[PATCH 6.6.y] ip6_gre: remove obsolete ERSPAN PMTU update

[Sasha Levin <sashal@kernel.org>]

From: Daehyeon Ko <4ncienth@gmail.com>

[ Upstream commit 8bc60db48e8cba35488bbd6e1f94aa990afa275a ]

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The update is obsolete.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>
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20261002033058.358137-1-4ncienth@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ sashal: Adapted the removal to the shorter PMTU block using dst->dev->mtu. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/ipv6/ip6_gre.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 06a1657755382..5e8c7071c638d 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -942,7 +942,6 @@ 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);
 	bool truncate = false;
 	int encap_limit = -1;
 	__u8 dsfield = false;
@@ -1074,10 +1073,6 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
 	/* Push GRE header. */
 	gre_build_header(skb, 8, TUNNEL_SEQ, proto, 0, htonl(atomic_fetch_inc(&t->o_seqno)));
 
-	/* TooBig packet may have updated dst->dev's mtu */
-	if (!t->parms.collect_md && dst && dst_mtu(dst) > dst->dev->mtu)
-		dst->ops->update_pmtu(dst, NULL, skb, dst->dev->mtu, false);
-
 	err = ip6_tnl_xmit(skb, dev, dsfield, &fl6, encap_limit, &mtu,
 			   NEXTHDR_GRE);
 	if (err != 0) {
-- 
2.53.0