[Intel-gfx] [PATCH] drm/i915: Fix vm use-after-free in vma destruction

[Thomas Hellström]

Hi, Andi

On 5/19/22 23:46, Andi Shyti wrote:
> Hi Thomas,
>
> On Thu, May 12, 2022 at 11:40:45AM +0200, Thomas Hellström wrote:
>> In vma destruction, the following race may occur:
>>
>> Thread 1:	    		  Thread 2:
>> i915_vma_destroy();
>>
>>    ...
>>    list_del_init(vma->vm_link);
>>    ...
>>    mutex_unlock(vma->vm->mutex);
>> 				  __i915_vm_release();
>> release_references();
>>
>> And in release_reference() we dereference vma->vm to get to the
>> vm gt pointer, leadin go a use-after free.
> leading to
Thanks, will fix.
>
> [...]
>
>> -static void release_references(struct i915_vma *vma, bool vm_ddestroy)
>> +static void release_references(struct i915_vma *vma, struct intel_gt *gt,
>> +			       bool vm_ddestroy)
>>   {
>>   	struct drm_i915_gem_object *obj = vma->obj;
>> -	struct intel_gt *gt = vma->vm->gt;
>>   
>>   	GEM_BUG_ON(i915_vma_is_active(vma));
> but then we have
>
> 	if (vm_ddestroy)
> 		i915_vm_resv_put(vma->vm);
>
> were we reference to a freed vm, right? Do we need to check it
> here, as well?

No, it's not needed, since if vm_ddestroy is true, we keep the vm alive 
using the vm
resv_ref until i915_vm_resv_put(). This is for the rare occasions where, 
during vm destruction, we fail to grab an object reference and therefore 
vma destruction is left for the object destructor. In those cases the 
vma needs to keep the vm in memory for it to be able to grab the vm mutex.

/Thomas




>
> Andi