Drivers: hv: utils: fix crash when device is removed from host side

The crash is observed when a service is being disabled host side while
userspace daemon is connected to the device:

[   90.244859] general protection fault: 0000 [#1] SMP
...
[   90.800082] Call Trace:
[   90.800082]  [<ffffffff81187008>] __fput+0xc8/0x1f0
[   90.800082]  [<ffffffff8118716e>] ____fput+0xe/0x10
...
[   90.800082]  [<ffffffff81015278>] do_signal+0x28/0x580
[   90.800082]  [<ffffffff81086656>] ? finish_task_switch+0xa6/0x180
[   90.800082]  [<ffffffff81443ebf>] ? __schedule+0x28f/0x870
[   90.800082]  [<ffffffffa01ebbaa>] ? hvt_op_read+0x12a/0x140 [hv_utils]
...

The problem is that hvutil_transport_destroy() which does misc_deregister()
freeing the appropriate device is reachable by two paths: module unload
and from util_remove(). While module unload path is protected by .owner in
struct file_operations util_remove() path is not. Freeing the device while
someone holds an open fd for it is a show stopper.

In general, it is not possible to revoke an fd from all users so the only
way to solve the issue is to defer freeing the hvutil_transport structure.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/hv_utils_transport.c

@@ -155,13 +155,22 @@ static int hvt_op_open(struct inode *inode, struct file *file)
 	return ret;
 }
 
+static void hvt_transport_free(struct hvutil_transport *hvt)
+{
+	misc_deregister(&hvt->mdev);
+	kfree(hvt->outmsg);
+	kfree(hvt);
+}
+
 static int hvt_op_release(struct inode *inode, struct file *file)
 {
 	struct hvutil_transport *hvt;
+	int mode_old;
 
 	hvt = container_of(file->f_op, struct hvutil_transport, fops);
 
 	mutex_lock(&hvt->lock);
+	mode_old = hvt->mode;
 	if (hvt->mode != HVUTIL_TRANSPORT_DESTROY)
 		hvt->mode = HVUTIL_TRANSPORT_INIT;
 	/*
@@ -171,6 +180,9 @@ static int hvt_op_release(struct inode *inode, struct file *file)
 	hvt_reset(hvt);
 	mutex_unlock(&hvt->lock);
 
+	if (mode_old == HVUTIL_TRANSPORT_DESTROY)
+		hvt_transport_free(hvt);
+
 	return 0;
 }
 
@@ -304,17 +316,25 @@ struct hvutil_transport *hvutil_transport_init(const char *name,
 
 void hvutil_transport_destroy(struct hvutil_transport *hvt)
 {
+	int mode_old;
+
 	mutex_lock(&hvt->lock);
+	mode_old = hvt->mode;
 	hvt->mode = HVUTIL_TRANSPORT_DESTROY;
 	wake_up_interruptible(&hvt->outmsg_q);
 	mutex_unlock(&hvt->lock);
 
+	/*
+	 * In case we were in 'chardev' mode we still have an open fd so we
+	 * have to defer freeing the device. Netlink interface can be freed
+	 * now.
+	 */
 	spin_lock(&hvt_list_lock);
 	list_del(&hvt->list);
 	spin_unlock(&hvt_list_lock);
 	if (hvt->cn_id.idx > 0 && hvt->cn_id.val > 0)
 		cn_del_callback(&hvt->cn_id);
-	misc_deregister(&hvt->mdev);
-	kfree(hvt->outmsg);
-	kfree(hvt);
+
+	if (mode_old != HVUTIL_TRANSPORT_CHARDEV)
+		hvt_transport_free(hvt);
 }