[PATCH v2 net] net: skip offload for NETIF_F_IPV6_CSUM if ipv6 header contains extension

["Benoît Monin" <benoit.monin@gmx.fr>]

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/26548921.1r3eYUQgxm@benoit.monin/T/#u
Fixes: aa3463d65e7b ("fou: Add encap ops for IPv6 tunnels")
Signed-off-by: Benoît Monin <benoit.monin@gmx.fr>
---
changelog
* v2:
    - patch against net instead of net-next
    - clarify documentation of NETIF_F_IPV6_CSUM
    - add link to thread describing the problem
    - add fixes tag
    - use vlan_get_protocol to check for IPv6
* v1:
    - https://lore.kernel.org/netdev/0dc0c2af98e96b1df20bd36aeaed4eb4e27d507e.1728056028.git.benoit.monin@gmx.fr/T/#u
---
 net/core/dev.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index ea5fbcd133ae..8453e14d301b 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3639,6 +3639,9 @@ 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):
@@ -3646,6 +3649,7 @@ int skb_csum_hwoffload_help(struct sk_buff *skb,
 		}
 	}

+sw_checksum:
 	return skb_checksum_help(skb);
 }
 EXPORT_SYMBOL(skb_csum_hwoffload_help);