Re: [PATCH net v2] ppp: defer channel free to an RCU grace period to fix pppol2tp RX UAF

[Breno Leitao <leitao@debian.org>]

On Fri, Jul 03, 2026 at 03:27:00PM +0800, Qingfang Deng wrote:
> Hi,
> 
> On 2026/7/2 2:12, Norbert Szetei wrote:
> > +/* Purge after the grace period: a late ppp_input() may still queue an
> > + * skb on pch->file.rq before the last RCU reader drains.
> > + */
> > +static void ppp_release_channel_free(struct rcu_head *rcu)
> > +{
> > +	struct channel *pch = container_of(rcu, struct channel, rcu);
> > +
> > +	skb_queue_purge(&pch->file.xq);
> > +	skb_queue_purge(&pch->file.rq);
> > +	kfree(pch);
> > +}
> > +
> >   /*
> >    * Drop a reference to a ppp channel and free its memory if the refcount reaches
> >    * zero.
> > @@ -3581,9 +3594,7 @@ static void ppp_release_channel(struct channel *pch)
> >   		pr_err("ppp: destroying undead channel %p !\n", pch);
> >   		return;
> >   	}
> > -	skb_queue_purge(&pch->file.xq);
> > -	skb_queue_purge(&pch->file.rq);
> > -	kfree(pch);
> > +	call_rcu(&pch->rcu, ppp_release_channel_free);
> >   }
> >   static void __exit ppp_cleanup(void)
> 
> AI-review found an issue: https://sashiko.dev/#/patchset/D9C0245B-608B-4884-8A09-F55BA4A9F948%40doyensec.com
> 
> An rcu_barrier() call is needed at the end of ppp_cleanup().

I was initially unclear why rcu_barrier() would be necessary on a kfree path,
but it appears to be required during module unload to ensure that
ppp_release_channel_free() completes before the module's struct rcu_head is
destroyed. Is that the correct understanding?