Re: [PATCH net v3] seg6: Fix unset mac header in SRv6 End.DT VRF decapsulation

[Hangbin Liu <hangbin.liu@linux.dev>]

On Wed, Oct 21, 2026 at 10:47:38AM +0800, Boyan Liu wrote:
> SRv6 End.DT4/End.DT6 VRF decapsulation unsets the mac header in
> end_dt_vrf_rcv() so that the VRF device can rebuild it for packet
> sniffers. The VRF rx handler only performs the rebuild when a
> sniffer is actually attached to the VRF device (ptype_all
> non-empty); otherwise the packet continues up the stack with
> mac_header == 0xffff.
> 
> Every other receive path delivers packets with a valid mac header:
> NIC drivers set it in eth_type_trans(), and tunnels without an L2
> header (ipip, sit, gre, tun) reset it to the network header after
> decapsulation. skb_mac_header() even warns on the unset state with
> DEBUG_NET_WARN_ON_ONCE().
> 
> If the decapsulated packet is an unexpected UDP GSO packet,
> udp_queue_rcv_skb() re-segments it after restoring skb->data to the
> mac header with __skb_push(skb, -skb_mac_offset(skb)). With the mac
> header unset, skb_mac_offset() returns a large positive value,
> which the unsigned __skb_push() argument turns into a huge length:
> skb->data moves far outside the buffer and skb->len is corrupted,
> so segmentation reads out of bounds:
> 
>     BUG: KASAN: slab-out-of-bounds in inet_gso_segment (net/ipv4/af_inet.c:1383)
>     Read of size 1 at addr ffff88800da7ffff by task exploit/149
>     Call Trace:
>      inet_gso_segment (net/ipv4/af_inet.c:1383)
>      skb_mac_gso_segment (net/core/gso.c:53)
>      __skb_gso_segment (net/core/gso.c:124)
>      udp_queue_rcv_skb (net/ipv4/udp.c:2476 net/ipv4/udp.c:2466)
>      udp_unicast_rcv_skb (net/ipv4/udp.c:2620)
>      ip_local_deliver (net/ipv4/ip_input.c:262)
>      seg6_local_input_core (net/ipv6/seg6_local.c:1640)
>      lwtunnel_input (net/core/lwtunnel.c:465)
>      ipv6_rcv (net/ipv6/ip6_input.c:351)
> 
> Restore a valid mac header once the VRF rx handler returns: point
> it at the network header, and reset mac_len, which still holds the
> ingress frame's L2 header length. The sniffer rebuild happens
> inside the handler while the header is still unset, so tap behavior
> is unchanged; when no rebuild happened, the packet now carries the
> same mac header state as any packet received on an L3 device.
> 
> Fixes: 664d6f86868b ("seg6: add support for the SRv6 End.DT4 behavior")
> Cc: stable@vger.kernel.org
> Reported-by: co+98d7de09cd94fec5@bugs.sh
> Signed-off-by: Boyan Liu <yymhvert@gmail.com>
> ---
> v3:
> - Also reset skb->mac_len with skb_reset_mac_len(), as suggested by
>   Andrea Mayer: it still held the ingress frame's L2 header length.
> v2: https://lore.kernel.org/netdev/20261017120144.3059-1-yymhvert@gmail.com/
> - Move the fix from the UDP GSO path to seg6local.
> - Restore the mac header after the VRF rx handler returns, preserving
>   the VRF sniffer behavior.
> - Verify the original reproducer, datagram delivery, and VRF sniffer
>   output under QEMU.
> v1: https://lore.kernel.org/netdev/20261010073015.7665-1-yymhvert@gmail.com/
> 
>  net/ipv6/seg6_local.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
> index 7b5212220185..4ef9d32907e5 100644
> --- a/net/ipv6/seg6_local.c
> +++ b/net/ipv6/seg6_local.c
> @@ -1113,6 +1113,16 @@ static struct sk_buff *end_dt_vrf_rcv(struct sk_buff *skb, u16 family,
>  		/* the skb buffer was consumed by the handler */
>  		return NULL;
>  
> +	/* the VRF device had its chance to rebuild the mac header for
> +	 * sniffers; restore a valid one pointing at the network header,
> +	 * like any L3 device without an L2 header, before the packet
> +	 * continues up the stack.
> +	 */
> +	if (!skb_mac_header_was_set(skb)) {
> +		skb_reset_mac_header(skb);
> +		skb_reset_mac_len(skb);
> +	}
> +
>  	/* when a packet is received by a VRF or by one of its slaves, the
>  	 * master device reference is set into the skb.
>  	 */
> -- 
> 2.43.0
> 

Thanks for the fix.

Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>