Re: [PATCH] amdgpu: prevent NULL pointer dereference if ATIF is not supported

[Antonio Quartulli <antonio@mandelbit.com>]

On 31/10/2024 15:41, Mario Limonciello wrote:
> On 10/30/2024 16:06, Antonio Quartulli wrote:
>> Hi Mario,
>>
>> On 30/10/2024 02:41, Mario Limonciello wrote:
>>> On 10/29/2024 18:32, Antonio Quartulli wrote:
>>>> acpi_evaluate_object() may return AE_NOT_FOUND (failure), which
>>>> would result in dereferencing buffer.pointer (obj) while being NULL.
>>>>
>>>> Bail out also when status is AE_NOT_FOUND with a proper error message.
>>>>
>>>> This fixes 1 FORWARD_NULL issue reported by Coverity
>>>> Report: CID 1600951:  Null pointer dereferences  (FORWARD_NULL)
>>>>
>>>> Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
>>>
>>> I'm not really sure how realistic this failure is.  Can you share the 
>>> full call trace that Coverity identified?
>>
>> I just checked Coverity Scan and it only says:
>>
>>      5. Condition status, taking true branch.
>>      6. Condition status != 5U /* (acpi_status)(5 | 0) */, taking 
>> false branch.
>>
>> The above points are related to:
>>
>>      if (ACPI_FAILURE(status) && status != AE_NOT_FOUND)
>>
>> It doesn't show how acpi_evaluate_object() is expected to return 
>> AE_NOT_FOUND.
>>
>> This said, if you think this case is unrealistic, why do you check for 
>> "status != AE_NOT_FOUND" at all?
>>
>> At this point maybe it would make more sense to simply drop this check 
>> and always bail out with the current error message.
>>
>> Basically a patch with the following only:
>>
>> -       /* Fail if calling the method fails and ATIF is supported */
>> -       if (ACPI_FAILURE(status) && status != AE_NOT_FOUND) {
>> +       /* Fail if calling the method fails */
>> +       if (ACPI_FAILURE(status)) {
>>
>> This way we don't make a fuzz for a possibly unrealistic case, while 
>> still protecting against bugs and null-dereferences.
> 
> Yeah I think that's a good idea.  Can you respin it as a v2?

Will do!
Thanks for your feedback, Mario.

Regards,

> 
>>
>>
>> Regards,
>>
>>>
>>> amdgpu_atif_pci_probe_handle() will check whether the handle is 
>>> available in the first place.  We'll never this this far if that failed.
>>>
>>> Because of that I don't follow how this could return AE_NOT_FOUND.
>>>> ---
>>>>   drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c | 11 +++++++----
>>>>   1 file changed, 7 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c b/drivers/gpu/ 
>>>> drm/amd/amdgpu/amdgpu_acpi.c
>>>> index cce85389427f..f10c3261a4ab 100644
>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
>>>> @@ -172,10 +172,13 @@ static union acpi_object 
>>>> *amdgpu_atif_call(struct amdgpu_atif *atif,
>>>>                         &buffer);
>>>>       obj = (union acpi_object *)buffer.pointer;
>>>> -    /* Fail if calling the method fails and ATIF is supported */
>>>> -    if (ACPI_FAILURE(status) && status != AE_NOT_FOUND) {
>>>> -        DRM_DEBUG_DRIVER("failed to evaluate ATIF got %s\n",
>>>> -                 acpi_format_exception(status));
>>>> +    /* Fail if calling the method fails */
>>>> +    if (ACPI_FAILURE(status)) {
>>>> +        if (status != AE_NOT_FOUND)
>>>> +            DRM_DEBUG_DRIVER("failed to evaluate ATIF got %s\n",
>>>> +                     acpi_format_exception(status));
>>>> +        else
>>>> +            DRM_DEBUG_DRIVER("ATIF not supported\n");
>>>>           kfree(obj);
>>>>           return NULL;
>>>>       }
>>>
>>
> 

-- 
Antonio Quartulli

CEO and Co-Founder
Mandelbit Srl
https://www.mandelbit.com