Date: Sat, 8 Jul 2017 23:43:52 +0200
From: Alexandru Moise <00moses.alexander00@gmail.com>
To: linux-kernel@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org, airlied@linux.ie,
        seanpaul@chromium.org, jani.nikula@linux.intel.com,
        daniel.vetter@intel.com
Subject: [PATCH] drm: inhibit drm drivers register to uninitialized drm core
Message-ID: <20170708214352.GA27205@gmail.com>
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If the DRM core fails to init for whatever reason, ensure that
no driver ever calls drm_dev_register().

This is best done at drm_dev_init() as it covers drivers that call
drm_dev_alloc() as well as drivers that prefer to embed struct
drm_device into their own device struct and call drm_dev_init()
themselves.

In my case I had so many dynamic device majors used that the major
number for DRM (226) was stolen, causing DRM core init to fail after
failing to register a chrdev, and ultimately calling debugfs_remove()
on drm_debugfs_root in drm_core_exit().

After drm core failed to init, VGEM was still calling drm_dev_register(),
ultimately leading to drm_debugfs_init(), with drm_debugfs_root passed
as the root for the new debugfs dir at debugfs_create_dir().

This led to a kernel panic once we were either derefencing root->d_inode
while it was NULL or calling root->d_inode->i_op->lookup() while it was
NULL in debugfs at inode_lock() or lookup_*().

Signed-off-by: Alexandru Moise <00moses.alexander00@gmail.com>
---
 drivers/gpu/drm/drm_drv.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 37b8ad3e30d8..2ed2d919beae 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -63,6 +63,15 @@ module_param_named(debug, drm_debug, int, 0600);
 static DEFINE_SPINLOCK(drm_minor_lock);
 static struct idr drm_minors_idr;
 
+/*
+ * If the drm core fails to init for whatever reason,
+ * we should prevent any drivers from registering with it.
+ * It's best to check this at drm_dev_init(), as some drivers
+ * prefer to embed struct drm_device into their own device
+ * structure and call drm_dev_init() themselves.
+ */
+static bool drm_core_init_complete = false;
+
 static struct dentry *drm_debugfs_root;
 
 #define DRM_PRINTK_FMT "[" DRM_NAME ":%s]%s %pV"
@@ -484,6 +493,11 @@ int drm_dev_init(struct drm_device *dev,
 {
 	int ret;
 
+	if (!drm_core_init_complete) {
+		DRM_ERROR("DRM core is not initialized\n");
+		return -ENODEV;
+	}
+
 	kref_init(&dev->ref);
 	dev->dev = parent;
 	dev->driver = driver;
@@ -966,6 +980,8 @@ static int __init drm_core_init(void)
 	if (ret < 0)
 		goto error;
 
+	drm_core_init_complete = true;
+
 	DRM_DEBUG("Initialized\n");
 	return 0;
 
-- 
2.13.2