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Message-ID: <20131209121347.GB2449@minipsycho.orion>
Date:	Mon, 9 Dec 2013 13:13:47 +0100
From:	Jiri Pirko <jiri@...nulli.us>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, jtluka@...hat.com,
	zhiguohong@...cent.com, bridge@...ts.linux-foundation.org,
	stephen@...workplumber.org, edumazet@...gle.com, laine@...hat.com,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [patch net/stable v2] br: fix use of ->rx_handler_data in code
 executed on non-rx_handler path

Mon, Dec 09, 2013 at 12:58:35PM CET, mst@...hat.com 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 <laine@...hat.com>
>> Debugged-by: Michael S. Tsirkin <mst@...hat.com>
>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>> Signed-off-by: Jiri Pirko <jiri@...nulli.us>
>> ---
>> 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.

Yes. That can be done. That would safe a barrier on some archs.

>
>
>> +}
>
>
>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?

I like this a lot.

Acked-by: Jiri Pirko <jiri@...nulli.us>

>
>---
>
>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 <mst@...hat.com>
>
>-->
>
>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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