drm/vkms: Fix use after free and double free on init error

[ Upstream commit ed15511 ]

If the driver initialization fails, the vkms_exit() function might
access an uninitialized or freed default_config pointer and it might
double free it.

Fix both possible errors by initializing default_config only when the
driver initialization succeeded.

Reported-by: Louis Chauvet <louis.chauvet@bootlin.com>
Closes: https://lore.kernel.org/all/Z5uDHcCmAwiTsGte@louis-chauvet-laptop/
Fixes: 2df7af9 ("drm/vkms: Add vkms_config type")
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmremann@suse.de>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250212084912.3196-1-jose.exposito89@gmail.com
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/gpu/drm/vkms/vkms_drv.c

@@ -243,17 +243,19 @@ static int __init vkms_init(void)
 	if (!config)
 		return -ENOMEM;
 
-	default_config = config;
-
 	config->cursor = enable_cursor;
 	config->writeback = enable_writeback;
 	config->overlay = enable_overlay;
 
 	ret = vkms_create(config);
-	if (ret)
+	if (ret) {
 		kfree(config);
+		return ret;
+	}
 
-	return ret;
+	default_config = config;
+
+	return 0;
 }
 
 static void vkms_destroy(struct vkms_config *config)
@@ -277,9 +279,10 @@ static void vkms_destroy(struct vkms_config *config)
 
 static void __exit vkms_exit(void)
 {
-	if (default_config->dev)
-		vkms_destroy(default_config);
+	if (!default_config)
+		return;
 
+	vkms_destroy(default_config);
 	kfree(default_config);
 }