Drivers: hv: vmbus: fix the race when querying & updating the percpu list

There is a rare race when we remove an entry from the global list
hv_context.percpu_list[cpu] in hv_process_channel_removal() ->
percpu_channel_deq() -> list_del(): at this time, if vmbus_on_event() ->
process_chn_event() -> pcpu_relid2channel() is trying to query the list,
we can get the kernel fault.

Similarly, we also have the issue in the code path: vmbus_process_offer() ->
percpu_channel_enq().

We can resolve the issue by disabling the tasklet when updating the list.

The patch also moves vmbus_release_relid() to a later place where
the channel has been removed from the per-cpu and the global lists.

Reported-by: Rolf Neugebauer <rolf.neugebauer@docker.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/channel.c

@@ -505,7 +505,6 @@ static void reset_channel_cb(void *arg)
 static int vmbus_close_internal(struct vmbus_channel *channel)
 {
 	struct vmbus_channel_close_channel *msg;
-	struct tasklet_struct *tasklet;
 	int ret;
 
 	/*
@@ -517,8 +516,7 @@ static int vmbus_close_internal(struct vmbus_channel *channel)
 	 * To resolve the race, we can serialize them by disabling the
 	 * tasklet when the latter is running here.
 	 */
-	tasklet = hv_context.event_dpc[channel->target_cpu];
-	tasklet_disable(tasklet);
+	hv_event_tasklet_disable(channel);
 
 	/*
 	 * In case a device driver's probe() fails (e.g.,
@@ -584,7 +582,7 @@ static int vmbus_close_internal(struct vmbus_channel *channel)
 		get_order(channel->ringbuffer_pagecount * PAGE_SIZE));
 
 out:
-	tasklet_enable(tasklet);
+	hv_event_tasklet_enable(channel);
 
 	return ret;
 }

drivers/hv/channel_mgmt.c

@@ -21,6 +21,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/kernel.h>
+#include <linux/interrupt.h>
 #include <linux/sched.h>
 #include <linux/wait.h>
 #include <linux/mm.h>
@@ -303,16 +304,32 @@ static void vmbus_release_relid(u32 relid)
 	vmbus_post_msg(&msg, sizeof(struct vmbus_channel_relid_released));
 }
 
+void hv_event_tasklet_disable(struct vmbus_channel *channel)
+{
+	struct tasklet_struct *tasklet;
+	tasklet = hv_context.event_dpc[channel->target_cpu];
+	tasklet_disable(tasklet);
+}
+
+void hv_event_tasklet_enable(struct vmbus_channel *channel)
+{
+	struct tasklet_struct *tasklet;
+	tasklet = hv_context.event_dpc[channel->target_cpu];
+	tasklet_enable(tasklet);
+
+	/* In case there is any pending event */
+	tasklet_schedule(tasklet);
+}
+
 void hv_process_channel_removal(struct vmbus_channel *channel, u32 relid)
 {
 	unsigned long flags;
 	struct vmbus_channel *primary_channel;
 
-	vmbus_release_relid(relid);
-
 	BUG_ON(!channel->rescind);
 	BUG_ON(!mutex_is_locked(&vmbus_connection.channel_mutex));
 
+	hv_event_tasklet_disable(channel);
 	if (channel->target_cpu != get_cpu()) {
 		put_cpu();
 		smp_call_function_single(channel->target_cpu,
@@ -321,6 +338,7 @@ void hv_process_channel_removal(struct vmbus_channel *channel, u32 relid)
 		percpu_channel_deq(channel);
 		put_cpu();
 	}
+	hv_event_tasklet_enable(channel);
 
 	if (channel->primary_channel == NULL) {
 		list_del(&channel->listentry);
@@ -341,6 +359,8 @@ void hv_process_channel_removal(struct vmbus_channel *channel, u32 relid)
 	cpumask_clear_cpu(channel->target_cpu,
 			  &primary_channel->alloced_cpus_in_node);
 
+	vmbus_release_relid(relid);
+
 	free_channel(channel);
 }
 
@@ -409,6 +429,7 @@ static void vmbus_process_offer(struct vmbus_channel *newchannel)
 
 	init_vp_index(newchannel, dev_type);
 
+	hv_event_tasklet_disable(newchannel);
 	if (newchannel->target_cpu != get_cpu()) {
 		put_cpu();
 		smp_call_function_single(newchannel->target_cpu,
@@ -418,6 +439,7 @@ static void vmbus_process_offer(struct vmbus_channel *newchannel)
 		percpu_channel_enq(newchannel);
 		put_cpu();
 	}
+	hv_event_tasklet_enable(newchannel);
 
 	/*
 	 * This state is used to indicate a successful open
@@ -463,12 +485,11 @@ static void vmbus_process_offer(struct vmbus_channel *newchannel)
 	return;
 
 err_deq_chan:
-	vmbus_release_relid(newchannel->offermsg.child_relid);
-
 	mutex_lock(&vmbus_connection.channel_mutex);
 	list_del(&newchannel->listentry);
 	mutex_unlock(&vmbus_connection.channel_mutex);
 
+	hv_event_tasklet_disable(newchannel);
 	if (newchannel->target_cpu != get_cpu()) {
 		put_cpu();
 		smp_call_function_single(newchannel->target_cpu,
@@ -477,6 +498,9 @@ err_deq_chan:
 		percpu_channel_deq(newchannel);
 		put_cpu();
 	}
+	hv_event_tasklet_enable(newchannel);
+
+	vmbus_release_relid(newchannel->offermsg.child_relid);
 
 err_free_chan:
 	free_channel(newchannel);

include/linux/hyperv.h

@@ -1357,6 +1357,9 @@ extern bool vmbus_prep_negotiate_resp(struct icmsg_hdr *,
 					struct icmsg_negotiate *, u8 *, int,
 					int);
 
+void hv_event_tasklet_disable(struct vmbus_channel *channel);
+void hv_event_tasklet_enable(struct vmbus_channel *channel);
+
 void hv_process_channel_removal(struct vmbus_channel *channel, u32 relid);
 
 /*