[media] media: fix media devnode ioctl/syscall and unregister race

Media devnode open/ioctl could be in progress when media device unregister
is initiated. System calls and ioctls check media device registered status
at the beginning, however, there is a window where unregister could be in
progress without changing the media devnode status to unregistered.

process 1				process 2
fd = open(/dev/media0)
media_devnode_is_registered()
	(returns true here)

					media_device_unregister()
						(unregister is in progress
						and devnode isn't
						unregistered yet)
					...
ioctl(fd, ...)
__media_ioctl()
media_devnode_is_registered()
	(returns true here)
					...
					media_devnode_unregister()
					...
					(driver releases the media device
					memory)

media_device_ioctl()
	(By this point
	devnode->media_dev does not
	point to allocated memory.
	use-after free in in mutex_lock_nested)

BUG: KASAN: use-after-free in mutex_lock_nested+0x79c/0x800 at addr
ffff8801ebe914f0

Fix it by clearing register bit when unregister starts to avoid the race.

process 1                               process 2
fd = open(/dev/media0)
media_devnode_is_registered()
        (could return true here)

                                        media_device_unregister()
                                                (clear the register bit,
						 then start unregister.)
                                        ...
ioctl(fd, ...)
__media_ioctl()
media_devnode_is_registered()
        (return false here, ioctl
	 returns I/O error, and
	 will not access media
	 device memory)
                                        ...
                                        media_devnode_unregister()
                                        ...
                                        (driver releases the media device
					 memory)

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Suggested-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reported-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Tested-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
index 33a9952..1795abe 100644 (file)
--- a/drivers/media/media-device.c
+++ b/drivers/media/media-device.c
@@ -732,6 +732,7 @@ int __must_check __media_device_register(struct media_device *mdev,
        if (ret < 0) {
                /* devnode free is handled in media_devnode_*() */
                mdev->devnode = NULL;
+               media_devnode_unregister_prepare(devnode);
                media_devnode_unregister(devnode);
                return ret;
        }
@@ -788,6 +789,9 @@ void media_device_unregister(struct media_device *mdev)
                return;
        }
 
+       /* Clear the devnode register bit to avoid races with media dev open */
+       media_devnode_unregister_prepare(mdev->devnode);
+
        /* Remove all entities from the media device */
        list_for_each_entry_safe(entity, next, &mdev->entities, graph_obj.list)
                __media_device_unregister_entity(entity);
@@ -808,13 +812,10 @@ void media_device_unregister(struct media_device *mdev)
 
        dev_dbg(mdev->dev, "Media device unregistered\n");
 
-       /* Check if mdev devnode was registered */
-       if (media_devnode_is_registered(mdev->devnode)) {
-               device_remove_file(&mdev->devnode->dev, &dev_attr_model);
-               media_devnode_unregister(mdev->devnode);
-               /* devnode free is handled in media_devnode_*() */
-               mdev->devnode = NULL;
-       }
+       device_remove_file(&mdev->devnode->dev, &dev_attr_model);
+       media_devnode_unregister(mdev->devnode);
+       /* devnode free is handled in media_devnode_*() */
+       mdev->devnode = NULL;
 }
 EXPORT_SYMBOL_GPL(media_device_unregister);
 

diff --git a/drivers/media/media-devnode.c b/drivers/media/media-devnode.c
index 5b605ff..f2772ba 100644 (file)
--- a/drivers/media/media-devnode.c
+++ b/drivers/media/media-devnode.c
@@ -287,7 +287,7 @@ cdev_add_error:
        return ret;
 }
 
-void media_devnode_unregister(struct media_devnode *devnode)
+void media_devnode_unregister_prepare(struct media_devnode *devnode)
 {
        /* Check if devnode was ever registered at all */
        if (!media_devnode_is_registered(devnode))
@@ -295,6 +295,12 @@ void media_devnode_unregister(struct media_devnode *devnode)
 
        mutex_lock(&media_devnode_lock);
        clear_bit(MEDIA_FLAG_REGISTERED, &devnode->flags);
+       mutex_unlock(&media_devnode_lock);
+}
+
+void media_devnode_unregister(struct media_devnode *devnode)
+{
+       mutex_lock(&media_devnode_lock);
        /* Delete the cdev on this minor as well */
        cdev_del(&devnode->cdev);
        mutex_unlock(&media_devnode_lock);

diff --git a/include/media/media-devnode.h b/include/media/media-devnode.h
index 5bb3b0e..f0b7dd7 100644 (file)
--- a/include/media/media-devnode.h
+++ b/include/media/media-devnode.h
@@ -125,6 +125,19 @@ int __must_check media_devnode_register(struct media_device *mdev,
                                        struct media_devnode *devnode,
                                        struct module *owner);
 
+/**
+ * media_devnode_unregister_prepare - clear the media device node register bit
+ * @devnode: the device node to prepare for unregister
+ *
+ * This clears the passed device register bit. Future open calls will be met
+ * with errors. Should be called before media_devnode_unregister() to avoid
+ * races with unregister and device file open calls.
+ *
+ * This function can safely be called if the device node has never been
+ * registered or has already been unregistered.
+ */
+void media_devnode_unregister_prepare(struct media_devnode *devnode);
+
 /**
  * media_devnode_unregister - unregister a media device node
  * @devnode: the device node to unregister
@@ -132,8 +145,7 @@ int __must_check media_devnode_register(struct media_device *mdev,
  * This unregisters the passed device. Future open calls will be met with
  * errors.
  *
- * This function can safely be called if the device node has never been
- * registered or has already been unregistered.
+ * Should be called after media_devnode_unregister_prepare()
  */
 void media_devnode_unregister(struct media_devnode *devnode);