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

[Ido Schimmel]

On Fri, Oct 02, 2026 at 12:30:58PM +0900, Daehyeon Ko wrote:
> 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 The update is obsolete.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@xxxxxxxxxxxxxxx
> Suggested-by: Ido Schimmel <idosch@xxxxxxxxxx>
> Assisted-by: LLM
> Signed-off-by: Daehyeon Ko <4ncienth@xxxxxxxxx>

Reviewed-by: Ido Schimmel <idosch@xxxxxxxxxx>