Re: [PATCH] drm: inhibit drm drivers register to uninitialized drm core

[Daniel Vetter]

On Sat, Jul 08, 2017 at 11:43:52PM +0200, Alexandru Moise wrote:
> 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@xxxxxxxxx>

Applied to drm-misc-next for 4.14. Since Greg has a pach in 4.13 already
for the chardev issue it feels like this isn't justified for pushing
through -rc/stable kernels since too much a corner-case of a bug (it
exists since decades after all).

Thanks, Daniel

> ---
>  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
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch