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

[Breno Leitao <leitao@debian.org>]

On Wed, Jul 01, 2026 at 02:14:39PM +0200, Norbert Szetei wrote:
> diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
> index 57c68efa5ff8..cb8fe37170d3 100644
> --- a/drivers/net/ppp/ppp_generic.c
> +++ b/drivers/net/ppp/ppp_generic.c
> @@ -184,6 +184,7 @@ struct channel {
>  	struct list_head clist;		/* link in list of channels per unit */
>  	spinlock_t	upl;		/* protects `ppp' and 'bridge' */
>  	struct channel __rcu *bridge;	/* "bridged" ppp channel */
> +	struct rcu_head	rcu;		/* for RCU-deferred free of the channel */
>  #ifdef CONFIG_PPP_MULTILINK
>  	u8		avail;		/* flag used in multilink stuff */
>  	u8		had_frag;	/* >= 1 fragments have been sent */
> @@ -3583,7 +3584,7 @@ static void ppp_release_channel(struct channel *pch)
>  	}
>  	skb_queue_purge(&pch->file.xq);
>  	skb_queue_purge(&pch->file.rq);
> -	kfree(pch);
> +	kfree_rcu(pch, rcu);

Why not use kfree_rcu_mightsleep() instead? That would eliminate the need
for the additional `struct rcu_head rcu;` field.