Commit 0b0860a3 authored by Desmond Cheong Zhi Xi's avatar Desmond Cheong Zhi Xi Committed by Daniel Vetter
Browse files

drm: serialize drm_file.master with a new spinlock 



Currently, drm_file.master pointers should be protected by
drm_device.master_mutex when being dereferenced. This is because
drm_file.master is not invariant for the lifetime of drm_file. If
drm_file is not the creator of master, then drm_file.is_master is
false, and a call to drm_setmaster_ioctl will invoke
drm_new_set_master, which then allocates a new master for drm_file and
puts the old master.

Thus, without holding drm_device.master_mutex, the old value of
drm_file.master could be freed while it is being used by another
concurrent process.

However, it is not always possible to lock drm_device.master_mutex to
dereference drm_file.master. Through the fbdev emulation code, this
might occur in a deep nest of other locks. But drm_device.master_mutex
is also the outermost lock in the nesting hierarchy, so this leads to
potential deadlocks.

To address this, we introduce a new spin lock at the bottom of the
lock hierarchy that only serializes drm_file.master. With this change,
the value of drm_file.master changes only when both
drm_device.master_mutex and drm_file.master_lookup_lock are
held. Hence, any process holding either of those locks can ensure that
the value of drm_file.master will not change concurrently.

Since no lock depends on the new drm_file.master_lookup_lock, when
drm_file.master is dereferenced, but drm_device.master_mutex cannot be
held, we can safely protect the master pointer with
drm_file.master_lookup_lock.

Reported-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: default avatarDesmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Signed-off-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210712043508.11584-5-desmondcheongzx@gmail.com
parent 1f7ef07c

drivers/gpu/drm/drm_auth.c

+11 −6
Original line number Diff line number Diff line
@@ -164,16 +164,18 @@ static void drm_set_master(struct drm_device *dev, struct drm_file *fpriv, 
static int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)

{

	struct drm_master *old_master;

	struct drm_master *new_master;



	lockdep_assert_held_once(&dev->master_mutex);



	WARN_ON(fpriv->is_master);

	old_master = fpriv->master;

	fpriv->master = drm_master_create(dev);

	if (!fpriv->master) {

		fpriv->master = old_master;

	new_master = drm_master_create(dev);

	if (!new_master)

		return -ENOMEM;

	}

	spin_lock(&fpriv->master_lookup_lock);

	fpriv->master = new_master;

	spin_unlock(&fpriv->master_lookup_lock);



	fpriv->is_master = 1;

	fpriv->authenticated = 1;
@@ -332,10 +334,13 @@ int drm_master_open(struct drm_file *file_priv) 
	 * any master object for render clients

	 */

	mutex_lock(&dev->master_mutex);

	if (!dev->master)

	if (!dev->master) {

		ret = drm_new_set_master(dev, file_priv);

	else

	} else {

		spin_lock(&file_priv->master_lookup_lock);

		file_priv->master = drm_master_get(dev->master);

		spin_unlock(&file_priv->master_lookup_lock);

	}

	mutex_unlock(&dev->master_mutex);



	return ret;

drivers/gpu/drm/drm_file.c

+1 −0
Original line number Diff line number Diff line
@@ -176,6 +176,7 @@ struct drm_file *drm_file_alloc(struct drm_minor *minor) 
	init_waitqueue_head(&file->event_wait);

	file->event_space = 4096; /* set aside 4k for event buffer */



	spin_lock_init(&file->master_lookup_lock);

	mutex_init(&file->event_read_lock);



	if (drm_core_check_feature(dev, DRIVER_GEM))

include/drm/drm_file.h

+9 −3
Original line number Diff line number Diff line
@@ -226,15 +226,21 @@ struct drm_file { 
	/**

	 * @master:

	 *

	 * Master this node is currently associated with. Only relevant if

	 * drm_is_primary_client() returns true. Note that this only

	 * matches &drm_device.master if the master is the currently active one.

	 * Master this node is currently associated with. Protected by struct

	 * &drm_device.master_mutex, and serialized by @master_lookup_lock.

	 *

	 * Only relevant if drm_is_primary_client() returns true. Note that

	 * this only matches &drm_device.master if the master is the currently

	 * active one.

	 *

	 * See also @authentication and @is_master and the :ref:`section on

	 * primary nodes and authentication <drm_primary_node>`.

	 */

	struct drm_master *master;



	/** @master_lock: Serializes @master. */

	spinlock_t master_lookup_lock;



	/** @pid: Process that opened this file. */

	struct pid *pid;