[PATCH net] ip6_gre: validate ERSPAN skb dst before PMTU update

[Daehyeon Ko <4ncienth@gmail.com>]

A tc tunnel_key action can attach a METADATA_IP_TUNNEL dst to an skb,
and mirred preserves it when redirecting the skb to a native ip6erspan
device. ip6erspan_tunnel_xmit() treats any non-NULL dst as a route for
PMTU updates. A metadata dst has no output device, so
dst_dev(dst)->mtu dereferences NULL.

On v7.2 with KASAN, an initial UID/GID 65534 process with CapEff 0
created user and network namespaces, used namespace-local CAP_NET_ADMIN,
and triggered:

  KASAN: null-ptr-deref
  RIP: 0010:ip6erspan_tunnel_xmit+0x10fc/0x2cc0
  Kernel panic - not syncing: Fatal exception in interrupt

The collect-metadata path already skips this block because its metadata
describes the outer tunnel. Native mode must still propagate PMTU updates
for real route destinations. Use skb_valid_dst(), matching IPv4 tunnel
PMTU handling, to exclude DST_METADATA without suppressing valid routes.

With this change, the same trigger processed three packets without a
sanitizer report, oops, or panic.

Fixes: 5a963eb61b7c ("ip6_gre: Add ERSPAN native tunnel support")
Cc: stable@vger.kernel.org
Assisted-by: Codex:GPT-5
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
Affected since v4.16-rc1; present in v7.2 and all four pinned
2026-09-30 upstream snapshots.

Tested with CONFIG_USER_NS=y, CONFIG_NET_NS=y, CONFIG_IPV6_GRE=y,
CONFIG_NET_CLS_ACT=y, CONFIG_NET_ACT_TUNNEL_KEY=y, and
CONFIG_NET_ACT_MIRRED=y. Config SHA-256:
f992c9fdb881ca95c467896791c52554719f6204cf06268cd92ac37c14c14f83

The reproducer is available privately on request and is omitted from this
public AI-assisted report.
---
 net/ipv6/ip6_gre.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index e61cb10b50dc..3ee7fad6089a 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -1059,7 +1059,7 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 			 htonl(atomic_fetch_inc(&t->o_seqno)));
 
 	/* TooBig packet may have updated dst->dev's mtu */
-	if (!t->parms.collect_md && dst) {
+	if (!t->parms.collect_md && skb_valid_dst(skb)) {
 		mtu = READ_ONCE(dst_dev(dst)->mtu);
 		if (dst_mtu(dst) > mtu)
 			dst->ops->update_pmtu(dst, NULL, skb, mtu, false);

base-commit: 54518e0e827f4ca9229ae657022c60bf60f5c1bf
-- 
2.55.0