Re: [PATCH net v2 1/2] net: Fix checksum update for ILA adj-transport

[Paolo Abeni <pabeni@redhat.com>]

On 5/27/25 11:48 AM, Paul Chaignon wrote:
> During ILA address translations, the L4 checksums can be handled in
> different ways. One of them, adj-transport, consist in parsing the
> transport layer and updating any found checksum. This logic relies on
> inet_proto_csum_replace_by_diff and produces an incorrect skb->csum when
> in state CHECKSUM_COMPLETE.
> 
> This bug can be reproduced with a simple ILA to SIR mapping, assuming
> packets are received with CHECKSUM_COMPLETE:
> 
>   $ ip a show dev eth0
>   14: eth0@if15: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
>       link/ether 62:ae:35:9e:0f:8d brd ff:ff:ff:ff:ff:ff link-netnsid 0
>       inet6 3333:0:0:1::c078/64 scope global
>          valid_lft forever preferred_lft forever
>       inet6 fd00:10:244:1::c078/128 scope global nodad
>          valid_lft forever preferred_lft forever
>       inet6 fe80::60ae:35ff:fe9e:f8d/64 scope link proto kernel_ll
>          valid_lft forever preferred_lft forever
>   $ ip ila add loc_match fd00:10:244:1 loc 3333:0:0:1 \
>       csum-mode adj-transport ident-type luid dev eth0
> 
> Then I hit [fd00:10:244:1::c078]:8000 with a server listening only on
> [3333:0:0:1::c078]:8000. With the bug, the SYN packet is dropped with
> SKB_DROP_REASON_TCP_CSUM after inet_proto_csum_replace_by_diff changed
> skb->csum. The translation and drop are visible on pwru [1] traces:
> 
>   IFACE   TUPLE                                                        FUNC
>   eth0:9  [fd00:10:244:3::3d8]:51420->[fd00:10:244:1::c078]:8000(tcp)  ipv6_rcv
>   eth0:9  [fd00:10:244:3::3d8]:51420->[fd00:10:244:1::c078]:8000(tcp)  ip6_rcv_core
>   eth0:9  [fd00:10:244:3::3d8]:51420->[fd00:10:244:1::c078]:8000(tcp)  nf_hook_slow
>   eth0:9  [fd00:10:244:3::3d8]:51420->[fd00:10:244:1::c078]:8000(tcp)  inet_proto_csum_replace_by_diff
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     tcp_v6_early_demux
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     ip6_route_input
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     ip6_input
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     ip6_input_finish
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     ip6_protocol_deliver_rcu
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     raw6_local_deliver
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     ipv6_raw_deliver
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     tcp_v6_rcv
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     __skb_checksum_complete
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     kfree_skb_reason(SKB_DROP_REASON_TCP_CSUM)
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     skb_release_head_state
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     skb_release_data
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     skb_free_head
>   eth0:9  [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp)     kfree_skbmem
> 
> This is happening because inet_proto_csum_replace_by_diff is updating
> skb->csum when it shouldn't. The L4 checksum is updated such that it
> "cancels" the IPv6 address change in terms of checksum computation, so
> the impact on skb->csum is null.
> 
> Note this would be different for an IPv4 packet since three fields
> would be updated: the IPv4 address, the IP checksum, and the L4
> checksum. Two would cancel each other and skb->csum would still need
> to be updated to take the L4 checksum change into account.
> 
> This patch fixes it by passing an ipv6 flag to
> inet_proto_csum_replace_by_diff, to skip the skb->csum update if we're
> in the IPv6 case. Note the behavior of the only other user of
> inet_proto_csum_replace_by_diff, the BPF subsystem, is left as is in
> this patch and fixed in the subsequent patch.
> 
> With the fix, using the reproduction from above, I can confirm
> skb->csum is not touched by inet_proto_csum_replace_by_diff and the TCP
> SYN proceeds to the application after the ILA translation.
> 
> 1 - https://github.com/cilium/pwru
> Fixes: 65d7ab8de582 ("net: Identifier Locator Addressing module")
> Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>

Patch 2 does not apply cleanly anymore, please rebase. While at it,
please also replace:

1 - https://github.com/cilium/pwru

with a more customary tag:

Link: https://github.com/cilium/pwru [1]

Thanks,

Paolo