Re: [PATCH 1/2] drm/ttm: set ttm_buffer_object pointer as null after it's freed

["Christian König" <ckoenig.leichtzumerken-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Am 10.09.2018 um 14:05 schrieb Huang Rui:
> On Mon, Sep 10, 2018 at 05:25:48PM +0800, Koenig, Christian wrote:
>> Am 10.09.2018 um 11:23 schrieb Huang Rui:
>>> On Mon, Sep 10, 2018 at 11:00:04AM +0200, Christian König wrote:
>>>> Hi Ray,
>>>>
>>>> well those patches doesn't make sense, the pointer is only local to
>>>> the function.
>>> You're right.
>>> I narrowed it with gdb dump from ttm_bo_bulk_move_lru_tail+0x2b, the
>>> use-after-free should be in below codes:
>>>
>>> man = &bulk->tt[i].first->bdev->man[TTM_PL_TT];
>>> ttm_bo_bulk_move_helper(&bulk->tt[i], &man->lru[i], false);
>>>
>>> Is there a case, when orignal bo is destroyed in the bulk pos, but it
>>> doesn't update pos->first pointer, then we still use it during the bulk
>>> moving?
>> Only when a per VM BO is freed or the VM destroyed.
>>
>> The first case should now be handled by "drm/amdgpu: set bulk_moveable
>> to false when a per VM is released" and when we use a destroyed VM we
>> would see other problems as well.
>>
> If a VM instance is teared down, all BOs which belong that VM should be
> removed from LRU. But how can we submit cmd based on a destroyed VM? You
> know, we do the bulk move at last step of submission.

Well exactly that's the point this can't happen :)

Otherwise we would crash because of using freed up memory much earlier 
in the command submission.

The best idea I have to track this down further is to add some 
trace_printk in ttm_bo_bulk_move_helper and amdgpu_bo_destroy and see 
why and when we are actually using a destroyed BO.

Christian.

>
>
> Thanks,
> Ray
>
>> BTW: Just pushed this commit to the repository, should show up any second.
>>
>> Christian.
>>
>>> Thanks,
>>> Ray
>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>> Am 10.09.2018 um 10:57 schrieb Huang Rui:
>>>>> It avoids to be refered again after freed.
>>>>>
>>>>> Signed-off-by: Huang Rui <ray.huang@amd.com>
>>>>> Cc: Christian König <christian.koenig@amd.com>
>>>>> Cc: Tom StDenis <Tom.StDenis@amd.com>
>>>>> ---
>>>>>    drivers/gpu/drm/ttm/ttm_bo.c | 1 +
>>>>>    1 file changed, 1 insertion(+)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
>>>>> index 138c989..d3ef5f8 100644
>>>>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>>>>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>>>>> @@ -54,6 +54,7 @@ static struct attribute ttm_bo_count = {
>>>>>    static void ttm_bo_default_destroy(struct ttm_buffer_object *bo)
>>>>>    {
>>>>>    	kfree(bo);
>>>>> +	bo = NULL;
>>>>>    }
>>>>>    static inline int ttm_mem_type_from_place(const struct ttm_place *place,
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