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

Michael S. Tsirkin
2013-12-09T11:58:35Z

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>

-->

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 *);