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

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Message-ID: <77ba52f4-dcdc-4fdd-97b7-0163e54e8836@mandelbit.com>
Date: Wed, 30 Oct 2024 22:06:36 +0100
From: Antonio Quartulli <antonio@...delbit.com>
To: Mario Limonciello <mario.limonciello@....com>
Cc: amd-gfx@...ts.freedesktop.org, alexander.deucher@....com,
 christian.koenig@....com, Xinhui.Pan@....com,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] amdgpu: prevent NULL pointer dereference if ATIF is not
 supported

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@...delbit.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.


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