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

Vlad Yasevich
2013-12-09T19:31:49Z

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 <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.
>
>
>> +}
>
>
> 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 <mst@redhat.com>

This looks very nice is a usefull API.

Acked-by: Vlad Yasevich <vyasevic@redhat.com>

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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