Re: [PATCH] acpi: cppc: Prevent cpc_desc_ptr points to the invalid data

[Hoan Tran]

Hi Ashwin,


On Tue, May 31, 2016 at 12:29 PM, Ashwin Chaugule
<ashwin.chaugule@xxxxxxxxxx> wrote:
> On 27 May 2016 at 12:41, Hoan Tran <hotran@xxxxxxx> wrote:
>> Hi Ashwin,
>
> Hi,
>
>>
>> Yes, I saw kernel crash.
>> As cpc_desc_ptr is not NULL, cppc_cpufreq_init() still can pass then
>> crash during cppc_get_perf_caps() access CPPC shared memory.
>>
>> It's not only "PCC channel request fail" can create this issue but
>> "acpi_get_psd() fail" also creates it
>>
>> Thanks
>> Hoan
>
>
> Just for future reference. :)
>
> https://web.archive.org/web/20080722025748/http://www.zip.com.au/~akpm/linux/patches/stuff/top-posting.txt

My mistake and will fix in future :)

>
>
>>
>> On Fri, May 27, 2016 at 9:10 AM, Ashwin Chaugule
>> <ashwin.chaugule@xxxxxxxxxx> wrote:
>>> On 25 May 2016 at 15:09, Hoan Tran <hotran@xxxxxxx> wrote:
>>>> When CPPC fails to request PCC channel, the CPC data is freed
>>>> and cpc_desc_ptr points to the invalid data. This change prevents
>>>> this issue by moving cpc_desc_ptr assignment after PCC channel
>>>> request.
>>>>
>>>> Signed-off-by: Hoan Tran <hotran@xxxxxxx>
>>>> ---
>>>>  drivers/acpi/cppc_acpi.c | 6 +++---
>>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
>>>> index 8adac69..85fd8f7 100644
>>>> --- a/drivers/acpi/cppc_acpi.c
>>>> +++ b/drivers/acpi/cppc_acpi.c
>>>> @@ -595,9 +595,6 @@ int acpi_cppc_processor_probe(struct acpi_processor *pr)
>>>>         /* Store CPU Logical ID */
>>>>         cpc_ptr->cpu_id = pr->id;
>>>>
>>>> -       /* Plug it into this CPUs CPC descriptor. */
>>>> -       per_cpu(cpc_desc_ptr, pr->id) = cpc_ptr;
>>>> -
>>>>         /* Parse PSD data for this CPU */
>>>>         ret = acpi_get_psd(cpc_ptr, handle);
>>>>         if (ret)
>>>> @@ -610,6 +607,9 @@ int acpi_cppc_processor_probe(struct acpi_processor *pr)
>>>>                         goto out_free;
>>>>         }
>>>>
>>>> +       /* Plug PSD data into this CPUs CPC descriptor. */
>>>> +       per_cpu(cpc_desc_ptr, pr->id) = cpc_ptr;
>>>> +
>>>
>>> Are you seeing a real problem without this change? I'm missing where
>>> this pointer is dereferenced if the PCC channel request fails.
>>>
>
> So, after freeing the cpc_ptr, we need to NULL the per-cpu pointer as
> well. Alternately, not assign it until everything passes and rely on
> the static declaration, which is the path you've taken here.
>
> Acked-by: Ashwin Chaugule <ashwin.chaugule@xxxxxxxxxx>

Thanks
Hoan