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: 65d7ab8de582 ("net: Identifier Locator Addressing module")
Signed-off-by: Paul Chaignon <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Link: https://patch.msgid.link/b5539[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/checksum.h

···
 			       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

···
 		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

···
 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

···
 
 			diff = get_csum_diff(ip6h, p);
 			inet_proto_csum_replace_by_diff(&th->check, skb,
-							diff, true);
+							diff, true, true);
 		}
 		break;
 	case NEXTHDR_UDP:
···
 			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;
 			}
···
 
 			diff = get_csum_diff(ip6h, p);
 			inet_proto_csum_replace_by_diff(&ih->icmp6_cksum, skb,
-							diff, true);
+							diff, true, true);
 		}
 		break;
 	}