RE: [PATCH] bridge: missing null bridge device check causing null pointer dereference (bugfix)

[박수현]

>-----Original Message-----
>From: Toshiaki Makita [mailto:makita.toshiaki@xxxxxxxxxxxxx]
>Sent: Thursday, November 06, 2014 4:07 PM
>To: 박수현; Stephen Hemminger; David S. Miller
>Cc: bridge@xxxxxxxxxxxxxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; linux-
>kernel@xxxxxxxxxxxxxxx
>Subject: Re: [PATCH] bridge: missing null bridge device check causing null
>pointer dereference (bugfix)
>
>On 2014/11/06 15:26, Su-Hyun Park wrote:
>> the bridge device can be null if the bridge is being deleted while
>> processing the packet, which causes the null pointer dereference in
>switch statement.
>
>How can this happen??
>It is guarded by rcu.
>netdev_rx_handler_unregister() ensures rx_handler_data is non NULL.
>

The RCU protect rx_handler_data, not the bridge member port. It can be NULL according to below code.

static inline struct net_bridge_port *br_port_get_rcu(const struct net_device *dev) {
	struct net_bridge_port *port = rcu_dereference(dev->rx_handler_data);
	return br_port_exists(dev) ? port : NULL; 
}

The crash happens at the below switch statement in br_handle_frame, where p is NULL.

	switch (p->state)

>>
>> crash dump snippet:
>>
>> <1>BUG: unable to handle kernel NULL pointer dereference at
>> 0000000000000021
>> <1>IP: [<ffffffff814179f6>] br_handle_frame+0xe6/0x270
>>
>> <0>Code: 4c 0f 44 f0 89 f8 66 33 15 32 52 24 00 66 33 05 29 52 24 00
>> 09 c2 89
>> f0 66 33 05 22 52 24 00 80 e4 f0 66 09 c2 0f 84 eb 00 00 00 <41> 0f b6
>> 46 21 3c 02 74 61 3c 03 74 1d 48 89 df e8 d5 bc f0 ff
>> ---
>>  net/bridge/br_input.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c index
>> 6fd5522..7e899ca 100644
>> --- a/net/bridge/br_input.c
>> +++ b/net/bridge/br_input.c
>> @@ -176,6 +176,8 @@ rx_handler_result_t br_handle_frame(struct sk_buff
>**pskb)
>>  		return RX_HANDLER_CONSUMED;
>>
>>  	p = br_port_get_rcu(skb->dev);
>> +	if (!p)
>> +		goto drop;
>>
>>  	if (unlikely(is_link_local_ether_addr(dest))) {
>>  		u16 fwd_mask = p->br->group_fwd_mask_required;
>>
>