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 <[email protected]>
Suggested-by: Sakari Ailus <[email protected]>
Reported-by: Mauro Carvalho Chehab <[email protected]>
---
Test Procedure and Results:
https://drive.google.com/file/d/0B0NIL0BQg-AlN1VxT2oyTXBPRHc/view?usp=sharing
drivers/media/media-device.c | 15 ++++++++-------
drivers/media/media-devnode.c | 8 +++++++-
include/media/media-devnode.h | 16 ++++++++++++++--
3 files changed, 29 insertions(+), 10 deletions(-)
diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
index 33a9952..1795abe 100644
--- 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
--- 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
--- a/include/media/media-devnode.h
+++ b/include/media/media-devnode.h
@@ -126,14 +126,26 @@ int __must_check media_devnode_register(struct
media_device *mdev,
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
*
* 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);
--
2.7.4
