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 08:37:22 +0200

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").

> Many thanks to Michal and Sabrina.
> 
> Fixes: 6471658dc66c ("udp: use skb_attempt_defer_free()")
> Reported-and-bisected-by: Michal Kubecek <[email-masked]>
> Closes: https://lore.kernel.org/netdev/gpjh4lrotyephiqpuldtxxizrsg6job7cvhiqrw72saz2ubs3h@g6fgbvexgl3r/
> Signed-off-by: Eric Dumazet <[email-masked]>
> Cc: Sabrina Dubroca <[email-masked]>
> ---
>  net/ipv4/udp.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 95241093b7f0..3f05ee70029c 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1709,6 +1709,8 @@ int __udp_enqueue_schedule_skb(struct sock *sk, struct sk_buff *skb)
>  	int dropcount;
>  	int nb = 0;
>  
> +	secpath_reset(skb);

See also the comment for udp_try_make_stateless:

/* all head states (dst, sk, nf conntrack) except skb extensions are
 * cleared by udp_rcv().
 *
 * We need to preserve secpath, if present, to eventually process
 * IP_CMSG_PASSSEC at recvmsg() time.
 *
 * Other extensions can be cleared.
 */

It looks like this patch would re-introduce the problem fixed by
dce4551cb2ad ("udp: preserve head state for IP_CMSG_PASSSEC").

> +
>  	rmem = atomic_read(&sk->sk_rmem_alloc);
>  	rcvbuf = READ_ONCE(sk->sk_rcvbuf);
>  	size = skb->truesize;
> -- 
> 2.51.0.788.g6d19910ace-goog
> 

-- 
Sabrina