[PATCH net v2] ip6_gre: remove obsolete ERSPAN PMTU update

A tc tunnel_key action can attach a METADATA_IP_TUNNEL dst to an skb,
and mirred can redirect it to a native ip6erspan device. The legacy PMTU
update in ip6erspan_tunnel_xmit() treats that metadata dst as a route.
Since it has no output device, dst_dev(dst)->mtu dereferences NULL.

On v7.2 with KASAN, an initial UID/GID 65534 process with CapEff 0 used
self-created user and network namespaces to trigger:

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

The update is obsolete. Commit fe1a4ca0a2b7 ("ip6_gre: process toobig
in a better way") changed IPv6 GRE Too Big handling to update the
underlay route and removed the equivalent block from __gre6_xmit().
ip6_tnl_xmit() then propagates the underlay PMTU to the overlay route.

Remove the stale ERSPAN copy and its now-unused dst variable. This
eliminates the metadata-dst dereference and keeps ERSPAN aligned with
IPv6 GRE PMTU handling.

Fixes: 5a963eb61b7c ("ip6_gre: Add ERSPAN native tunnel support")
Cc: stable@vger.kernel.org
Suggested-by: Ido Schimmel <idosch@nvidia.com>
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <redacted>
---
v2:
  - Remove the complete obsolete PMTU block and unused dst variable, per
    Ido Schimmel.
  - Drop Eric Dumazet's Reviewed-by because the hunk changed.
  - Use the generic Assisted-by label.
v1: https://lore.kernel.org/netdev/20260930075320.760328-1-4ncienth@gmail.com/ (local)

No new build or VM run was performed for v2. The retained v1 runtime
tested the same metadata-dst trigger while skipping this block; v2 deletes
the block as requested in review.

The reproducer is available privately on request and is omitted from this
public AI-assisted report.
---
 net/ipv6/ip6_gre.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index e61cb10b50dc9..04f7c70b7320e 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c

@@ -923,7 +923,6 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 {
 	struct ip_tunnel_info *tun_info = NULL;
 	struct ip6_tnl *t = netdev_priv(dev);
-	struct dst_entry *dst = skb_dst(skb);
 	IP_TUNNEL_DECLARE_FLAGS(flags) = { };
 	bool truncate = false;
 	int encap_limit = -1;

@@ -1058,12 +1057,6 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 	gre_build_header(skb, 8, flags, proto, 0,
 			 htonl(atomic_fetch_inc(&t->o_seqno)));
 
-	/* TooBig packet may have updated dst->dev's mtu */
-	if (!t->parms.collect_md && dst) {
-		mtu = READ_ONCE(dst_dev(dst)->mtu);
-		if (dst_mtu(dst) > mtu)
-			dst->ops->update_pmtu(dst, NULL, skb, mtu, false);
-	}
 	err = ip6_tnl_xmit(skb, dev, dsfield, &fl6, encap_limit, &mtu,
 			   NEXTHDR_GRE);
 	if (err != 0) {

-- 
2.55.0