linux-kernel - Re: [PATCH] cpuidle: Avoid possible NULL pointer dereference in __cpuidle_register

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Message-ID: <4F7B007C.50201@linaro.org>
Date:	Tue, 03 Apr 2012 15:51:56 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
CC:	lenb@...nel.org, khilman@...com, deepthi@...ux.vnet.ibm.com,
	g.trinabh@...il.com, arjan@...radead.org,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	amit.kucheria@...aro.org
Subject: Re: [PATCH] cpuidle: Avoid possible NULL pointer dereference in __cpuidle_register_device()

On 04/03/2012 03:15 PM, Srivatsa S. Bhat wrote:
> On 04/03/2012 05:38 PM, Daniel Lezcano wrote:
>
>> On 04/03/2012 01:51 PM, Srivatsa S. Bhat wrote:
>>> On 04/03/2012 01:01 AM, Daniel Lezcano wrote:
>>>
>>>> On 04/02/2012 04:44 PM, Srivatsa S. Bhat wrote:
>>>>> In __cpuidle_register_device(), "dev->cpu" is used before checking if
>>>>> dev is
>>>>> non-NULL. Fix it.
>>>>>
>>>>> Signed-off-by: Srivatsa S. Bhat<srivatsa.bhat@...ux.vnet.ibm.com>
>>>>> ---
>>>>
>>>> That should be fixed at the caller level. Usually, static function does
>>>> not check the function parameters, it is up to the exported function to
>>>> do that. It is supposed the static functions are called with valid
>>>> parameters.
>>>>
>>>
>>>
>>> Ok, good point! I hadn't thought about that.. I just happened to notice
>>> that in __cpuidle_register_device(), the dev == NULL check is performed
>>> _after_ dereferencing it, which made the check useless. So I tried to
>>> fix that within that function. But thanks for pointing out the
>>> semantics..
>>>
>>>> There are two callers for __cpuidle_register_device:
>>>>    * cpuidle_register_device
>>>>    * cpuidle_enable_device
>>>>
>>>> Both of them do not check 'dev' is a valid parameter. They should as
>>>> they are exported and could be used by an external module. IMHO, BUG_ON
>>>> could be used here if dev == NULL.
>>>>
>>>
>>>
>>> BUG_ON? That would crash the system.. which might be unnecessary..
>>
>> Mmh, yes, I agree. never mind.
>>
>>> How about checking if dev == NULL in the 2 callers like you suggested
>>> and returning -EINVAL if dev is indeed NULL?
>>> (And of course no checks for dev == NULL in __cpuidle_register_device).
>>
>> Ok for me.
>>
>
>
> Great! Here is the updated patch:
>
> ---
>
> From: Srivatsa S. Bhat<srivatsa.bhat@...ux.vnet.ibm.com>
> Subject: [PATCH v2] cpuidle: Add checks to avoid NULL pointer dereference
>
> The existing check for dev == NULL in __cpuidle_register_device() is rendered
> useless because dev is dereferenced before the check itself. Moreover,
> correctly speaking, it is the job of the callers of this function, i.e.,
> cpuidle_register_device()&  cpuidle_enable_device() (which also happen to be
> exported functions) to ensure that __cpuidle_register_device() is called with
> a non-NULL dev.
>
> So add the necessary dev == NULL checks in the two callers and remove the
> (useless) check from __cpuidle_register_device().
>
> Signed-off-by: Srivatsa S. Bhat<srivatsa.bhat@...ux.vnet.ibm.com>

Acked-by: Daniel Lezcano <daniel.lezcano@...aro.org>

Thanks
   -- Daniel

ps : shouldn't this patch be sent in a separate email ?

> ---
>
>   drivers/cpuidle/cpuidle.c |    8 ++++++--
>   1 files changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index 87411ce..eae2f11 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -291,6 +291,9 @@ int cpuidle_enable_device(struct cpuidle_device *dev)
>   	int ret, i;
>   	struct cpuidle_driver *drv = cpuidle_get_driver();
>
> +	if (!dev)
> +		return -EINVAL;
> +
>   	if (dev->enabled)
>   		return 0;
>   	if (!drv || !cpuidle_curr_governor)
> @@ -375,8 +378,6 @@ static int __cpuidle_register_device(struct cpuidle_device *dev)
>   	struct device *cpu_dev = get_cpu_device((unsigned long)dev->cpu);
>   	struct cpuidle_driver *cpuidle_driver = cpuidle_get_driver();
>
> -	if (!dev)
> -		return -EINVAL;
>   	if (!try_module_get(cpuidle_driver->owner))
>   		return -EINVAL;
>
> @@ -401,6 +402,9 @@ int cpuidle_register_device(struct cpuidle_device *dev)
>   {
>   	int ret;
>
> +	if (!dev)
> +		return -EINVAL;
> +
>   	mutex_lock(&cpuidle_lock);
>
>   	if ((ret = __cpuidle_register_device(dev))) {
>
>


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