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

[Jeffrey Hugo <quic_jhugo@quicinc.com>]

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