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

Jiri Pirko
2013-12-09T12:13:50Z

Mon, Dec 09, 2013 at 12:58:35PM CET, mst@redhat.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@redhat.com>
>> Debugged-by: Michael S. Tsirkin <mst@redhat.com>
>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>> Signed-off-by: Jiri Pirko <jiri@resnulli.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@resnulli.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@redhat.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 *);