Subject: Re: [patch net/stable v2] br: fix use of ->rx_handler_data in code executed on non-rx_handler path

From: Vlad Yasevich <[email-masked]>
Date: Mon, 09 Dec 2013 14:31:49 -0500

On 12/09/2013 06:58 AM, Michael S. Tsirkin wrote:
> On Thu, Dec 05, 2013 at 04:27:37PM +0100, Jiri Pirko wrote:
>> br_stp_rcv() is reached by non-rx_handler path. That means there is no
>> guarantee that dev is bridge port and therefore simple NULL check of
>> ->rx_handler_data is not enough. There is need to check if dev is really
>> bridge port and since only rcu read lock is held here, do it by checking
>> ->rx_handler pointer.
>>
>> Note that synchronize_net() in netdev_rx_handler_unregister() ensures
>> this approach as valid.
>>
>> Introduced originally by:
>> commit f350a0a87374418635689471606454abc7beaa3a
>>   "bridge: use rx_handler_data pointer to store net_bridge_port pointer"
>>
>> Fixed but not in the best way by:
>> commit b5ed54e94d324f17c97852296d61a143f01b227a
>>   "bridge: fix RCU races with bridge port"
>>
>> Reintroduced by:
>> commit 716ec052d2280d511e10e90ad54a86f5b5d4dcc2
>>   "bridge: fix NULL pointer deref of br_port_get_rcu"
>>
>> Please apply to stable trees as well. Thanks.
>>
>> RH bugzilla reference: https://bugzilla.redhat.com/show_bug.cgi?id=1025770
>>
>> Reported-by: Laine Stump <[email-masked]>
>> Debugged-by: Michael S. Tsirkin <[email-masked]>
>> Signed-off-by: Michael S. Tsirkin <[email-masked]>
>> Signed-off-by: Jiri Pirko <[email-masked]>
>> ---
>> v1->v2: moved br_port_get_check_rcu definition below br_handle_frame definition
>>
>>  net/bridge/br_private.h  | 10 ++++++++++
>>  net/bridge/br_stp_bpdu.c |  2 +-
>>  2 files changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
>> index 229d820..045d56e 100644
>> --- a/net/bridge/br_private.h
>> +++ b/net/bridge/br_private.h
>> @@ -426,6 +426,16 @@ netdev_features_t br_features_recompute(struct net_bridge *br,
>>  int br_handle_frame_finish(struct sk_buff *skb);
>>  rx_handler_result_t br_handle_frame(struct sk_buff **pskb);
>>  
>> +static inline bool br_rx_handler_check_rcu(const struct net_device *dev)
>> +{
>> +	return rcu_dereference(dev->rx_handler) == br_handle_frame;
> 
> Actually this started to bother me.
> rcu_dereference is for when we dereference, isn't it?
> I think we should use rcu_access_pointer here.
> 
> 
>> +}
> 
> 
> Given all the confusion, how about we create an API to
> access rx handler data outside rx handler itself in a
> safe, documented way?
> 
> If everyone agrees, we can then re-implement
> br_port_get_check_rcu on top of this API.
> 
> What do others think?
> 
> ---
> 
> netdevice: allow access to rx_handler_data outside rx handler
> 
> rx_handler_data is easy to use correctly within
> rx handler itself. Outside of that context, one must
> validate the handler first.
> 
> Add an API to do this in a uniform way.
> 
> Signed-off-by: Michael S. Tsirkin <[email-masked]>

This looks very nice is a usefull API.

Acked-by: Vlad Yasevich <[email-masked]>

however, as I mentioned to Jiri, I've been trying to understand why
Stephen's patch is insufficient and so far I can't come up with a race
scenario that would break a simple check for dev->priv_flags.

So, I've decided to look at the history that Jiri mentioned in his
commit.  In particular, I was reading
    commit b5ed54e94d324f17c97852296d61a143f01b227a
    "bridge: fix RCU races with bridge port"

that claimed that there is a race in RCU section when just checking
the priv_flags for IFF_BRIDGE_PORT flag.  Doing a little more digging
shows that at the time that commit was added, there was no call to
synchronise_net() in netdev_rx_handler_unregister().  So, at the time
of that commit there truly was a race, and the race still was not fixed
until Eric submitted
    commit 00cfec37484761a44a3b6f4675a54caa618210ae
    net: add a synchronize_net() in netdev_rx_handler_unregister()

So, I think now it is perfectly safe to simply use the construct
    if (!br_port_exists(dev))
         return;

    port = br_port_get_rcu(dev);

under rcu protection.  In fact, we are guaranteed to have a valid
bridge port in this situation due to the fact that the the flag is
is turned off before netdev_rx_handler_unregister() is called.

-vlad

-vlad

> 
> -->
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 7f0ed42..7a353b1 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -1320,6 +1320,9 @@ struct net_device {
>  #endif
>  
>  	rx_handler_func_t __rcu	*rx_handler;
> +	/* rx_handler itself can use rx_handler_data directly.
> +	 * Others must use netdev_rx_handler_data_rcu_dereference.
> +	 */
>  	void __rcu		*rx_handler_data;
>  
>  	struct netdev_queue __rcu *ingress_queue;
> @@ -2399,6 +2402,31 @@ int netdev_rx_handler_register(struct net_device *dev,
>  			       void *rx_handler_data);
>  void netdev_rx_handler_unregister(struct net_device *dev);
>  
> +/**
> + *	netdev_rx_handler_data_rcu_dereference - access receive handler data
> + *	@dev: device to get handler data for
> + *	@rx_handler: receive handler used to register this data
> + *
> + *	Check that the receive handler is valid for the device.
> + *	Return handler data if it is, NULL otherwise.
> + *
> + *	Use this function if you want to access rx handler data
> + *	outside rx handler itself.
> + *
> + *	The caller must invoke this function under RCU read lock.
> + *
> + *	For a general description of rx_handler, see enum rx_handler_result.
> + */
> +static inline
> +void *netdev_rx_handler_data_rcu_dereference(struct net_device *dev,
> +					     rx_handler_func_t *rx_handler)
> +{
> +	if (rcu_access_pointer(dev->rx_handler) != rx_handler)
> +		return NULL;
> +
> +	return rcu_dereference(dev->rx_handler_data);
> +}
> +
>  bool dev_valid_name(const char *name);
>  int dev_ioctl(struct net *net, unsigned int cmd, void __user *);
>  int dev_ethtool(struct net *net, struct ifreq *);
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