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

[Benoît Monin]

Devices with NETIF_F_IP_CSUM capability can checksum TCP and UDP over
IPv4 with an IP header that may contains options; whereas devices with
NETIF_F_IPV6_CSUM capability can only checksum TCP and UDP over IPv6 if
the IP header does not contains extension.

Enforce that in skb_csum_hwoffload_help by checking the network header
length in the case where the IP header version is 6. We cannot simply
rely on the network header length since the IPv4 header can from 20 to
60 bytes whereas the IPv6 header must be 40 bytes. So we check the
version field which is common to IPv4 and IPv6 headers.

This fixes checksumming errors seen with ip6_tunnel and fou6
encapsulation, for example with GRE-in-UDP over IPv6:
* 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).

Signed-off-by: Benoît Monin <benoit.monin@xxxxxx>
---
 net/core/dev.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/core/dev.c b/net/core/dev.c
index ea5fbcd133ae..199831d86ec1 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 (ip_hdr(skb)->version == 6 &&
+		    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);