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Date: Wed, 1 Jul 2026 15:25:52 +0200
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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Norbert Szetei <norbert@doyensec.com>
Cc: netdev@vger.kernel.org, Andrew Lunn <andrew+netdev@lunn.ch>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Qingfang Deng <qingfang.deng@linux.dev>,
	Taegu Ha <hataegu0826@gmail.com>,
	Yue Haibing <yuehaibing@huawei.com>, Kees Cook <kees@kernel.org>,
	linux-ppp@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH net] ppp: defer channel free to an RCU grace period to
 fix pppol2tp RX UAF
Message-ID: <20260701132552.nFP2AZrJ@linutronix.de>
References: <C954A7EA-AA98-4E3C-80B5-42C34B3183A3@doyensec.com>
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In-Reply-To: <C954A7EA-AA98-4E3C-80B5-42C34B3183A3@doyensec.com>

On 2026-07-01 14:14:39 [+0200], Norbert Szetei wrote:
> --- 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);

>From looking at ppp_input(), what ensures that the skb in-flight is not
added skb_queue which is purged above?

>  }

Sebastian