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 general protection fault:

general protection fault: 0000 [#1] SMP
...
RIP: 0010:[<ffffffff81461b6b>]  [<ffffffff81461b6b>] vmbus_on_event+0xc4/0x149

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.

Reported-by Rolf Neugebauer <[email protected]>

Signed-off-by Dexuan Cui <[email protected]>

---
 drivers/hv/channel.c      |  3 +++
 drivers/hv/channel_mgmt.c | 20 +++++++++-----------
 include/linux/hyperv.h    |  3 +++
 3 files changed, 15 insertions(+), 11 deletions(-)

drivers/hv/channel.c +3 -0

index 56dd261..7811cf9 100644
@@ -546,8 +546,11 @@ static int vmbus_close_internal(struct vmbus_channel *channel)
 		put_cpu();
 		smp_call_function_single(channel->target_cpu, reset_channel_cb,
 					 channel, true);
+		smp_call_function_single(channel->target_cpu,
+					 percpu_channel_deq, channel, true);
 	} else {
 		reset_channel_cb(channel);
+		percpu_channel_deq(channel);
 		put_cpu();
 	}
 

drivers/hv/channel_mgmt.c +6 -11

index 38b682ba..c4b5c07 100644
@@ -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>

@@ -277,7 +278,7 @@ static void free_channel(struct vmbus_channel *channel)
 	kfree(channel);
 }
 
-static void percpu_channel_enq(void *arg)
+void percpu_channel_enq(void *arg)
 {
 	struct vmbus_channel *channel = arg;
 	int cpu = smp_processor_id();

@@ -285,7 +286,7 @@ static void percpu_channel_enq(void *arg)
 	list_add_tail(&channel->percpu_list, &hv_context.percpu_list[cpu]);
 }
 
-static void percpu_channel_deq(void *arg)
+void percpu_channel_deq(void *arg)
 {
 	struct vmbus_channel *channel = arg;
 

@@ -313,15 +314,6 @@ void hv_process_channel_removal(struct vmbus_channel *channel, u32 relid)
 	BUG_ON(!channel->rescind);
 	BUG_ON(!mutex_is_locked(&vmbus_connection.channel_mutex));
 
-	if (channel->target_cpu != get_cpu()) {
-		put_cpu();
-		smp_call_function_single(channel->target_cpu,
-					 percpu_channel_deq, channel, true);
-	} else {
-		percpu_channel_deq(channel);
-		put_cpu();
-	}
-
 	if (channel->primary_channel == NULL) {
 		list_del(&channel->listentry);
 

@@ -363,6 +355,7 @@ void vmbus_free_channels(void)
  */
 static void vmbus_process_offer(struct vmbus_channel *newchannel)
 {
+	struct tasklet_struct *tasklet;
 	struct vmbus_channel *channel;
 	bool fnew = true;
 	unsigned long flags;

@@ -409,6 +402,8 @@ static void vmbus_process_offer(struct vmbus_channel *newchannel)
 
 	init_vp_index(newchannel, dev_type);
 
+	tasklet = hv_context.event_dpc[newchannel->target_cpu];
+	tasklet_disable(tasklet);
 	if (newchannel->target_cpu != get_cpu()) {
 		put_cpu();
 		smp_call_function_single(newchannel->target_cpu,

drivers/hv/channel_mgmt.c +2 -0

 	/*
 	 * This state is used to indicate a successful open

@@ -469,6 +465,7 @@ static void vmbus_process_offer(struct vmbus_channel *newchannel)
 	list_del(&newchannel->listentry);
 	mutex_unlock(&vmbus_connection.channel_mutex);
 
+	tasklet_disable(tasklet);
 	if (newchannel->target_cpu != get_cpu()) {
 		put_cpu();
 		smp_call_function_single(newchannel->target_cpu,

@@ -477,6 +474,7 @@ static void vmbus_process_offer(struct vmbus_channel *newchannel)
 		percpu_channel_deq(newchannel);
 		put_cpu();
 	}
+	tasklet_enable(tasklet);
 
 err_free_chan:
 	free_channel(newchannel);

include/linux/hyperv.h +3 -0

index 7be7237..95aea09 100644
@@ -1328,6 +1328,9 @@ extern bool vmbus_prep_negotiate_resp(struct icmsg_hdr *,
 					struct icmsg_negotiate *, u8 *, int,
 					int);
 
+void percpu_channel_enq(void *arg);
+void percpu_channel_deq(void *arg);
+
 void hv_process_channel_removal(struct vmbus_channel *channel, u32 relid);
 
 /*

From: Vitaly Kuznetsov [mailto:[email protected]]
Sent: Tuesday, May 17, 2016 16:15
To: Dexuan Cui <[email protected]>
Cc: [email protected]; [email protected]; driverdev- [email protected]; [email protected]; [email protected]; [email protected]; KY Srinivasan <[email protected]>; Haiyang Zhang <[email protected]>
Subject: Re: [PATCH] Drivers: hv: vmbus: fix the race when querying & updating the percpu list

Review static void vmbus_process_offer(struct

Quoted 10 lines · Dexuan Cui

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 general protection fault:
...

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

(quoted diff) +0 -0

@@ -418,6 +413,7 @@ static void vmbus_process_offer(struct

vmbus_channel *newchannel)

drivers/hv/channel_mgmt.c static void vmbus_process_offer(struct vmbus_channel *newchannel) · line 416

Quoted 4 lines

drivers/hv/channel_mgmt.c +1 -0

 		percpu_channel_enq(newchannel);
 		put_cpu();
 	}
+	tasklet_enable(tasklet);

Do we need to do tasklet_schedule() to make sure there are no events pending? This is probably not a big issue as some other event will trigger scheduling but in some corner cases it may bite. Same question applies to the code below and to vmbus_close_internal().

From: devel [mailto:[email protected]] On Behalf Of Dexuan Cui

Quoted 5 lines

Do we need to do tasklet_schedule() to make sure there are no events pending? This is probably not a big issue as some other event will trigger scheduling but in some corner cases it may bite. Same question applies to the code below and to vmbus_close_internal().

Hi Vitaly,
Thanks for spotting this!
I think you're correct.
I'll add tasklet_schedul() before the tasklet_enable() in the 2 places.