Subject: Re: [PATCH net] tun, bpf: fix suspicious RCU usage in tun_{attach,detach}_filter

From: Hannes Frederic Sowa <[email-masked]>
Date: Thu, 31 Mar 2016 21:48:27 +0200

On 31.03.2016 21:36, David Miller wrote:
> From: Hannes Frederic Sowa <[email-masked]>
> Date: Thu, 31 Mar 2016 21:24:12 +0200
>
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 4b81b71171b4ce..8ab270d5ce5507 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -1166,7 +1166,8 @@ static int __sk_attach_prog(struct bpf_prog
>> *prog, struct sock *sk)
>>   	}
>>
>>   	old_fp = rcu_dereference_protected(sk->sk_filter,
>> -					   sock_owned_by_user(sk));
>> +					   lockdep_rtnl_is_held() ||
>> +					   lockdep_sock_is_held(sk));
>>   	rcu_assign_pointer(sk->sk_filter, fp);
>>
>>   	if (old_fp)
>
> I have the same objections Daniel did.
>
> Not all socket filter clients use RTNL as the synchornization
> mechanism.  The caller, or some descriptive element, should tell us
> what that synchronizing element is.
>
> Yes, I understand how these RTNL checks can pass "accidently" but
> the opposite is true too.  A socket locking synchornizing user,
> who didn't lock the socket, might now pass because RTNL happens
> to be held elsewhere.

Actually lockdep_rtnl_is_held checks if this specific code/thread holds 
the lock and no other cpu/thread. So it will not pass here in case 
another cpu has the lock.

lockdep stores the current held locks in current->held_locks, if we 
preempt we switch current pointer, if we take a spin_lock we can't sleep 
thus not preempt. Thus we always know that this specific code has the lock.

Using sock_owned_by_user actually has this problem, and thus I am 
replacing it. We don't know who has the socket locked.

Tightest solution would probably be to combine both patches.

bool called_by_tuntap;

old_fp = rcu_dereference_protected(sk->sk_filter, called_by_tuntap ? 
lockdep_rtnl_is_held() : lockdep_sock_is_held());

Bye,
Hannes