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

[Eric Dumazet <eric.dumazet@gmail.com>]

On Fri, 2017-12-08 at 10:54 +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@gmail.com>
> Cc: Eric Dumazet <eric.dumazet@gmail.com>
> Cc: Willem de Bruijn <willemb@google.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  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..f7ccd79 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(&tun->lock);
> +	old = rcu_dereference_protected(tun->steering_prog,
> +					lock_is_held(&tun->lock));
>  	rcu_assign_pointer(tun->steering_prog, new);
> +	spin_unlock(&tun->lock);
> 

Hi Jason, thank you for the following up.

Have you tested this code path with lockdep enabled ?

My gut feeling is that you need spin_lock_bh() here.

Thanks