[PATCH v5 16/19] drm: fix race between partial drm_dev_register() failure and ioctl

[Danilo Krummrich <dakr@kernel.org>]

If drm_dev_register() fails after registering a minor (e.g. render minor
registered, primary minor fails), userspace could have opened the first
minor and entered a drm_dev_enter() critical section. Since the
unplugged flag was never set, the ioctl proceeds while the error path
tears down device resources.

Fix this by introducing drm_dev_synchronize_unplug(), which sets the
unplugged flag and waits for the SRCU barrier, ensuring all in-flight
drm_dev_enter() critical sections complete before cleanup proceeds; call
it on the error path of drm_dev_register().

Fixes: bee330f3d672 ("drm: Use srcu to protect drm_device.unplugged")
Cc: stable@vger.kernel.org
Reported-by: sashiko-bot@kernel.org
Closes: https://lore.kernel.org/all/20260620190648.2E9F61F000E9@smtp.kernel.org/
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 drivers/gpu/drm/drm_drv.c | 34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 675675480da4..e890052061f3 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -473,6 +473,22 @@ void drm_dev_exit(int idx)
 }
 EXPORT_SYMBOL(drm_dev_exit);
 
+/*
+ * Mark the device as unplugged and wait for any in-flight drm_dev_enter()
+ * critical sections to complete.
+ */
+static void drm_dev_synchronize_unplug(struct drm_device *dev)
+{
+	/*
+	 * After synchronizing any critical read section is guaranteed to see
+	 * the new value of ->unplugged, and any critical section which might
+	 * still have seen the old value of ->unplugged is guaranteed to have
+	 * finished.
+	 */
+	dev->unplugged = true;
+	synchronize_srcu(&drm_unplug_srcu);
+}
+
 /**
  * drm_dev_unplug - unplug a DRM device
  * @dev: DRM device
@@ -485,15 +501,7 @@ EXPORT_SYMBOL(drm_dev_exit);
  */
 void drm_dev_unplug(struct drm_device *dev)
 {
-	/*
-	 * After synchronizing any critical read section is guaranteed to see
-	 * the new value of ->unplugged, and any critical section which might
-	 * still have seen the old value of ->unplugged is guaranteed to have
-	 * finished.
-	 */
-	dev->unplugged = true;
-	synchronize_srcu(&drm_unplug_srcu);
-
+	drm_dev_synchronize_unplug(dev);
 	drm_dev_unregister(dev);
 
 	/* Clear all CPU mappings pointing to this device */
@@ -1091,6 +1099,7 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
 		goto err_minors;
 
 	dev->registered = true;
+	dev->unplugged = false;
 
 	if (driver->load) {
 		ret = driver->load(dev, flags);
@@ -1118,6 +1127,13 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
 	if (dev->driver->unload)
 		dev->driver->unload(dev);
 err_minors:
+	/*
+	 * If a minor was registered before the failure, userspace could have
+	 * opened it and entered a drm_dev_enter() critical section. Ensure all
+	 * such sections complete before we clean up.
+	 */
+	drm_dev_synchronize_unplug(dev);
+
 	remove_compat_control_link(dev);
 	drm_minor_unregister(dev, DRM_MINOR_ACCEL);
 	drm_minor_unregister(dev, DRM_MINOR_PRIMARY);
-- 
2.54.0