Re: [PATCH net v2] net: bridge: Fix locking in br_fdb_find_port()

[Nikolay Aleksandrov <nikolay@cumulusnetworks.com>]

On 08/06/18 16:35, Nikolay Aleksandrov wrote:
> On 08/06/18 16:11, Petr Machata wrote:
>> Callers of br_fdb_find() need to hold the hash lock, which
>> br_fdb_find_port() doesn't do. However, since br_fdb_find_port() is not
>> doing any actual FDB manipulation, the hash lock is not really needed at
>> all. So convert to br_fdb_find_rcu(), surrounded by rcu_read_lock() /
>> _unlock() pair.
>>
>> The device pointer copied from inside the FDB entry is then kept alive
>> by the RTNL lock, which br_fdb_find_port() asserts.
>>
>> Fixes: 4d4fd36126d6 ("net: bridge: Publish bridge accessor functions")
>> Signed-off-by: Petr Machata <petrm@mellanox.com>
>> ---
>>
>> Notes:
>>     Changes from v1 to v2:
>>     
>>     - Instead of taking hash lock, take RCU lock and call br_fdb_find_rcu().
>>
>>  net/bridge/br_fdb.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/bridge/br_fdb.c b/net/bridge/br_fdb.c
>> index b19e310..502f663 100644
>> --- a/net/bridge/br_fdb.c
>> +++ b/net/bridge/br_fdb.c
>> @@ -135,9 +135,11 @@ struct net_device *br_fdb_find_port(const struct net_device *br_dev,
>>  		return NULL;
>>  
>>  	br = netdev_priv(br_dev);
>> -	f = br_fdb_find(br, addr, vid);
>> +	rcu_read_lock();
>> +	f = br_fdb_find_rcu(br, addr, vid);
>>  	if (f && f->dst)
>>  		dev = f->dst->dev;
>> +	rcu_read_unlock();
>>  
>>  	return dev;
>>  }
>>
> 
> Important note: the only reason this will not dereference a NULL pointer
> when getting f->dst is because RTNL is held in all of its current
> callers. I missed the comments on the previous version, but using RCU
> here is dangerous if someone decides to use this without rtnl they will
> get a false sense of security, that is why I acked the previous version.
> I'd suggest to use READ_ONCE() for f->dst to avoid reading it again.
> 

Nevermind the READ_ONCE part, I missed that there's ASSERT_RTNL() in the
beginning of this function, so it'll always be used with RTNL. :-)
It's good as it stands, I need to get some coffee.

Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>