[PATCH] PM / devfreq: Check NULL governor in available_governors_show

[PATCH] PM / devfreq: Check NULL governor in available_governors_show

[Leonard Crestez]

The governor is initialized after sysfs attributes become visible so in
theory the governor field can be NULL here.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
---
 drivers/devfreq/devfreq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Found this by hacking device core to call attribute "show" functions
from inside device_add.

diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
index 00fc23fea5b2..896fb2312f2f 100644
--- a/drivers/devfreq/devfreq.c
+++ b/drivers/devfreq/devfreq.c
@@ -1322,11 +1322,11 @@ static ssize_t available_governors_show(struct device *d,
 
 	/*
 	 * The devfreq with immutable governor (e.g., passive) shows
 	 * only own governor.
 	 */
-	if (df->governor->immutable) {
+	if (df->governor && df->governor->immutable) {
 		count = scnprintf(&buf[count], DEVFREQ_NAME_LEN,
 				  "%s ", df->governor_name);
 	/*
 	 * The devfreq device shows the registered governor except for
 	 * immutable governors such as passive governor .
-- 
2.17.1


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Re: [PATCH] PM / devfreq: Check NULL governor in available_governors_show

[Matthias Kaehlcke]

On Mon, Sep 23, 2019 at 07:34:43PM +0300, Leonard Crestez wrote:
> The governor is initialized after sysfs attributes become visible so in
> theory the governor field can be NULL here.
> 
> Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
> ---
>  drivers/devfreq/devfreq.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Found this by hacking device core to call attribute "show" functions
> from inside device_add.
> 
> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
> index 00fc23fea5b2..896fb2312f2f 100644
> --- a/drivers/devfreq/devfreq.c
> +++ b/drivers/devfreq/devfreq.c
> @@ -1322,11 +1322,11 @@ static ssize_t available_governors_show(struct device *d,
>  
>  	/*
>  	 * The devfreq with immutable governor (e.g., passive) shows
>  	 * only own governor.
>  	 */
> -	if (df->governor->immutable) {
> +	if (df->governor && df->governor->immutable) {
>  		count = scnprintf(&buf[count], DEVFREQ_NAME_LEN,
>  				  "%s ", df->governor_name);
>  	/*
>  	 * The devfreq device shows the registered governor except for
>  	 * immutable governors such as passive governor .

Reviewed-by: Matthias Kaehlcke <mka@chromium.org>

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Re: [PATCH] PM / devfreq: Check NULL governor in available_governors_show

[Chanwoo Choi]

Hi,

On 19. 9. 24. 오전 1:34, Leonard Crestez wrote:
> The governor is initialized after sysfs attributes become visible so in
> theory the governor field can be NULL here.
> 
> Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
> ---
>  drivers/devfreq/devfreq.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Found this by hacking device core to call attribute "show" functions
> from inside device_add.
> 
> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
> index 00fc23fea5b2..896fb2312f2f 100644
> --- a/drivers/devfreq/devfreq.c
> +++ b/drivers/devfreq/devfreq.c
> @@ -1322,11 +1322,11 @@ static ssize_t available_governors_show(struct device *d,
>  
>  	/*
>  	 * The devfreq with immutable governor (e.g., passive) shows
>  	 * only own governor.
>  	 */
> -	if (df->governor->immutable) {
> +	if (df->governor && df->governor->immutable) {
>  		count = scnprintf(&buf[count], DEVFREQ_NAME_LEN,
>  				  "%s ", df->governor_name);
>  	/*
>  	 * The devfreq device shows the registered governor except for
>  	 * immutable governors such as passive governor .
> 

As you mentioned, it create sysfs and then initialize the governor instance
as following:

	device_register()
		device_add()
			device_add_attrs()
				creating sysfs entries.
	governor = try_then_request_governor(...)


Thanks for fix-up.
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>


Additionally, you have to add the following 'fixes' tag
and then send it to stable mailing list(stable@vger.kernel.org).
- Fixes: bcf23c79c4e46 ("PM / devfreq: Fix available_governor sysfs")

-- 
Best Regards,
Chanwoo Choi
Samsung Electronics

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Re: [PATCH] PM / devfreq: Check NULL governor in available_governors_show

[Leonard Crestez]

On 2019-09-24 4:48 AM, Chanwoo Choi wrote:
> On 19. 9. 24. 오전 1:34, Leonard Crestez wrote:
>> The governor is initialized after sysfs attributes become visible so in
>> theory the governor field can be NULL here.
>>
>> Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
>> ---
>>   drivers/devfreq/devfreq.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> Found this by hacking device core to call attribute "show" functions
>> from inside device_add.
>>
>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>> index 00fc23fea5b2..896fb2312f2f 100644
>> --- a/drivers/devfreq/devfreq.c
>> +++ b/drivers/devfreq/devfreq.c
>> @@ -1322,11 +1322,11 @@ static ssize_t available_governors_show(struct device *d,
>>   
>>   	/*
>>   	 * The devfreq with immutable governor (e.g., passive) shows
>>   	 * only own governor.
>>   	 */
>> -	if (df->governor->immutable) {
>> +	if (df->governor && df->governor->immutable) {
>>   		count = scnprintf(&buf[count], DEVFREQ_NAME_LEN,
>>   				  "%s ", df->governor_name);
>>   	/*
>>   	 * The devfreq device shows the registered governor except for
>>   	 * immutable governors such as passive governor .
>>
> 
> As you mentioned, it create sysfs and then initialize the governor instance
> as following:
> 
> 	device_register()
> 		device_add()
> 			device_add_attrs()
> 				creating sysfs entries.
> 	governor = try_then_request_governor(...)
> 
> 
> Thanks for fix-up.
> Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
> 
> Additionally, you have to add the following 'fixes' tag
> and then send it to stable mailing list(stable@vger.kernel.org).
> - Fixes: bcf23c79c4e46 ("PM / devfreq: Fix available_governor sysfs")

OK, but I'm not sure this meets the criteria for inclusion into linux 
stable:

* It must fix a real bug that bothers people (not a, "This could be a 
problem..." type thing).
* No "theoretical race condition" issues, unless an explanation of how 
the race can be exploited is also provided.

This patch fixes a theoretical race condition which has been there since 
the start.

--
Regards,
Leonard
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Re: [PATCH] PM / devfreq: Check NULL governor in available_governors_show

[Chanwoo Choi]

On 19. 9. 24. 오후 4:17, Leonard Crestez wrote:
> On 2019-09-24 4:48 AM, Chanwoo Choi wrote:
>> On 19. 9. 24. 오전 1:34, Leonard Crestez wrote:
>>> The governor is initialized after sysfs attributes become visible so in
>>> theory the governor field can be NULL here.
>>>
>>> Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
>>> ---
>>>   drivers/devfreq/devfreq.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> Found this by hacking device core to call attribute "show" functions
>>> from inside device_add.
>>>
>>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>>> index 00fc23fea5b2..896fb2312f2f 100644
>>> --- a/drivers/devfreq/devfreq.c
>>> +++ b/drivers/devfreq/devfreq.c
>>> @@ -1322,11 +1322,11 @@ static ssize_t available_governors_show(struct device *d,
>>>   
>>>   	/*
>>>   	 * The devfreq with immutable governor (e.g., passive) shows
>>>   	 * only own governor.
>>>   	 */
>>> -	if (df->governor->immutable) {
>>> +	if (df->governor && df->governor->immutable) {
>>>   		count = scnprintf(&buf[count], DEVFREQ_NAME_LEN,
>>>   				  "%s ", df->governor_name);
>>>   	/*
>>>   	 * The devfreq device shows the registered governor except for
>>>   	 * immutable governors such as passive governor .
>>>
>>
>> As you mentioned, it create sysfs and then initialize the governor instance
>> as following:
>>
>> 	device_register()
>> 		device_add()
>> 			device_add_attrs()
>> 				creating sysfs entries.
>> 	governor = try_then_request_governor(...)
>>
>>
>> Thanks for fix-up.
>> Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
>>
>> Additionally, you have to add the following 'fixes' tag
>> and then send it to stable mailing list(stable@vger.kernel.org).
>> - Fixes: bcf23c79c4e46 ("PM / devfreq: Fix available_governor sysfs")
> 
> OK, but I'm not sure this meets the criteria for inclusion into linux 
> stable:
> 
> * It must fix a real bug that bothers people (not a, "This could be a 
> problem..." type thing).
> * No "theoretical race condition" issues, unless an explanation of how 
> the race can be exploited is also provided.

OK. If you think that it is not necessary to send it to stable mailing list,
don't send it. Thanks.

> 
> This patch fixes a theoretical race condition which has been there since 
> the start.
> 
> --
> Regards,
> Leonard
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics

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