Re: [PATCH net] netfilter: flowtable: Saturate 16-bit flow_tuple->mtu

[Pablo Neira Ayuso <pablo@netfilter.org>]

Hi,

On Thu, Sep 10, 2026 at 07:20:52PM +0300, Alice Mikityanska wrote:
> From: Alice Mikityanska <alice@isovalent.com>
> 
> ip6_dst_mtu_maybe_forward clamps its return value to IP6_MAX_MTU, which
> is 40 bytes bigger than U16_MAX. While it can be a valid IPv6 MTU (IPv6
> payload_len doesn't include the header), this value is assigned to the
> 16-bit flow_tuple->mtu in flow_offload_fill_route.
> 
> To avoid the overflow, use U16_MAX for all MTUs bigger than 65535. Since
> flow_tuple->mtu is just a threshold for the software flowtable fast
> path, packets exceeding it won't be dropped, but rather punted to the
> normal forwarding path: nf_flow_offload_ipv6_forward returns 0, and
> nf_flow_offload_ipv6_hook returns NF_ACCEPT.
> 
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260901195714.673548-1-alice.kernel%40fastmail.im
> Fixes: 4f3780c004ef ("netfilter: nf_flow_table: cache mtu in struct flow_offload_tuple")
> Assisted-by: Codex:gpt-5.6-sol
> Signed-off-by: Alice Mikityanska <alice@isovalent.com>
> ---
>  net/netfilter/nf_flow_table_core.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/nf_flow_table_core.c b/net/netfilter/nf_flow_table_core.c
> index 03241d4bfd5e..8f15540ae206 100644
> --- a/net/netfilter/nf_flow_table_core.c
> +++ b/net/netfilter/nf_flow_table_core.c
> @@ -107,7 +107,8 @@ static int flow_offload_fill_route(struct flow_offload *flow,
>  		flow_tuple->mtu = ip_dst_mtu_maybe_forward(dst, true);
>  		break;
>  	case NFPROTO_IPV6:
> -		flow_tuple->mtu = ip6_dst_mtu_maybe_forward(dst, true);
> +		flow_tuple->mtu = min(ip6_dst_mtu_maybe_forward(dst, true),
> +				      U16_MAX);

This reminds me we should not catch the mtu.

>  		break;
>  	}
>  
> -- 
> 2.55.0
>