net: skip offload for NETIF_F_IPV6_CSUM if ipv6 header contains extension

As documented in skbuff.h, devices with NETIF_F_IPV6_CSUM capability
can only checksum TCP and UDP over IPv6 if the IP header does not
contains extension.

This is enforced for UDP packets emitted from user-space to an IPv6
address as they go through ip6_make_skb(), which calls
__ip6_append_data() where a check is done on the header size before
setting CHECKSUM_PARTIAL.

But the introduction of UDP encapsulation with fou6 added a code-path
where it is possible to get an skb with a partial UDP checksum and an
IPv6 header with extension:
* fou6 adds a UDP header with a partial checksum if the inner packet
does not contains a valid checksum.
* ip6_tunnel adds an IPv6 header with a destination option extension
header if encap_limit is non-zero (the default value is 4).

The thread linked below describes in more details how to reproduce the
problem with GRE-in-UDP tunnel.

Add a check on the network header size in skb_csum_hwoffload_help() to
make sure no IPv6 packet with extension header is handed to a network
device with NETIF_F_IPV6_CSUM capability.

Link: https://lore.kernel.org/netdev/[email protected]/T/#u
Fixes: aa3463d ("fou: Add encap ops for IPv6 tunnels")
Signed-off-by: Benoît Monin <[email protected]>
Reviewed-by: Willem de Bruijn <[email protected]>
Link: https://patch.msgid.link/5fbeecfc311ea182aa1d1c771725ab8b4cac515e.1729778144.git.benoit.monin@gmx.fr
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Benoît Monin

net/core/dev.c

@@ -3639,13 +3639,17 @@ int skb_csum_hwoffload_help(struct sk_buff *skb,
 		return 0;
 
 	if (features & (NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM)) {
+		if (vlan_get_protocol(skb) == htons(ETH_P_IPV6) &&
+		    skb_network_header_len(skb) != sizeof(struct ipv6hdr))
+			goto sw_checksum;
 		switch (skb->csum_offset) {
 		case offsetof(struct tcphdr, check):
 		case offsetof(struct udphdr, check):
 			return 0;
 		}
 	}
 
+sw_checksum:
 	return skb_checksum_help(skb);
 }
 EXPORT_SYMBOL(skb_csum_hwoffload_help);