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 10:28:25 +0200

2025-10-14, 01:06:04 -0700, Eric Dumazet wrote:
> On Tue, Oct 14, 2025 at 1:01 AM Eric Dumazet <[email-masked]> wrote:
> >
> > On Tue, Oct 14, 2025 at 12:43 AM Eric Dumazet <[email-masked]> wrote:
> > >
> > > On Tue, Oct 14, 2025 at 12:32 AM Paolo Abeni <[email-masked]> 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.
> > > >
> > > > 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;
> >
> > We probably also want to clear CONNTRACK state as well.
> 
> Perhaps not use skb_release_head_state() ?
> 
> We know there is no dst, and no destructor.

Then, do we need to do anything before calling skb_attempt_defer_free()?
skb_attempt_defer_free() only wants no dst and no destructor, and the
secpath issue that we dealt with in TCP is not a problem anymore.

Can we just drop the udp_skb_has_head_state() special handling and
simply call skb_attempt_defer_free()?

> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 95241093b7f0..98628486c4c5 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)))
> -                       skb_release_head_state(skb);
> +               if (unlikely(udp_skb_has_head_state(skb))) {
> +                       nf_reset_ct(skb);
> +                       skb_ext_reset(skb);
> +               }
>                 skb_attempt_defer_free(skb);
>                 return;
>         }

-- 
Sabrina