Message-ID: <689de08861510_18aa6c29471@willemb.c.googlers.com.notmuch>
Date: Thu, 14 Aug 2025 09:11:36 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jakub Ramaseuski <jramaseu@...hat.com>, 
 netdev@...r.kernel.org
Cc: kuba@...nel.org, 
 horms@...nel.org, 
 pabeni@...hat.com, 
 edumazet@...gle.com, 
 sdf@...ichev.me, 
 ahmed.zaki@...el.com, 
 aleksander.lobakin@...el.com, 
 benoit.monin@....fr, 
 willemb@...gle.com, 
 Jakub Ramaseuski <jramaseu@...hat.com>, 
 Tianhao Zhao <tizhao@...hat.com>, 
 Michal Schmidt <mschmidt@...hat.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Subject: Re: [PATCH net v3] net: gso: Forbid IPv6 TSO with extensions on
 devices with only IPV6_CSUM

Jakub Ramaseuski wrote:
> When performing Generic Segmentation Offload (GSO) on an IPv6 packet that
> contains extension headers, the kernel incorrectly requests checksum offload
> if the egress device only advertises NETIF_F_IPV6_CSUM feature, which has 
> a strict contract: it supports checksum offload only for plain TCP or UDP 
> over IPv6 and explicitly does not support packets with extension headers.
> The current GSO logic violates this contract by failing to disable the feature
> for packets with extension headers, such as those used in GREoIPv6 tunnels.
> 
> This violation results in the device being asked to perform an operation
> it cannot support, leading to a `skb_warn_bad_offload` warning and a collapse
> of network throughput. While device TSO/USO is correctly bypassed in favor
> of software GSO for these packets, the GSO stack must be explicitly told not 
> to request checksum offload.
> 
> Mask NETIF_F_IPV6_CSUM, NETIF_F_TSO6 and NETIF_F_GSO_UDP_L4
> in gso_features_check if the IPv6 header contains extension headers to compute
> checksum in software.
> 
> The exception is a BIG TCP extension, which, as stated in commit
> 68e068cabd2c6c53 ("net: reenable NETIF_F_IPV6_CSUM offload for BIG TCP packets"):
> "The feature is only enabled on devices that support BIG TCP TSO.
> The header is only present for PF_PACKET taps like tcpdump,
> and not transmitted by physical devices."
> 
> kernel log output (truncated):
> WARNING: CPU: 1 PID: 5273 at net/core/dev.c:3535 skb_warn_bad_offload+0x81/0x140
> ...
> Call Trace:
>  <TASK>
>  skb_checksum_help+0x12a/0x1f0
>  validate_xmit_skb+0x1a3/0x2d0
>  validate_xmit_skb_list+0x4f/0x80
>  sch_direct_xmit+0x1a2/0x380
>  __dev_xmit_skb+0x242/0x670
>  __dev_queue_xmit+0x3fc/0x7f0
>  ip6_finish_output2+0x25e/0x5d0
>  ip6_finish_output+0x1fc/0x3f0
>  ip6_tnl_xmit+0x608/0xc00 [ip6_tunnel]
>  ip6gre_tunnel_xmit+0x1c0/0x390 [ip6_gre]
>  dev_hard_start_xmit+0x63/0x1c0
>  __dev_queue_xmit+0x6d0/0x7f0
>  ip6_finish_output2+0x214/0x5d0
>  ip6_finish_output+0x1fc/0x3f0
>  ip6_xmit+0x2ca/0x6f0
>  ip6_finish_output+0x1fc/0x3f0
>  ip6_xmit+0x2ca/0x6f0
>  inet6_csk_xmit+0xeb/0x150
>  __tcp_transmit_skb+0x555/0xa80
>  tcp_write_xmit+0x32a/0xe90
>  tcp_sendmsg_locked+0x437/0x1110
>  tcp_sendmsg+0x2f/0x50
> ...
> skb linear:   00000000: e4 3d 1a 7d ec 30 e4 3d 1a 7e 5d 90 86 dd 60 0e
> skb linear:   00000010: 00 0a 1b 34 3c 40 20 11 00 00 00 00 00 00 00 00
> skb linear:   00000020: 00 00 00 00 00 12 20 11 00 00 00 00 00 00 00 00
> skb linear:   00000030: 00 00 00 00 00 11 2f 00 04 01 04 01 01 00 00 00
> skb linear:   00000040: 86 dd 60 0e 00 0a 1b 00 06 40 20 23 00 00 00 00
> skb linear:   00000050: 00 00 00 00 00 00 00 00 00 12 20 23 00 00 00 00
> skb linear:   00000060: 00 00 00 00 00 00 00 00 00 11 bf 96 14 51 13 f9
> skb linear:   00000070: ae 27 a0 a8 2b e3 80 18 00 40 5b 6f 00 00 01 01
> skb linear:   00000080: 08 0a 42 d4 50 d5 4b 70 f8 1a
> 
> Fixes: 04c20a9356f283da ("net: skip offload for NETIF_F_IPV6_CSUM if ipv6 header contains extension")
> Reported-by: Tianhao Zhao <tizhao@...hat.com>
> Suggested-by: Michal Schmidt <mschmidt@...hat.com>
> Suggested-by: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> Signed-off-by: Jakub Ramaseuski <jramaseu@...hat.com>

Reviewed-by: Willem de Bruijn <willemb@...gle.com>

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