Commit 6043b794 authored May 29, 2025 by Paul Chaignon Committed by Jakub Kicinski May 30, 2025

net: Fix checksum update for ILA adj-transport

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.

Link: https://github.com/cilium/pwru

 [1]
Fixes: 65d7ab8d ("net: Identifier Locator Addressing module")
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://patch.msgid.link/b5539869e3550d46068504feb02d37653d939c0b.1748509484.git.paul.chaignon@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>

parent 44abca1f

include/net/checksum.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -152,7 +152,7 @@ void inet_proto_csum_replace16(__sum16 sum, struct sk_buff skb,
		const __be32 from, const __be32 to,
		bool pseudohdr);
		void inet_proto_csum_replace_by_diff(__sum16 sum, struct sk_buff skb,
		__wsum diff, bool pseudohdr);
		__wsum diff, bool pseudohdr, bool ipv6);

		static __always_inline
		void inet_proto_csum_replace2(__sum16 sum, struct sk_buff skb,

net/core/filter.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -1987,7 +1987,7 @@ BPF_CALL_5(bpf_l4_csum_replace, struct sk_buff *, skb, u32, offset,
		if (unlikely(from != 0))
		return -EINVAL;

		inet_proto_csum_replace_by_diff(ptr, skb, to, is_pseudo);
		inet_proto_csum_replace_by_diff(ptr, skb, to, is_pseudo, false);
		break;
		case 2:
		inet_proto_csum_replace2(ptr, skb, from, to, is_pseudo);

net/core/utils.c

+2 −2

Original line number	Diff line number	Diff line
		@@ -473,11 +473,11 @@ void inet_proto_csum_replace16(__sum16 sum, struct sk_buff skb,
		EXPORT_SYMBOL(inet_proto_csum_replace16);

		void inet_proto_csum_replace_by_diff(__sum16 sum, struct sk_buff skb,
		__wsum diff, bool pseudohdr)
		__wsum diff, bool pseudohdr, bool ipv6)
		{
		if (skb->ip_summed != CHECKSUM_PARTIAL) {
		csum_replace_by_diff(sum, diff);
		if (skb->ip_summed == CHECKSUM_COMPLETE && pseudohdr)
		if (skb->ip_summed == CHECKSUM_COMPLETE && pseudohdr && !ipv6)
		skb->csum = ~csum_sub(diff, skb->csum);
		} else if (pseudohdr) {
		sum = ~csum_fold(csum_add(diff, csum_unfold(sum)));

net/ipv6/ila/ila_common.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -86,7 +86,7 @@ static void ila_csum_adjust_transport(struct sk_buff *skb,

		diff = get_csum_diff(ip6h, p);
		inet_proto_csum_replace_by_diff(&th->check, skb,
		diff, true);
		diff, true, true);
		}
		break;
		case NEXTHDR_UDP:
		@@ -97,7 +97,7 @@ static void ila_csum_adjust_transport(struct sk_buff *skb,
		if (uh->check \|\| skb->ip_summed == CHECKSUM_PARTIAL) {
		diff = get_csum_diff(ip6h, p);
		inet_proto_csum_replace_by_diff(&uh->check, skb,
		diff, true);
		diff, true, true);
		if (!uh->check)
		uh->check = CSUM_MANGLED_0;
		}
		@@ -111,7 +111,7 @@ static void ila_csum_adjust_transport(struct sk_buff *skb,

		diff = get_csum_diff(ip6h, p);
		inet_proto_csum_replace_by_diff(&ih->icmp6_cksum, skb,
		diff, true);
		diff, true, true);
		}
		break;
		}