vmbus: use rcu for per-cpu channel list

The per-cpu channel list is now referred to in the interrupt
routine. This is mostly safe since the host will not normally generate
an interrupt when channel is being deleted but if it did then there
would be a use after free problem.

To solve, this use RCU protection on ther per-cpu list.

Fixes: 631e63a9f346 ("vmbus: change to per channel tasklet")

Signed-off-by: Stephen Hemminger <[e-mail]>
Signed-off-by: K. Y. Srinivasan <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/hv/channel_mgmt.c

···
 static void free_channel(struct vmbus_channel *channel)
 {
 	tasklet_kill(&channel->callback_event);
-	kfree(channel);
+
+	kfree_rcu(channel, rcu);
 }
 
 static void percpu_channel_enq(void *arg)
···
 	struct hv_per_cpu_context *hv_cpu
 		= this_cpu_ptr(hv_context.cpu_context);
 
-	list_add_tail(&channel->percpu_list, &hv_cpu->chan_list);
+	list_add_tail_rcu(&channel->percpu_list, &hv_cpu->chan_list);
 }
 
 static void percpu_channel_deq(void *arg)
 {
 	struct vmbus_channel *channel = arg;
 
-	list_del(&channel->percpu_list);
+	list_del_rcu(&channel->percpu_list);
 }
 
 

drivers/hv/vmbus_drv.c

···
 		if (relid == 0)
 			continue;
 
+		rcu_read_lock();
+
 		/* Find channel based on relid */
-		list_for_each_entry(channel, &hv_cpu->chan_list, percpu_list) {
+		list_for_each_entry_rcu(channel, &hv_cpu->chan_list, percpu_list) {
 			if (channel->offermsg.child_relid != relid)
 				continue;
 
···
 				tasklet_schedule(&channel->callback_event);
 			}
 		}
+
+		rcu_read_unlock();
 	}
 }
 

include/linux/hyperv.h

···
 	 * link up channels based on their CPU affinity.
 	 */
 	struct list_head percpu_list;
+
+	/*
+	 * Defer freeing channel until after all cpu's have
+	 * gone through grace period.
+	 */
+	struct rcu_head rcu;
+
 	/*
 	 * For performance critical channels (storage, networking
 	 * etc,), Hyper-V has a mechanism to enhance the throughput