[Intel-gfx] i915: NULL pointer dereference in i915_update_dri1_breadcrumb() during shutdown

[Eugene Shatokhin]

On 12/10/2013 04:23 PM, Daniel Vetter wrote:
> On Tue, Dec 10, 2013 at 12:27:55PM +0400, Eugene Shatokhin wrote:
>> Hi,
>>
>> I have recently observed a NULL pointer dereference in i915 driver
>> on my Eee PC running ROSA Linux with kernel 3.10.21.
>>
>> The crash occurs during shutdown but quite rarely, not each time.
>>
>> The system log is lost but here is what I extracted from the info
>> displayed on the screen.
>>
>> NULL pointer dereference at 0x4
>> EIP is at i915_update_dri1_breadcrumb+0x25/0x70
>> comm: systemd-journal
>>
>> i915_update_dri1_breadcrumb+0x25:
>> 	mov    0x4(%eax),%ebx // %eax contains 0, the list of register
>> values confirms that.
>>
>> That is the reading of 'master_priv->sarea_priv':
>>
>> void i915_update_dri1_breadcrumb(struct drm_device *dev)
>> {
>> 	drm_i915_private_t *dev_priv = dev->dev_private;
>> 	struct drm_i915_master_private *master_priv;
>>
>> 	if (dev->primary->master) {
>> 		master_priv = dev->primary->master->driver_priv;
>> 		if (master_priv->sarea_priv) // <<< crashes here
>> 			master_priv->sarea_priv->last_dispatch =
>> 				READ_BREADCRUMB(dev_priv);
>> 	}
>> }
>
> Indeed, that's fairly hapzardous. But also only needed for legacy ums
> support. I'll send out a patch to block this in kms mode quickly. Please
> test it.
>
> Thanks, Daniel

Thanks a lot!

I am building kernel 3.10.23 now with this patch applied and will let 
you know if something goes wrong.

Regards,
Eugene

-- 
Eugene Shatokhin, ROSA Laboratory.
www.rosalab.com