Subject: Re: [PATCH v2 1/1] l2tp: pppol2tp: hold reference to session in pppol2tp_ioctl()

From: Lee Jones <[email-masked]>
Date: Wed, 27 May 2026 14:46:14 +0100

On Wed, 27 May 2026, Lee Jones wrote:

> pppol2tp_ioctl() read sock->sk->sk_user_data directly without any
> locks or reference counting.  If a controllable sleep was induced during
> copy_from_user() (e.g. via a userfaultfd page fault sleep), a concurrent
> socket close could trigger pppol2tp_session_close() asynchronously.  This
> frees the l2tp_session structure via the l2tp_session_del_work workqueue.
> Upon resuming, the ioctl thread dereferences the stale session pointer,
> resulting in a Use-After-Free (UAF).
> 
> Fix this by securely fetching the session reference using the RCU-safe,
> refcounted helper pppol2tp_sock_to_session(sk) on entry.  This locks the
> session's refcount across the sleep.  We structured the function to exit
> via standard err breaks, guaranteeing that l2tp_session_put() is cleanly
> called on all return paths to drop the reference.
> 
> To preserve existing behavior we validate the session and its magic
> signature only for the specific L2TP commands that require it.  This
> ensures that generic/unknown ioctls called on an unconnected socket
> still return -ENOIOCTLCMD and correctly fall back to generic handlers
> (e.g. in sock_do_ioctl()).

Fixes: fd558d186df2 ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")

> Signed-off-by: Lee Jones <[email-masked]>
> 
> ---
> v1 => v2: Restrict session validation in pppol2tp_ioctl() to L2TP specific
>           commands to preserve generic socket ioctl fallback and perform
>           checks silently to mitigate WARN_ON() DoS risks.
> 
>  net/l2tp/l2tp_ppp.c | 82 +++++++++++++++++++++++++++------------------
>  1 file changed, 50 insertions(+), 32 deletions(-)
> 
> diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
> index 99d6582f41de..e0b1915be1a6 100644
> --- a/net/l2tp/l2tp_ppp.c
> +++ b/net/l2tp/l2tp_ppp.c
> @@ -1045,64 +1045,76 @@ static int pppol2tp_ioctl(struct socket *sock, unsigned int cmd,
>  {
>  	struct pppol2tp_ioc_stats stats;
>  	struct l2tp_session *session;
> +	int err = 0;
> +
> +	session = pppol2tp_sock_to_session(sock->sk);
>  
> +	/* Validate session presence and magic integrity ONLY for commands
> +	 * that belong to L2TP and require a valid session.
> +	 */
>  	switch (cmd) {
>  	case PPPIOCGMRU:
>  	case PPPIOCGFLAGS:
> -		session = sock->sk->sk_user_data;
> +	case PPPIOCSMRU:
> +	case PPPIOCSFLAGS:
> +	case PPPIOCGL2TPSTATS:
>  		if (!session)
>  			return -ENOTCONN;
>  
> -		if (WARN_ON(session->magic != L2TP_SESSION_MAGIC))
> +		if (session->magic != L2TP_SESSION_MAGIC) {
> +			l2tp_session_put(session);
>  			return -EBADF;
> +		}
> +		break;
> +	default:
> +		break;
> +	}
>  
> +	switch (cmd) {
> +	case PPPIOCGMRU:
> +	case PPPIOCGFLAGS:
>  		/* Not defined for tunnels */
> -		if (!session->session_id && !session->peer_session_id)
> -			return -ENOSYS;
> +		if (!session->session_id && !session->peer_session_id) {
> +			err = -ENOSYS;
> +			break;
> +		}
>  
> -		if (put_user(0, (int __user *)arg))
> -			return -EFAULT;
> +		if (put_user(0, (int __user *)arg)) {
> +			err = -EFAULT;
> +			break;
> +		}
>  		break;
>  
>  	case PPPIOCSMRU:
>  	case PPPIOCSFLAGS:
> -		session = sock->sk->sk_user_data;
> -		if (!session)
> -			return -ENOTCONN;
> -
> -		if (WARN_ON(session->magic != L2TP_SESSION_MAGIC))
> -			return -EBADF;
> -
>  		/* Not defined for tunnels */
> -		if (!session->session_id && !session->peer_session_id)
> -			return -ENOSYS;
> +		if (!session->session_id && !session->peer_session_id) {
> +			err = -ENOSYS;
> +			break;
> +		}
>  
> -		if (!access_ok((int __user *)arg, sizeof(int)))
> -			return -EFAULT;
> +		if (!access_ok((int __user *)arg, sizeof(int))) {
> +			err = -EFAULT;
> +			break;
> +		}
>  		break;
>  
>  	case PPPIOCGL2TPSTATS:
> -		session = sock->sk->sk_user_data;
> -		if (!session)
> -			return -ENOTCONN;
> -
> -		if (WARN_ON(session->magic != L2TP_SESSION_MAGIC))
> -			return -EBADF;
> -
>  		/* Session 0 represents the parent tunnel */
>  		if (!session->session_id && !session->peer_session_id) {
>  			u32 session_id;
> -			int err;
>  
>  			if (copy_from_user(&stats, (void __user *)arg,
> -					   sizeof(stats)))
> -				return -EFAULT;
> +					   sizeof(stats))) {
> +				err = -EFAULT;
> +				break;
> +			}
>  
>  			session_id = stats.session_id;
>  			err = pppol2tp_tunnel_copy_stats(&stats,
>  							 session->tunnel);
>  			if (err < 0)
> -				return err;
> +				break;
>  
>  			stats.session_id = session_id;
>  		} else {
> @@ -1112,15 +1124,21 @@ static int pppol2tp_ioctl(struct socket *sock, unsigned int cmd,
>  		stats.tunnel_id = session->tunnel->tunnel_id;
>  		stats.using_ipsec = l2tp_tunnel_uses_xfrm(session->tunnel);
>  
> -		if (copy_to_user((void __user *)arg, &stats, sizeof(stats)))
> -			return -EFAULT;
> +		if (copy_to_user((void __user *)arg, &stats, sizeof(stats))) {
> +			err = -EFAULT;
> +			break;
> +		}
>  		break;
>  
>  	default:
> -		return -ENOIOCTLCMD;
> +		err = -ENOIOCTLCMD;
> +		break;
>  	}
>  
> -	return 0;
> +	if (session)
> +		l2tp_session_put(session);
> +
> +	return err;
>  }
>  
>  /*****************************************************************************
> -- 
> 2.54.0.746.g67dd491aae-goog
> 

-- 
Lee Jones