Re: [PATCH net-next V2] tuntap: fix possible deadlock when fail to register netdev

[Michael S. Tsirkin]

On Fri, Dec 08, 2017 at 12:02:30PM +0800, Jason Wang wrote:
> Private destructor could be called when register_netdev() fail with
> rtnl lock held. This will lead deadlock in tun_free_netdev() who tries
> to hold rtnl_lock. Fixing this by switching to use spinlock to
> synchronize.
> 
> Fixes: 96f84061620c ("tun: add eBPF based queue selection method")
> Reported-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Cc: Willem de Bruijn <willemb@xxxxxxxxxx>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>

Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

> ---
>  drivers/net/tun.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 787cc35..8d85163 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -2050,8 +2050,11 @@ static int __tun_set_steering_ebpf(struct tun_struct *tun,
>  		new->prog = prog;
>  	}
>  
> -	old = rtnl_dereference(tun->steering_prog);
> +	spin_lock_bh(&tun->lock);
> +	old = rcu_dereference_protected(tun->steering_prog,
> +					lockdep_is_held(&tun->lock));
>  	rcu_assign_pointer(tun->steering_prog, new);
> +	spin_unlock_bh(&tun->lock);
>  
>  	if (old)
>  		call_rcu(&old->rcu, tun_steering_prog_free);
> @@ -2067,9 +2070,7 @@ static void tun_free_netdev(struct net_device *dev)
>  	free_percpu(tun->pcpu_stats);
>  	tun_flow_uninit(tun);
>  	security_tun_dev_free_security(tun->security);
> -	rtnl_lock();
>  	__tun_set_steering_ebpf(tun, NULL);
> -	rtnl_unlock();
>  }
>  
>  static void tun_setup(struct net_device *dev)
> -- 
> 2.7.4