linux-kernel - Re: [PATCH] acpi: cppc: Prevent cpc_desc

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Date:	Fri, 27 May 2016 09:41:40 -0700
From:	Hoan Tran <hotran@....com>
To:	Ashwin Chaugule <ashwin.chaugule@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	linux acpi <linux-acpi@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>, Duc Dang <dhdang@....com>,
	Loc Ho <lho@....com>
Subject: Re: [PATCH] acpi: cppc: Prevent cpc_desc_ptr points to the invalid data

Hi Ashwin,

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

On Fri, May 27, 2016 at 9:10 AM, Ashwin Chaugule
<ashwin.chaugule@...aro.org> wrote:
> On 25 May 2016 at 15:09, Hoan Tran <hotran@....com> 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@....com>
>> ---
>>  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.
>
> Thanks,
> Ashwin.