[PATCH net] net: correctly handle tunneled traffic on IPV6_CSUM GSO fallback

[PATCH net] net: correctly handle tunneled traffic on IPV6_CSUM GSO fallback

[Willem de Bruijn]

From: Willem de Bruijn <willemb@google.com>

NETIF_F_IPV6_CSUM only advertises support for checksum offload of
packets without IPv6 extension headers. Packets with extension
headers must fall back onto software checksumming. Since TSO
depends on checksum offload, those must revert to GSO.

The below commit introduces that fallback. It always checks
network header length. For tunneled packets, the inner header length
must be checked instead. Extend the check accordingly.

A special case is tunneled packets without inner IP protocol. Such as
RFC 6951 SCTP in UDP. Those are not standard IPv6 followed by
transport header either, so also must revert to the software GSO path.

Cc: stable@vger.kernel.org
Fixes: 864e3396976e ("net: gso: Forbid IPv6 TSO with extensions on devices with only IPV6_CSUM")
Reported-by: Tangxin Xie <xietangxin@yeah.net>
Closes: https://lore.kernel.org/netdev/0414e7e2-9a1c-4d7c-a99d-b9039cf68f40@yeah.net/
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
---
 net/core/dev.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 14a83f2035b9..fc5557062414 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3769,6 +3769,22 @@ static netdev_features_t dflt_features_check(struct sk_buff *skb,
 	return vlan_features_check(skb, features);
 }
 
+static bool skb_gso_has_extension_hdr(const struct sk_buff *skb)
+{
+	if (!skb->encapsulation)
+		return ((skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 ||
+			 (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
+			  vlan_get_protocol(skb) == htons(ETH_P_IPV6))) &&
+			skb_transport_header_was_set(skb) &&
+			skb_network_header_len(skb) != sizeof(struct ipv6hdr));
+	else
+		return (!skb_inner_network_header_was_set(skb) ||
+			((skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 ||
+			  (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
+			   inner_ip_hdr(skb)->version == 6)) &&
+			 skb_inner_network_header_len(skb) != sizeof(struct ipv6hdr)));
+}
+
 static netdev_features_t gso_features_check(const struct sk_buff *skb,
 					    struct net_device *dev,
 					    netdev_features_t features)
@@ -3816,11 +3832,7 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb,
 	 * so neither does TSO that depends on it.
 	 */
 	if (features & NETIF_F_IPV6_CSUM &&
-	    (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6 ||
-	     (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4 &&
-	      vlan_get_protocol(skb) == htons(ETH_P_IPV6))) &&
-	    skb_transport_header_was_set(skb) &&
-	    skb_network_header_len(skb) != sizeof(struct ipv6hdr))
+	    skb_gso_has_extension_hdr(skb))
 		features &= ~(NETIF_F_IPV6_CSUM | NETIF_F_TSO6 | NETIF_F_GSO_UDP_L4);
 
 	return features;
-- 
2.53.0.959.g497ff81fa9-goog

Re: [PATCH net] net: correctly handle tunneled traffic on IPV6_CSUM GSO fallback

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Paolo Abeni <pabeni@redhat.com>:

On Fri, 20 Mar 2026 15:01:46 -0400 you wrote:
> From: Willem de Bruijn <willemb@google.com>
> 
> NETIF_F_IPV6_CSUM only advertises support for checksum offload of
> packets without IPv6 extension headers. Packets with extension
> headers must fall back onto software checksumming. Since TSO
> depends on checksum offload, those must revert to GSO.
> 
> [...]

Here is the summary with links:
  - [net] net: correctly handle tunneled traffic on IPV6_CSUM GSO fallback
    https://git.kernel.org/netdev/net/c/c4336a07eb6b

You are awesome, thank you!
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