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

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Message-ID: <20170710071453.GA16847@gmail.com>
Date:   Mon, 10 Jul 2017 09:14:53 +0200
From:   Alexandru Moise <00moses.alexander00@...il.com>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     linux-kernel@...r.kernel.org, daniel.vetter@...el.com,
        dri-devel@...ts.freedesktop.org, airlied@...ux.ie,
        seanpaul@...omium.org, jani.nikula@...ux.intel.com
Subject: Re: [PATCH] drm: inhibit drm drivers register to uninitialized drm
 core

On Mon, Jul 10, 2017 at 08:52:46AM +0200, Daniel Vetter wrote:
> 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@...il.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;
> 
> Isn't the correct fix to pass down the error value, which iirc should
> make the kmod stuff unload the module again? Or does this not work'
> -Daniel

What if everything is built in?

../Alex

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