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

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Message-ID: <8ad598ed-90dd-c2e4-02f4-7cc87bea3e8c@redhat.com>
Date:   Fri, 8 Dec 2017 11:27:22 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     mst@...hat.com, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] tuntap: fix possible deadlock when fail to
 register netdev



On 2017年12月08日 11:11, Eric Dumazet wrote:
> 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@...il.com>
>> Cc: Eric Dumazet <eric.dumazet@...il.com>
>> Cc: Willem de Bruijn <willemb@...gle.com>
>> Signed-off-by: Jason Wang <jasowang@...hat.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 ?

No I test without it.

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

Yes, I miss the fact this the lock is used by e.g flow caches too. Will 
post V2.

Thanks