Subject: Re: [PATCH net] udp: drop secpath before storing an skb in a receive queue

From: Sabrina Dubroca <[email-masked]>
Date: Tue, 14 Oct 2025 09:45:08 +0200

2025-10-14, 09:32:09 +0200, Paolo Abeni wrote:
> 
> 
> On 10/14/25 8:37 AM, Sabrina Dubroca wrote:
> > 2025-10-14, 06:04:54 +0000, Eric Dumazet wrote:
> >> Michal reported and bisected an issue after recent adoption
> >> of skb_attempt_defer_free() in UDP.
> >>
> >> We had the same issue for TCP, that Sabrina fixed in commit 9b6412e6979f
> >> ("tcp: drop secpath at the same time as we currently drop dst")
> > 
> > I'm not convinced this is the same bug. The TCP one was a "leaked"
> > reference (delayed put). This looks more like a double put/missing
> > hold to me (we get to the destroy path without having done the proper
> > delete, which would set XFRM_STATE_DEAD).
> > 
> > And this shouldn't be an issue after b441cf3f8c4b ("xfrm: delete
> > x->tunnel as we delete x").
> 
> I think Sabrina is right. If the skb carries a secpath,
> UDP_SKB_IS_STATELESS is not set, and skb_release_head_state() will be
> called by skb_consume_udp().
> 
> skb_ext_put() does not clear skb->extensions nor ext->refcnt, if
> skb_attempt_defer_free() enters the slow path (kfree_skb_napi_cache()),
> the skb will go through again skb_release_head_state(), with a double free.

Thanks Paolo, I was going down the same path, you got there faster :)

> I think something alike the following (completely untested) should work:
> ---
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 95241093b7f0..4a308fd6aa6c 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1851,8 +1851,10 @@ void skb_consume_udp(struct sock *sk, struct
> sk_buff *skb, int len)
>  		sk_peek_offset_bwd(sk, len);
> 
>  	if (!skb_shared(skb)) {
> -		if (unlikely(udp_skb_has_head_state(skb)))
> +		if (unlikely(udp_skb_has_head_state(skb))) {
>  			skb_release_head_state(skb);
> +			skb->active_extensions = 0;
> +		}
>  		skb_attempt_defer_free(skb);
>  		return;
>  	}

-- 
Sabrina