Re: [PATCH V3] cpufreq: fix a NULL pointer dereference triggered by _PPC changed notification

[ethan zhao <ethan.zhao@oracle.com>]

Viresh,

On 2014/12/18 14:29, Viresh Kumar wrote:
> On 18 December 2014 at 11:58, Ethan Zhao <ethan.zhao@oracle.com> wrote:
>> If _PPC changed notification happens before governor was initiated while kernel
>> is booting, a NULL pointer dereference will be triggered:
>>
>> BUG: unable to handle kernel NULL pointer dereference at 0000000000000030
>>   IP: [<ffffffff81470453>] __cpufreq_governor+0x23/0x1e0
>>   PGD 0
>>   Oops: 0000 [#1] SMP
>>   ... ...
>>   RIP: 0010:[<ffffffff81470453>]  [<ffffffff81470453>]
>>   __cpufreq_governor+0x23/0x1e0
>>   RSP: 0018:ffff881fcfbcfbb8  EFLAGS: 00010286
>>   RAX: 0000000000000000 RBX: ffff881fd11b3980 RCX: ffff88407fc20000
>>   RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff881fd11b3980
>>   RBP: ffff881fcfbcfbd8 R08: 0000000000000000 R09: 000000000000000f
>>   R10: ffffffff818068d0 R11: 0000000000000043 R12: 0000000000000004
>>   R13: 0000000000000000 R14: ffffffff8196cae0 R15: 0000000000000000
>>   FS:  0000000000000000(0000) GS:ffff881fffc00000(0000) knlGS:0000000000000000
>>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>   CR2: 0000000000000030 CR3: 00000000018ae000 CR4: 00000000000407f0
>>   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>   DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
>>   Process kworker/0:3 (pid: 750, threadinfo ffff881fcfbce000, task
>>   ffff881fcf556400)
>>   Stack:
>>    ffff881fffc17d00 ffff881fcfbcfc18 ffff881fd11b3980 0000000000000000
>>    ffff881fcfbcfc08 ffffffff81470d08 ffff881fd11b3980 0000000000000007
>>    ffff881fcfbcfc18 ffff881fffc17d00 ffff881fcfbcfd28 ffffffff81472e9a
>>   Call Trace:
>>    [<ffffffff81470d08>] __cpufreq_set_policy+0x1b8/0x2e0
>>    [<ffffffff81472e9a>] cpufreq_update_policy+0xca/0x150
>>    [<ffffffff81472f20>] ? cpufreq_update_policy+0x150/0x150
>>    [<ffffffff81324a96>] acpi_processor_ppc_has_changed+0x71/0x7b
>>    [<ffffffff81320bcd>] acpi_processor_notify+0x55/0x115
>>    [<ffffffff812f9c29>] acpi_device_notify+0x19/0x1b
>>    [<ffffffff813084ca>] acpi_ev_notify_dispatch+0x41/0x5f
>>    [<ffffffff812f64a4>] acpi_os_execute_deferred+0x27/0x34
>>
>> The root cause is a race conditon -- cpufreq core and acpi-cpufreq driver
>> were initiated, but cpufreq_governor wasn't and _PPC changed notification
>> happened, __cpufreq_governor() was called within acpi_os_execute_deferred
>> kernel thread context.
>>
>> To fix this panic issue, add pointer checking code in __cpufreq_governor()
>> before pointer policy->governor is to be dereferenced.
>>
>> Signed-off-by: Ethan Zhao <ethan.zhao@oracle.com>
>> Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
>> ---
>>   v2&v3: correct comment style.
>>
>>   drivers/cpufreq/cpufreq.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> index 4473eba..b75735c 100644
>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -2021,6 +2021,11 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
>>          /* Don't start any governor operations if we are entering suspend */
>>          if (cpufreq_suspended)
>>                  return 0;
>> +       /*
>> +        * Governor might not be initiated here if _PPC changed notification
>> +        * happened, check it.
>> +        */
>> +       if (!policy->governor)
>> +               return -EINVAL;
>>
>>          if (policy->governor->max_transition_latency &&
>>              policy->cpuinfo.transition_latency >
> It will take me sometime to get cpufreq core simplified, till that time
>
> Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
>
> to yet another band-aid :)
  Maybe more band-aid needed before it is be done.

  Thanks,
  Ethan