RE: [PATCH] cpufreq: pcc-cpufreq: sanity check to prevent a NULLpointer dereference

[Chumbalkar, Nagananda]

>-----Original Message-----
>From: Andrew Morton [mailto:akpm@xxxxxxxxxxxxxxxxxxxx]
>Sent: Thursday, September 08, 2011 6:53 PM
>To: Chumbalkar, Nagananda
>Cc: davej@xxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
>cpufreq@xxxxxxxxxxxxxxx; stable@xxxxxxxxxx
>Subject: Re: [PATCH] cpufreq: pcc-cpufreq: sanity check to prevent a
>NULL pointer dereference
>
>On Wed, 31 Aug 2011 20:08:45 +0000 (UTC)
>Naga Chumbalkar <nagananda.chumbalkar@xxxxxx> wrote:
>
>> If, for whatever reason, "pr" ends up being NULL we would end up in a
>PANIC
>> as seen below:
>>
>> Loading CPUFreq modules[  437.661360] BUG: unable to handle kernel
>NULL pointer
>> dereference at (null)
>> IP: [<ffffffffa0434314>] pcc_cpufreq_cpu_init+0x74/0x220 [pcc_cpufreq]
>>
>> It's better to prevent the PANIC by failing the driver, and allowing
>the system to boot.
>>
>> Signed-off-by: Naga Chumbalkar <nagananda.chumbalkar@xxxxxx>
>> Cc: stable@xxxxxxxxxx
>>
>> diff --git a/drivers/cpufreq/pcc-cpufreq.c b/drivers/cpufreq/pcc-
>cpufreq.c
>> index 7b0603e..cdc02ac 100644
>> --- a/drivers/cpufreq/pcc-cpufreq.c
>> +++ b/drivers/cpufreq/pcc-cpufreq.c
>> @@ -261,6 +261,9 @@ static int pcc_get_offset(int cpu)
>>  	pr = per_cpu(processors, cpu);
>>  	pcc_cpu_data = per_cpu_ptr(pcc_cpu_info, cpu);
>>
>> +	if (!pr)
>> +		return -ENODEV;
>> +
>>  	status = acpi_evaluate_object(pr->handle, "PCCP", NULL, &buffer);
>>  	if (ACPI_FAILURE(status))
>>  		return -ENODEV;
>
>hm, from reading drivers/acpi/processor_driver.c it appears that
>per_cpu(processors, n)==NULL is an expected and valid state.  Heaven
>knows what that state actually *means* - apparently this is a secret.
>
>I assume that you've hit this crash in real live code, hence your
>suggestion of a -stable backport?

Yes - the above oops can be reproduced on a shipping Linux distro when, 
for x2APIC implementations, the BIOS does not use the ACPI "Device" 
declaration for processors with Local APIC ID >= 255. Such a declaration 
is necessitated by Sec 8.4 of ACPI 4.0a.

- naga -


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