Re: [PATCH net] udp: clear skb->dev before running a sockmap verdict

[Alexander Lobakin]

From: Sechang Lim <rhkrqnwk98@xxxxxxxxx>
Date: Wed,  3 Jun 2026 12:26:37 +0000

> On the UDP receive path skb->dev is repurposed as dev_scratch (the
> truesize/state cache set by udp_set_dev_scratch()), through the
> union { struct net_device *dev; unsigned long dev_scratch; } in sk_buff.

[...]

> Fixes: 965b57b469a5 ("net: Introduce a new proto_ops ->read_skb()")

I think you need to add

Cc: stable@xxxxxxxxxxxxxxx

here to mark it as a candidate for backporting.

> Signed-off-by: Sechang Lim <rhkrqnwk98@xxxxxxxxx>
> ---
>  net/ipv4/udp.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 0ac2bf4f8759..f5ba36e721cb 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -2011,6 +2011,11 @@ int udp_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
>  	}
>  
>  	WARN_ON_ONCE(!skb_set_owner_sk_safe(skb, sk));
> +	/* skb->dev still aliases the UDP rx dev_scratch (its charge was freed

Pls favor generic block comment style

	/*
	 * skb->dev ...

> +	 * on dequeue above); a sockmap verdict program may deref it via
> +	 * bpf_sk_lookup_*(), so clear it -> bpf_skc_lookup() uses skb->sk
> +	 */
> +	skb->dev = NULL;
>  	return recv_actor(sk, skb);

I'd put newlines before and after the added piece for readability.

	WARN_ON_ONCE(...);

	/*
	 * skb->dev ...
	 * ...
	 */
	skb->dev = NULL;

	return recv_actor(sk, skb);

>  }
Thanks,
Olek