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

["Das, Nirmoy" <nirmoy.das@intel.com>]

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 ?


Thanks,

Nirmoy

>
> Regards,
>
> Tvrtko