.edu.cn>
> Reported-by: Xuewei Feng <fengxw06@126.com>
> Reported-by: Qi Li <qli01@tsinghua.edu.cn>
> Reported-by: Ke Xu <xuke@tsinghua.edu.cn>
> Assisted-by: Claude-Code:GLM-5.2
> Signed-off-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>

	I'm late for this patch, in case there is another iteration:

Acked-by: Julian Anastasov <ja@ssi.bg>

	BTW, I prepared followup patch to add the needed pskb_may_pull()
as suggested by Sashiko:

https://sashiko.dev/#/patchset/20260702073430.67680-1-zhaoyz24%40mails.tsinghua.edu.cn

	I'll wait this patch to be applied before posting it...

> ---
> Changes in v2:
>   - Use the short first hunk context so patch applies without fuzz.
>   - Adjust Assisted-by to checkpatch's agent-name format.
>   - Suggested by Julian.
>   - Link to v1: https://lore.kernel.org/netdev/20260701065941.46249-1-zhaoyz24@mails.tsinghua.edu.cn/
> ---
>  net/netfilter/ipvs/ip_vs_core.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/net/netfilter/ipvs/ip_vs_core.c b/net/netfilter/ipvs/ip_vs_core.c
> index d40b404c1bf6..906f2c361676 100644
> --- a/net/netfilter/ipvs/ip_vs_core.c
> +++ b/net/netfilter/ipvs/ip_vs_core.c
> @@ -1767,6 +1767,7 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
>  	bool tunnel, new_cp = false;
>  	union nf_inet_addr *raddr;
>  	char *outer_proto = "IPIP";
> +	int ulen = 0;
>  
>  	*related = 1;
>  
> @@ -1831,7 +1832,6 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
>  		   /* Error for our tunnel must arrive at LOCAL_IN */
>  		   (skb_rtable(skb)->rt_flags & RTCF_LOCAL)) {
>  		__u8 iproto;
> -		int ulen;
>  
>  		/* Non-first fragment has no UDP/GRE header */
>  		if (unlikely(cih->frag_off & htons(IP_OFFSET)))
> @@ -1936,8 +1936,8 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff *skb, int *related,
>  				if (dest_dst)
>  					mtu = dst_mtu(dest_dst->dst_cache);
>  			}
> -			if (mtu > 68 + sizeof(struct iphdr))
> -				mtu -= sizeof(struct iphdr);
> +			if (mtu > 68 + sizeof(struct iphdr) + ulen)
> +				mtu -= sizeof(struct iphdr) + ulen;
>  			info = htonl(mtu);
>  		}
>  		/* Strip outer IP, ICMP and IPIP/UDP/GRE, go to IP header of

Regards

--
Julian Anastasov <ja@ssi.bg>