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

[Zwane Mwaikambo]

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().

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;
}