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

[Eric Dumazet]

On Wed, Sep 30, 2026 at 9:53 AM Daehyeon Ko <4ncienth@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxx
> Assisted-by: Codex:GPT-5
> Signed-off-by: Daehyeon Ko <4ncienth@xxxxxxxxx>

SGTM, thanks.

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>