Re: [PATCH v2] l2tp: Avoid possible recursive deadlock in l2tp_tunnel_register()

[Alexander Lobakin]

From: Shigeru Yoshida <syoshida@xxxxxxxxxx>
Date: Mon, 13 Feb 2023 01:26:23 +0900

> When a file descriptor of pppol2tp socket is passed as file descriptor
> of UDP socket, a recursive deadlock occurs in l2tp_tunnel_register().
> This situation is reproduced by the following program:

[...]

> +static struct l2tp_tunnel *pppol2tp_tunnel_get(struct net *net,
> +					       struct l2tp_connect_info *info,
> +					       bool *new_tunnel)
> +{
> +	struct l2tp_tunnel *tunnel;
> +	int error;
> +
> +	*new_tunnel = false;
> +
> +	tunnel = l2tp_tunnel_get(net, info->tunnel_id);
> +
> +	/* Special case: create tunnel context if session_id and
> +	 * peer_session_id is 0. Otherwise look up tunnel using supplied
> +	 * tunnel id.
> +	 */
> +	if (!info->session_id && !info->peer_session_id) {
> +		if (!tunnel) {

This `if` is the sole thing the outer `if` contains, could we combine them?

> +			struct l2tp_tunnel_cfg tcfg = {
> +				.encap = L2TP_ENCAPTYPE_UDP,
> +			};
> +
> +			/* Prevent l2tp_tunnel_register() from trying to set up
> +			 * a kernel socket.
> +			 */
> +			if (info->fd < 0)
> +				return ERR_PTR(-EBADF);
> +
> +			error = l2tp_tunnel_create(info->fd,
> +						   info->version,

This fits into the prev line.

> +						   info->tunnel_id,
> +						   info->peer_tunnel_id, &tcfg,
> +						   &tunnel);
> +			if (error < 0)
> +				return ERR_PTR(error);
> +
> +			l2tp_tunnel_inc_refcount(tunnel);
> +			error = l2tp_tunnel_register(tunnel, net, &tcfg);
> +			if (error < 0) {
> +				kfree(tunnel);
> +				return ERR_PTR(error);
> +			}
> +
> +			*new_tunnel = true;
> +		}
> +	} else {
> +		/* Error if we can't find the tunnel */
> +		if (!tunnel)
> +			return ERR_PTR(-ENOENT);

[...]

Thanks,
Olek