[PATCH] Drivers: hv: vmbus: Fix a race condition when unregistering a device

[PATCH] Drivers: hv: vmbus: Fix a race condition when unregistering a device

[Vitaly Kuznetsov]

When build with Debug the following crash is sometimes observed:
Call Trace:
 [<ffffffff812b9600>] string+0x40/0x100
 [<ffffffff812bb038>] vsnprintf+0x218/0x5e0
 [<ffffffff810baf7d>] ? trace_hardirqs_off+0xd/0x10
 [<ffffffff812bb4c1>] vscnprintf+0x11/0x30
 [<ffffffff8107a2f0>] vprintk+0xd0/0x5c0
 [<ffffffffa0051ea0>] ? vmbus_process_rescind_offer+0x0/0x110 [hv_vmbus]
 [<ffffffff8155c71c>] printk+0x41/0x45
 [<ffffffffa004ebac>] vmbus_device_unregister+0x2c/0x40 [hv_vmbus]
 [<ffffffffa0051ecb>] vmbus_process_rescind_offer+0x2b/0x110 [hv_vmbus]
...

This happens due to the following race: between 'if (channel->device_obj)' check
in vmbus_process_rescind_offer() and pr_debug() in vmbus_device_unregister() the
device can disappear. Fix the issue by taking an additional reference to the
device before proceeding to vmbus_device_unregister().

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 drivers/hv/channel_mgmt.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index a2d1a96..d36ce68 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -216,9 +216,16 @@ static void vmbus_process_rescind_offer(struct work_struct *work)
 	unsigned long flags;
 	struct vmbus_channel *primary_channel;
 	struct vmbus_channel_relid_released msg;
+	struct device *dev;
+
+	if (channel->device_obj) {
+		dev = get_device(&channel->device_obj->device);
+		if (dev) {
+			vmbus_device_unregister(channel->device_obj);
+			put_device(dev);
+		}
+	}
 
-	if (channel->device_obj)
-		vmbus_device_unregister(channel->device_obj);
 	memset(&msg, 0, sizeof(struct vmbus_channel_relid_released));
 	msg.child_relid = channel->offermsg.child_relid;
 	msg.header.msgtype = CHANNELMSG_RELID_RELEASED;
-- 
1.9.3

RE: [PATCH] Drivers: hv: vmbus: Fix a race condition when unregistering a device

[KY Srinivasan]

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com]
> Sent: Tuesday, November 4, 2014 4:40 AM
> To: KY Srinivasan; Haiyang Zhang
> Cc: Greg Kroah-Hartman; devel@linuxdriverproject.org; linux-
> kernel@vger.kernel.org
> Subject: [PATCH] Drivers: hv: vmbus: Fix a race condition when unregistering
> a device
> 
> When build with Debug the following crash is sometimes observed:
> Call Trace:
>  [<ffffffff812b9600>] string+0x40/0x100
>  [<ffffffff812bb038>] vsnprintf+0x218/0x5e0  [<ffffffff810baf7d>] ?
> trace_hardirqs_off+0xd/0x10  [<ffffffff812bb4c1>] vscnprintf+0x11/0x30
> [<ffffffff8107a2f0>] vprintk+0xd0/0x5c0  [<ffffffffa0051ea0>] ?
> vmbus_process_rescind_offer+0x0/0x110 [hv_vmbus]  [<ffffffff8155c71c>]
> printk+0x41/0x45  [<ffffffffa004ebac>] vmbus_device_unregister+0x2c/0x40
> [hv_vmbus]  [<ffffffffa0051ecb>]
> vmbus_process_rescind_offer+0x2b/0x110 [hv_vmbus] ...
> 
> This happens due to the following race: between 'if (channel->device_obj)'
> check in vmbus_process_rescind_offer() and pr_debug() in
> vmbus_device_unregister() the device can disappear. Fix the issue by taking
> an additional reference to the device before proceeding to
> vmbus_device_unregister().
> 
Thank you.
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Cc: <stable@vger.kernel.org>
> ---
>  drivers/hv/channel_mgmt.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c index
> a2d1a96..d36ce68 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -216,9 +216,16 @@ static void vmbus_process_rescind_offer(struct
> work_struct *work)
>  	unsigned long flags;
>  	struct vmbus_channel *primary_channel;
>  	struct vmbus_channel_relid_released msg;
> +	struct device *dev;
> +
> +	if (channel->device_obj) {
> +		dev = get_device(&channel->device_obj->device);
> +		if (dev) {
> +			vmbus_device_unregister(channel->device_obj);
> +			put_device(dev);
> +		}
> +	}
> 
> -	if (channel->device_obj)
> -		vmbus_device_unregister(channel->device_obj);
>  	memset(&msg, 0, sizeof(struct vmbus_channel_relid_released));
>  	msg.child_relid = channel->offermsg.child_relid;
>  	msg.header.msgtype = CHANNELMSG_RELID_RELEASED;
> --
> 1.9.3