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

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

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.

Regards,

Nirmoy



>
> Regards,
>
> Tvrtko