[PATCH] drm: assure aux_dev is nonzero before using it

[Daniel Vetter]

On Mon, Aug 10, 2020 at 10:11:50AM -0700, Zwane Mwaikambo wrote:
> Hi Folks,
> 	I know this thread eventually dropped off due to not identifying 
> the underlying issue. It's still occuring on 5.8 and in my case it 
> happened because the udev device nodes for the DP aux devices were not 
> cleaned up whereas the kernel had no association with them. I can 
> reproduce the bug just by creating a device node for a non-existent minor 
> device and calling open().

Hm I don't have that thread anymore, but generally these bugs are solved
by not registering the device before it's ready for use. We do have
drm_connector->late_register for that stuff. Just a guess since I'm not
seeing full details here.
-Daniel

> 
> To me it still makes sense to just check aux_dev because the chardev has 
> no way to check before calling.
> 
> (gdb) list *drm_dp_aux_dev_get_by_minor+0x29
> 0x17b39 is in drm_dp_aux_dev_get_by_minor (drivers/gpu/drm/drm_dp_aux_dev.c:65).
> 60      static struct drm_dp_aux_dev *drm_dp_aux_dev_get_by_minor(unsigned index)
> 61      {
> 62              struct drm_dp_aux_dev *aux_dev = NULL;
> 63
> 64              mutex_lock(&aux_idr_mutex);
> 65              aux_dev = idr_find(&aux_idr, index);
> 66              if (!kref_get_unless_zero(&aux_dev->refcount))
> 67                      aux_dev = NULL;
> 68              mutex_unlock(&aux_idr_mutex);
> 69
> (gdb) p/x &((struct drm_dp_aux_dev *)(0x0))->refcount
> $8 = 0x18
> 
> static int auxdev_open(struct inode *inode, struct file *file)
> {
>     unsigned int minor = iminor(inode);
>     struct drm_dp_aux_dev *aux_dev;
> 
>     aux_dev = drm_dp_aux_dev_get_by_minor(minor);
>     if (!aux_dev)
>         return -ENODEV;
> 
>     file->private_data = aux_dev;
>     return 0;
> }
> 
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch