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

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Message-ID: <4F7AF7FE.5070307@linux.vnet.ibm.com>
Date:	Tue, 03 Apr 2012 18:45:42 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	lenb@...nel.org
CC:	Daniel Lezcano <daniel.lezcano@...aro.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 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>
---

 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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