Re: [Intel-gfx] [PATCH 1/2] drm/i915: Fix a potential UAF at device unload

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 22/09/2022 13:11, Das, Nirmoy wrote:
> 
> On 9/22/2022 11:37 AM, Tvrtko Ursulin wrote:
>>
>> On 21/09/2022 16:53, Das, Nirmoy wrote:
>>>
>>> On 9/9/2022 10:55 AM, Tvrtko Ursulin wrote:
>>>>
>>>> On 08/09/2022 21:07, Nirmoy Das wrote:
>>>>> i915_gem_drain_freed_objects() might not be enough to
>>>>> free all the objects and RCU delayed work might get
>>>>> scheduled after the i915 device struct gets freed.
>>>>>
>>>>> Call i915_gem_drain_workqueue() to catch all RCU delayed work.
>>>>>
>>>>> Suggested-by: Chris Wilson <chris.p.wilson@intel.com>
>>>>> Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
>>>>> ---
>>>>>   drivers/gpu/drm/i915/i915_gem.c | 2 +-
>>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/i915/i915_gem.c 
>>>>> b/drivers/gpu/drm/i915/i915_gem.c
>>>>> index 0f49ec9d494a..e8a053eaaa89 100644
>>>>> --- a/drivers/gpu/drm/i915/i915_gem.c
>>>>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>>>>> @@ -1254,7 +1254,7 @@ void i915_gem_init_early(struct 
>>>>> drm_i915_private *dev_priv)
>>>>>     void i915_gem_cleanup_early(struct drm_i915_private *dev_priv)
>>>>>   {
>>>>> -    i915_gem_drain_freed_objects(dev_priv);
>>>>> +    i915_gem_drain_workqueue(dev_priv);
>>>>> GEM_BUG_ON(!llist_empty(&dev_priv->mm.free_list));
>>>>> GEM_BUG_ON(atomic_read(&dev_priv->mm.free_count));
>>>>>       drm_WARN_ON(&dev_priv->drm, dev_priv->mm.shrink_count);
>>>>
>>>>
>>>> Help me spot the place where RCU free worker schedules itself back 
>>>> to free more objects - if I got the rationale here right?
>>> (Sorry for late reply, was on leave last week.)
>>>
>>> I had to clarify this with Chris. So when driver frees a obj, it does 
>>> dma_resv_fini() which will drop reference
>>>
>>> for all the fences in it and a fence might  reference to an object 
>>> and upon release of that fence can trigger a  release reference to an 
>>> object.
>>
>> Hmm I couldn't find that in code but never mind. It's just a stronger 
>> version of the same flushing and it's not on a path where speed 
>> matters so feel free to go with it.
> 
> 
> Can I get a Ack from you for this, Tvrtko ?

Sorry yes, forgot to be explicit.

Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko