[PATCH net v2] net: extend IPv6 exthdr detection of tunneled packets

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

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

Commit c4336a07eb6b ("net: correctly handle tunneled traffic on IPV6_CSUM
GSO fallback") split skb_gso_has_extension_hdr() into mutually exclusive
branches on skb->encapsulation. This did not yet address all paths:

1. With skb->encapsulation set, the outer header is not checked. IPv6
   tunnels such as ip6_gre and ip6_tunnel add an outer Destination
   Options header by default (encap_limit). Their GSO packets skip
   software GSO, then skb_csum_hwoffload_help() sees the outer extension
   header and calls skb_checksum_help() on the GSO skb, which warns and
   drops it.

2. Tunnels over IPv6 without an outer transport header, such as
   ip6_tunnel, leave skb->transport_header at the inner transport
   header. skb_network_header_len() then spans the outer IPv6, tunnel
   and inner IP headers, a false positive.

3. UDP tunnels without an inner network header, such as SCTP-in-UDP or
   PSP, have no inner IPv6 header to check. Decide on the outer header
   alone.

4. Directly dereferencing inner_ip_hdr(skb)->version without
   skb_header_pointer() is unsafe.

Instead, check ipv6_ext_hdr(nexthdr) on the outer IPv6 header and, if
set, on the inner IPv6 header. Read the headers with skb_header_pointer().
Keep the skb_network_header_len() check when !skb->encapsulation to also
catch encapsulated packets without skb->encapsulation (e.g., virtio).

Use the same helper in skb_csum_hwoffload_help(). Its open coded test
has the false positive of (2) and ignores the inner header.

Background: checksum offload of tunneled packets invariants:

Non-GSO skb:
- If the inner packet is CHECKSUM_PARTIAL, Local Checksum Offload computes
  the outer checksum in software and the device offloads only the inner L4
  checksum.
- If the inner packet is CHECKSUM_NONE (e.g., SCTP-in-UDP, ESP-in-UDP,
  Remote Checksum Offload), the device offloads the outer UDP or GRE
  checksum instead.

GSO skb:
- A device with NETIF_F_GSO_UDP_TUNNEL_CSUM or NETIF_F_GSO_GRE_CSUM
  computes both inner and outer checksums per segment.
  The outer checksum is seeded with only the pseudo-header checksum.

A NETIF_F_IPV6_CSUM device must parse through the outer headers to
reach the inner ones.

Fixes: c4336a07eb6b ("net: correctly handle tunneled traffic on IPV6_CSUM GSO fallback")
Cc: stable@vger.kernel.org
Signed-off-by: Willem de Bruijn <willemb@google.com>

---

  v1 -> v2
  - Keep skb_network_header_len() check when !skb->encapsulation to
    support tunnels without skb->encapsulation (e.g., virtio).

  v1: https://lore.kernel.org/netdev/20260924192128.1197118-1-willemdebruijn.kernel@gmail.com/

Stable: trees before commit 1676ebba391d ("net/ipv6: Remove jumbo_remove
step from TX path") must keep the BIG TCP jumbo exemption on the outer
check, or BIG TCP loses TSO (see 68e068cabd2c):

	if (vlan_get_protocol(skb) == htons(ETH_P_IPV6) &&
	    !ipv6_has_hopopt_jumbo(skb)) {
		if (__skb_has_ipv6_ext_hdr(skb, skb_network_offset(skb)))
			return true;

		/* Catch tunnels without skb->encapsulation (e.g., virtio). */
		if (!skb->encapsulation &&
		    skb_transport_header_was_set(skb) &&
		    skb_network_header_len(skb) != sizeof(struct ipv6hdr))
			return true;
	}

Tested with gre_gso.sh over veth with NETIF_F_IPV6_CSUM:
- GREv6 without extension headers
- GREv6 with inner dstopts
- GREv6 with outer encaplimit, and
- SCTP-in-UDPv6.

That needs a test-only feature added to veth to advertise
NETIF_F_IPV6_CSUM (mutually exclusive with NETIF_F_HW_CSUM).

If no one objects, we can follow up with those veth and selftest patches to
net-next later.
---
 net/core/dev.c | 49 ++++++++++++++++++++++++++++++++-----------------
 1 file changed, 32 insertions(+), 17 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 0292a16e16c2..91b8a57fad96 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3818,20 +3818,36 @@ 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 bool __skb_has_ipv6_ext_hdr(const struct sk_buff *skb, int nhoff)
+{
+	const struct ipv6hdr *ip6h;
+	struct ipv6hdr _ip6h;
+
+	ip6h = skb_header_pointer(skb, nhoff, sizeof(_ip6h), &_ip6h);
+	return ip6h && ip6h->version == 6 && ipv6_ext_hdr(ip6h->nexthdr);
+}
+
+static bool skb_has_ipv6_extension_hdr(const struct sk_buff *skb)
+{
+	if (vlan_get_protocol(skb) == htons(ETH_P_IPV6)) {
+		if (__skb_has_ipv6_ext_hdr(skb, skb_network_offset(skb)))
+			return true;
+
+		/* Catch tunnels without skb->encapsulation (e.g., virtio). */
+		if (!skb->encapsulation &&
+		    skb_transport_header_was_set(skb) &&
+		    skb_network_header_len(skb) != sizeof(struct ipv6hdr))
+			return true;
+	}
+
+	/* Tunnels without an inner network header, such as SCTP-in-UDP or
+	 * PSP, have no inner IP header and thus no inner extension header.
+	 */
+	if (skb->encapsulation && skb_inner_network_header_was_set(skb) &&
+	    __skb_has_ipv6_ext_hdr(skb, skb_inner_network_offset(skb)))
+		return true;
+
+	return false;
 }
 
 static netdev_features_t gso_features_check(const struct sk_buff *skb,
@@ -3886,7 +3902,7 @@ 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_gso_has_extension_hdr(skb))
+	    skb_has_ipv6_extension_hdr(skb))
 		features &= ~(NETIF_F_IPV6_CSUM | NETIF_F_TSO6 | NETIF_F_GSO_UDP_L4);
 
 	return features;
@@ -3988,8 +4004,7 @@ 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))
+		if (skb_has_ipv6_extension_hdr(skb))
 			goto sw_checksum;
 
 		switch (skb->csum_offset) {
-- 
2.56.0.rc1.315.gc6ed9934b7-goog