netdev - Re: [PATCH net-next] tun: fix memory leak in error path

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Date:   Tue, 8 Oct 2019 12:31:37 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jiri Pirko <jiri@...lanox.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net-next] tun: fix memory leak in error path

On Mon,  7 Oct 2019 12:21:05 -0700, Eric Dumazet wrote:
> syzbot reported a warning [1] that triggered after recent Jiri patch.
> 
> This exposes a bug that we hit already in the past (see commit
> ff244c6b29b1 ("tun: handle register_netdevice() failures properly")
> for details)
> 
> tun uses priv->destructor without an ndo_init() method.
> 
> register_netdevice() can return an error, but will
> not call priv->destructor() in some cases. Jiri recent
> patch added one more.
> 
> A long term fix would be to transfer the initialization
> of what we destroy in ->destructor() in the ndo_init()
> 
> This looks a bit risky given the complexity of tun driver.
> 
> A simpler fix is to detect after the failed register_netdevice()
> if the tun_free_netdev() function was called already.
> 
> [...]
> 
> Fixes: ff92741270bf ("net: introduce name_node struct to be used in hashlist")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Jiri Pirko <jiri@...lanox.com>
> Reported-by: syzbot <syzkaller@...glegroups.com>

Looks good, obviously. Presumably we could remove the workaround added
by commit 0ad646c81b21 ("tun: call dev_get_valid_name() before
register_netdevice()") at this point? What are your thoughts on that?

> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 812dc3a65efbb9d1ee2724e73978dbc4803ec171..1e541b08b136364302aa524e31efb62062c43faa 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -2290,7 +2290,13 @@ static void tun_free_netdev(struct net_device *dev)
>  	struct tun_struct *tun = netdev_priv(dev);
>  
>  	BUG_ON(!(list_empty(&tun->disabled)));
> +
>  	free_percpu(tun->pcpu_stats);
> +	/* We clear pcpu_stats so that tun_set_iff() can tell if
> +	 * tun_free_netdev() has been called from register_netdevice().
> +	 */
> +	tun->pcpu_stats = NULL;
> +
>  	tun_flow_uninit(tun);
>  	security_tun_dev_free_security(tun->security);
>  	__tun_set_ebpf(tun, &tun->steering_prog, NULL);
> @@ -2859,8 +2865,12 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
>  
>  err_detach:
>  	tun_detach_all(dev);
> -	/* register_netdevice() already called tun_free_netdev() */
> -	goto err_free_dev;
> +	/* We are here because register_netdevice() has failed.
> +	 * If register_netdevice() already called tun_free_netdev()
> +	 * while dealing with the error, tun->pcpu_stats has been cleared.
> +	 */
> +	if (!tun->pcpu_stats)
> +		goto err_free_dev;
>  
>  err_free_flow:
>  	tun_flow_uninit(tun);