drm/panthor: Fix UAF in panthor_gem_create_with_handle() debugfs code

The object is potentially already gone after the drm_gem_object_put().
In general the object should be fully constructed before calling
drm_gem_handle_create(), except the debugfs tracking uses a separate
lock and list and separate flag to denotate whether the object is
actually initialized.

Since I'm touching this all anyway simplify this by only adding the
object to the debugfs when it's ready for that, which allows us to
delete that separate flag. panthor_gem_debugfs_bo_rm() already checks
whether we've actually been added to the list or this is some error
path cleanup.

v2: Fix build issues for !CONFIG_DEBUGFS (Adrián)

v3: Add linebreak and remove outdated comment (Liviu)

Fixes: a3707f53eb3f ("drm/panthor: show device-wide list of DRM GEM objects over DebugFS")
Cc: Adrián Larumbe <[e-mail]>
Cc: Boris Brezillon <[e-mail]>
Cc: Steven Price <[e-mail]>
Cc: Liviu Dudau <[e-mail]>
Reviewed-by: Liviu Dudau <[e-mail]>
Signed-off-by: Simona Vetter <[e-mail]>
Signed-off-by: Simona Vetter <[e-mail]>
Reviewed-by: Steven Price <[e-mail]>
Signed-off-by: Steven Price <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/panthor/panthor_gem.c

···
 #include "panthor_mmu.h"
 
 #ifdef CONFIG_DEBUG_FS
-static void panthor_gem_debugfs_bo_add(struct panthor_device *ptdev,
-				       struct panthor_gem_object *bo)
+static void panthor_gem_debugfs_bo_init(struct panthor_gem_object *bo)
 {
 	INIT_LIST_HEAD(&bo->debugfs.node);
+}
+
+static void panthor_gem_debugfs_bo_add(struct panthor_gem_object *bo)
+{
+	struct panthor_device *ptdev = container_of(bo->base.base.dev,
+						    struct panthor_device, base);
 
 	bo->debugfs.creator.tgid = current->group_leader->pid;
 	get_task_comm(bo->debugfs.creator.process_name, current->group_leader);
···
 
 static void panthor_gem_debugfs_set_usage_flags(struct panthor_gem_object *bo, u32 usage_flags)
 {
-	bo->debugfs.flags = usage_flags | PANTHOR_DEBUGFS_GEM_USAGE_FLAG_INITIALIZED;
+	bo->debugfs.flags = usage_flags;
+	panthor_gem_debugfs_bo_add(bo);
 }
 #else
-static void panthor_gem_debugfs_bo_add(struct panthor_device *ptdev,
-				       struct panthor_gem_object *bo)
-{}
 static void panthor_gem_debugfs_bo_rm(struct panthor_gem_object *bo) {}
 static void panthor_gem_debugfs_set_usage_flags(struct panthor_gem_object *bo, u32 usage_flags) {}
+static void panthor_gem_debugfs_bo_init(struct panthor_gem_object *bo) {}
 #endif
 
 static void panthor_gem_free_object(struct drm_gem_object *obj)
···
 	drm_gem_gpuva_set_lock(&obj->base.base, &obj->gpuva_list_lock);
 	mutex_init(&obj->label.lock);
 
-	panthor_gem_debugfs_bo_add(ptdev, obj);
+	panthor_gem_debugfs_bo_init(obj);
 
 	return &obj->base.base;
 }
···
 		bo->base.base.resv = bo->exclusive_vm_root_gem->resv;
 	}
 
+	panthor_gem_debugfs_set_usage_flags(bo, 0);
+
 	/*
 	 * Allocate an id of idr table where the obj is registered
 	 * and handle has the id what user can see.
···
 
 	/* drop reference from allocate - handle holds it now. */
 	drm_gem_object_put(&shmem->base);
-
-	/*
-	 * No explicit flags are needed in the call below, since the
-	 * function internally sets the INITIALIZED bit for us.
-	 */
-	panthor_gem_debugfs_set_usage_flags(bo, 0);
 
 	return ret;
 }
···
 	unsigned int refcount = kref_read(&bo->base.base.refcount);
 	char creator_info[32] = {};
 	size_t resident_size;
-	u32 gem_usage_flags = bo->debugfs.flags & (u32)~PANTHOR_DEBUGFS_GEM_USAGE_FLAG_INITIALIZED;
+	u32 gem_usage_flags = bo->debugfs.flags;
 	u32 gem_state_flags = 0;
 
 	/* Skip BOs being destroyed. */
···
 
 	scoped_guard(mutex, &ptdev->gems.lock) {
 		list_for_each_entry(bo, &ptdev->gems.node, debugfs.node) {
-			if (bo->debugfs.flags & PANTHOR_DEBUGFS_GEM_USAGE_FLAG_INITIALIZED)
-				panthor_gem_debugfs_bo_print(bo, m, &totals);
+			panthor_gem_debugfs_bo_print(bo, m, &totals);
 		}
 	}
 

drivers/gpu/drm/panthor/panthor_gem.h

···
 
 	/** @PANTHOR_DEBUGFS_GEM_USAGE_FLAG_FW_MAPPED: BO is mapped on the FW VM. */
 	PANTHOR_DEBUGFS_GEM_USAGE_FLAG_FW_MAPPED = BIT(PANTHOR_DEBUGFS_GEM_USAGE_FW_MAPPED_BIT),
-
-	/** @PANTHOR_DEBUGFS_GEM_USAGE_FLAG_INITIALIZED: BO is ready for DebugFS display. */
-	PANTHOR_DEBUGFS_GEM_USAGE_FLAG_INITIALIZED = BIT(31),
 };
 
 /**