Commit 1815d9c8 authored by Desmond Cheong Zhi Xi's avatar Desmond Cheong Zhi Xi Committed by Daniel Vetter
Browse files

drm: add a locked version of drm_is_current_master 



While checking the master status of the DRM file in
drm_is_current_master(), the device's master mutex should be
held. Without the mutex, the pointer fpriv->master may be freed
concurrently by another process calling drm_setmaster_ioctl(). This
could lead to use-after-free errors when the pointer is subsequently
dereferenced in drm_lease_owner().

The callers of drm_is_current_master() from drm_auth.c hold the
device's master mutex, but external callers do not. Hence, we implement
drm_is_current_master_locked() to be used within drm_auth.c, and
modify drm_is_current_master() to grab the device's master mutex
before checking the master status.

Reported-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: default avatarDesmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Reviewed-by: default avatarEmil Velikov <emil.l.velikov@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210620110327.4964-2-desmondcheongzx@gmail.com
parent e541845a

drivers/gpu/drm/drm_auth.c

+32 −19
Original line number Diff line number Diff line
@@ -61,6 +61,35 @@ 
 * trusted clients.

 */



static bool drm_is_current_master_locked(struct drm_file *fpriv)

{

	lockdep_assert_held_once(&fpriv->master->dev->master_mutex);



	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;

}



/**

 * drm_is_current_master - checks whether @priv is the current master

 * @fpriv: DRM file private

 *

 * Checks whether @fpriv is current master on its device. This decides whether a

 * client is allowed to run DRM_MASTER IOCTLs.

 *

 * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting

 * - the current master is assumed to own the non-shareable display hardware.

 */

bool drm_is_current_master(struct drm_file *fpriv)

{

	bool ret;



	mutex_lock(&fpriv->master->dev->master_mutex);

	ret = drm_is_current_master_locked(fpriv);

	mutex_unlock(&fpriv->master->dev->master_mutex);



	return ret;

}

EXPORT_SYMBOL(drm_is_current_master);



int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)

{

	struct drm_auth *auth = data;
@@ -223,7 +252,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data, 
	if (ret)

		goto out_unlock;



	if (drm_is_current_master(file_priv))

	if (drm_is_current_master_locked(file_priv))

		goto out_unlock;



	if (dev->master) {
@@ -272,7 +301,7 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data, 
	if (ret)

		goto out_unlock;



	if (!drm_is_current_master(file_priv)) {

	if (!drm_is_current_master_locked(file_priv)) {

		ret = -EINVAL;

		goto out_unlock;

	}
@@ -321,7 +350,7 @@ void drm_master_release(struct drm_file *file_priv) 
	if (file_priv->magic)

		idr_remove(&file_priv->master->magic_map, file_priv->magic);



	if (!drm_is_current_master(file_priv))

	if (!drm_is_current_master_locked(file_priv))

		goto out;



	drm_legacy_lock_master_cleanup(dev, master);
@@ -342,22 +371,6 @@ void drm_master_release(struct drm_file *file_priv) 
	mutex_unlock(&dev->master_mutex);

}



/**

 * drm_is_current_master - checks whether @priv is the current master

 * @fpriv: DRM file private

 *

 * Checks whether @fpriv is current master on its device. This decides whether a

 * client is allowed to run DRM_MASTER IOCTLs.

 *

 * Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting

 * - the current master is assumed to own the non-shareable display hardware.

 */

bool drm_is_current_master(struct drm_file *fpriv)

{

	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;

}

EXPORT_SYMBOL(drm_is_current_master);



/**

 * drm_master_get - reference a master pointer

 * @master: &struct drm_master