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

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Message-ID: <CANn89iK6w0CNzMqRJiA7QN2Ap3AFWpqWYhbB55RcHPeLq6xzyg@mail.gmail.com>
Date: Tue, 14 Oct 2025 01:06:04 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Sabrina Dubroca <sd@...asysnail.net>, "David S . Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>, 
	Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Michal Kubecek <mkubecek@...e.cz>
Subject: Re: [PATCH net] udp: drop secpath before storing an skb in a receive queue

On Tue, Oct 14, 2025 at 1:01 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Oct 14, 2025 at 12:43 AM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Tue, Oct 14, 2025 at 12:32 AM Paolo Abeni <pabeni@...hat.com> 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.

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;
        }