Re: [PATCH] accel/qaic: Fix dereferencing freed memory

[Pranjal Ramajor Asha Kanojiya <quic_pkanojiy@quicinc.com>]

On 6/12/2023 8:39 PM, Jeffrey Hugo wrote:
> On 6/12/2023 7:21 AM, Christian König wrote:
>> Am 12.06.23 um 15:03 schrieb Pranjal Ramajor Asha Kanojiya:
>>>
>>>
>>> On 6/12/2023 4:52 PM, Christian König wrote:
>>>>
>>>>
>>>> Am 10.06.23 um 04:12 schrieb Sukrut Bellary:
>>>>> smatch warning:
>>>>>     drivers/accel/qaic/qaic_data.c:620 qaic_free_object() error:
>>>>>         dereferencing freed memory 'obj->import_attach'
>>>>>
>>>>> obj->import_attach is detached and freed using dma_buf_detach().
>>>>> But used after free to decrease the dmabuf ref count using
>>>>> dma_buf_put().
>>>>>
>>>>> Fixes: ff13be830333 ("accel/qaic: Add datapath")
>>>>> Signed-off-by: Sukrut Bellary <sukrut.bellary@linux.com>
>>>>> ---
>>>>>   drivers/accel/qaic/qaic_data.c | 4 +++-
>>>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/accel/qaic/qaic_data.c 
>>>>> b/drivers/accel/qaic/qaic_data.c
>>>>> index e42c1f9ffff8..7cba4d680ea8 100644
>>>>> --- a/drivers/accel/qaic/qaic_data.c
>>>>> +++ b/drivers/accel/qaic/qaic_data.c
>>>>> @@ -613,11 +613,13 @@ static int qaic_gem_object_mmap(struct 
>>>>> drm_gem_object *obj, struct vm_area_struc
>>>>>   static void qaic_free_object(struct drm_gem_object *obj)
>>>>>   {
>>>>>       struct qaic_bo *bo = to_qaic_bo(obj);
>>>>> +    struct dma_buf *dmabuf;
>>>>
>>>> Maybe move that variable into the if.
>>>>
>>>>>       if (obj->import_attach) {
>>>>>           /* DMABUF/PRIME Path */
>>>>> +        dmabuf = obj->import_attach->dmabuf;
>>>>>           dma_buf_detach(obj->import_attach->dmabuf, 
>>>>> obj->import_attach);
>>>>> -        dma_buf_put(obj->import_attach->dmabuf);
>>>>> +        dma_buf_put(dmabuf);
>>>>
>>>> I strongly assume you are not using the GEM prime helpers for this?
>>>>
>>>> Christian.
>>>
>>> Driver uses drm_gem_prime_fd_to_handle() helper function but it also 
>>> registers for ->gem_prime_import() which is internally called by 
>>> drm_gem_prime_fd_to_handle(). All the operations done in 
>>> gem_prime_import() are undone here.
>>
>> Then why don't you use drm_prime_gem_destroy() which is the cleanup 
>> helper for imports created by ->gem_prime_import() ?
>>
>> That looks pretty much identical to what you do here manually.
> 
> I think destroy() wasn't used because we are new to DRM and sometimes 
> confused by the multitude of options.  I appreciate the suggestion and 
> will follow up to see if destroy() will work here, or identify what 
> would prevent the use of it.
> 
> -Jeff

Thank you Christian for your suggestion. I agree with you driver can use 
  drm_prime_gem_destroy() here.