FAILED: patch "[PATCH] ip6_gre: remove obsolete ERSPAN PMTU update" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 8bc60db48e8cba35488bbd6e1f94aa990afa275a
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026101243-shrivel-chrome-e622@gregkh' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 8bc60db48e8cba35488bbd6e1f94aa990afa275a Mon Sep 17 00:00:00 2001
From: Daehyeon Ko <[email protected]>
Date: Fri, 2 Oct 2026 12:30:58 +0900
Subject: [PATCH] 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The update is obsolete.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.

index e61cb10b50dc..04f7c70b7320 100644
@@ -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) {

From: Daehyeon Ko <[email protected]>

[ 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.

---
 net/ipv6/ip6_gre.c | 5 -----
 1 file changed, 5 deletions(-)

index 06a1657755382..5e8c7071c638d 100644
@@ -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