rtc: s5m: Remove VLA usage

In preparation to enabling -Wvla, remove VLAs and replace them
with fixed-length arrays instead.

From a security viewpoint, the use of Variable Length Arrays can be
a vector for stack overflow attacks. Also, in general, as the code
evolves it is easy to lose track of how big a VLA can get. Thus, we
can end up having segfaults that are hard to debug.

Also, fixed as part of the directive to remove all VLAs from
the kernel: https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
 drivers/rtc/rtc-s5m.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

On Thu, Mar 8, 2018 at 9:20 AM, Gustavo A. R. Silva
<gustavo@embeddedor.com> wrote:
> In preparation to enabling -Wvla, remove VLAs and replace them
> with fixed-length arrays instead.
>
> From a security viewpoint, the use of Variable Length Arrays can be
> a vector for stack overflow attacks. Also, in general, as the code
> evolves it is easy to lose track of how big a VLA can get. Thus, we
> can end up having segfaults that are hard to debug.
>
> Also, fixed as part of the directive to remove all VLAs from
> the kernel: https://lkml.org/lkml/2018/3/7/621
>
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> ---
>  drivers/rtc/rtc-s5m.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/rtc/rtc-s5m.c b/drivers/rtc/rtc-s5m.c
> index 6deae10..2b5f4f7 100644
> --- a/drivers/rtc/rtc-s5m.c
> +++ b/drivers/rtc/rtc-s5m.c
> @@ -38,6 +38,9 @@
>   */
>  #define UDR_READ_RETRY_CNT     5
>
> +/* Maximum number of registers for setting time/alarm0/alarm1 */
> +#define MAX_NUM_TIME_REGS      8

I would adjust the various const struct s5m_rtc_reg_config's
.regs_count to be represented by this new define, so the stack and the
structures stay in sync. Something like:

static const struct s5m_rtc_reg_config s2mps13_rtc_regs = {
        .regs_count             = MAX_NUM_TIME_REGS - 1,

?

-Kees

> +
>  /*
>   * Registers used by the driver which are different between chipsets.
>   *
> @@ -367,7 +370,7 @@ static void s5m8763_tm_to_data(struct rtc_time *tm, u8 *data)
>  static int s5m_rtc_read_time(struct device *dev, struct rtc_time *tm)
>  {
>         struct s5m_rtc_info *info = dev_get_drvdata(dev);
> -       u8 data[info->regs->regs_count];
> +       u8 data[MAX_NUM_TIME_REGS];
>         int ret;
>
>         if (info->regs->read_time_udr_mask) {
> @@ -413,7 +416,7 @@ static int s5m_rtc_read_time(struct device *dev, struct rtc_time *tm)
>  static int s5m_rtc_set_time(struct device *dev, struct rtc_time *tm)
>  {
>         struct s5m_rtc_info *info = dev_get_drvdata(dev);
> -       u8 data[info->regs->regs_count];
> +       u8 data[MAX_NUM_TIME_REGS];
>         int ret = 0;
>
>         switch (info->device_type) {
> @@ -450,7 +453,7 @@ static int s5m_rtc_set_time(struct device *dev, struct rtc_time *tm)
>  static int s5m_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alrm)
>  {
>         struct s5m_rtc_info *info = dev_get_drvdata(dev);
> -       u8 data[info->regs->regs_count];
> +       u8 data[MAX_NUM_TIME_REGS];
>         unsigned int val;
>         int ret, i;
>
> @@ -500,7 +503,7 @@ static int s5m_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alrm)
>
>  static int s5m_rtc_stop_alarm(struct s5m_rtc_info *info)
>  {
> -       u8 data[info->regs->regs_count];
> +       u8 data[MAX_NUM_TIME_REGS];
>         int ret, i;
>         struct rtc_time tm;
>
> @@ -545,7 +548,7 @@ static int s5m_rtc_stop_alarm(struct s5m_rtc_info *info)
>  static int s5m_rtc_start_alarm(struct s5m_rtc_info *info)
>  {
>         int ret;
> -       u8 data[info->regs->regs_count];
> +       u8 data[MAX_NUM_TIME_REGS];
>         u8 alarm0_conf;
>         struct rtc_time tm;
>
> @@ -598,7 +601,7 @@ static int s5m_rtc_start_alarm(struct s5m_rtc_info *info)
>  static int s5m_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
>  {
>         struct s5m_rtc_info *info = dev_get_drvdata(dev);
> -       u8 data[info->regs->regs_count];
> +       u8 data[MAX_NUM_TIME_REGS];
>         int ret;
>
>         switch (info->device_type) {
> --
> 2.7.4
>

On 03/08/2018 11:58 AM, Kees Cook wrote:
> On Thu, Mar 8, 2018 at 9:20 AM, Gustavo A. R. Silva
> <gustavo@embeddedor.com> wrote:
>> In preparation to enabling -Wvla, remove VLAs and replace them
>> with fixed-length arrays instead.
>>
>>  From a security viewpoint, the use of Variable Length Arrays can be
>> a vector for stack overflow attacks. Also, in general, as the code
>> evolves it is easy to lose track of how big a VLA can get. Thus, we
>> can end up having segfaults that are hard to debug.
>>
>> Also, fixed as part of the directive to remove all VLAs from
>> the kernel: https://lkml.org/lkml/2018/3/7/621
>>
>> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
>> ---
>>   drivers/rtc/rtc-s5m.c | 15 +++++++++------
>>   1 file changed, 9 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/rtc/rtc-s5m.c b/drivers/rtc/rtc-s5m.c
>> index 6deae10..2b5f4f7 100644
>> --- a/drivers/rtc/rtc-s5m.c
>> +++ b/drivers/rtc/rtc-s5m.c
>> @@ -38,6 +38,9 @@
>>    */
>>   #define UDR_READ_RETRY_CNT     5
>>
>> +/* Maximum number of registers for setting time/alarm0/alarm1 */
>> +#define MAX_NUM_TIME_REGS      8
> 
> I would adjust the various const struct s5m_rtc_reg_config's
> .regs_count to be represented by this new define, so the stack and the
> structures stay in sync. Something like:
> 
> static const struct s5m_rtc_reg_config s2mps13_rtc_regs = {
>          .regs_count             = MAX_NUM_TIME_REGS - 1,
> 
> ?
> 

Yep. I thought about that and decided to wait for some feedback first. 
But yeah, I think is that'd be a good change.

--
Gustavo

> -Kees
> 
>> +
>>   /*
>>    * Registers used by the driver which are different between chipsets.
>>    *
>> @@ -367,7 +370,7 @@ static void s5m8763_tm_to_data(struct rtc_time *tm, u8 *data)
>>   static int s5m_rtc_read_time(struct device *dev, struct rtc_time *tm)
>>   {
>>          struct s5m_rtc_info *info = dev_get_drvdata(dev);
>> -       u8 data[info->regs->regs_count];
>> +       u8 data[MAX_NUM_TIME_REGS];
>>          int ret;
>>
>>          if (info->regs->read_time_udr_mask) {
>> @@ -413,7 +416,7 @@ static int s5m_rtc_read_time(struct device *dev, struct rtc_time *tm)
>>   static int s5m_rtc_set_time(struct device *dev, struct rtc_time *tm)
>>   {
>>          struct s5m_rtc_info *info = dev_get_drvdata(dev);
>> -       u8 data[info->regs->regs_count];
>> +       u8 data[MAX_NUM_TIME_REGS];
>>          int ret = 0;
>>
>>          switch (info->device_type) {
>> @@ -450,7 +453,7 @@ static int s5m_rtc_set_time(struct device *dev, struct rtc_time *tm)
>>   static int s5m_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alrm)
>>   {
>>          struct s5m_rtc_info *info = dev_get_drvdata(dev);
>> -       u8 data[info->regs->regs_count];
>> +       u8 data[MAX_NUM_TIME_REGS];
>>          unsigned int val;
>>          int ret, i;
>>
>> @@ -500,7 +503,7 @@ static int s5m_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alrm)
>>
>>   static int s5m_rtc_stop_alarm(struct s5m_rtc_info *info)
>>   {
>> -       u8 data[info->regs->regs_count];
>> +       u8 data[MAX_NUM_TIME_REGS];
>>          int ret, i;
>>          struct rtc_time tm;
>>
>> @@ -545,7 +548,7 @@ static int s5m_rtc_stop_alarm(struct s5m_rtc_info *info)
>>   static int s5m_rtc_start_alarm(struct s5m_rtc_info *info)
>>   {
>>          int ret;
>> -       u8 data[info->regs->regs_count];
>> +       u8 data[MAX_NUM_TIME_REGS];
>>          u8 alarm0_conf;
>>          struct rtc_time tm;
>>
>> @@ -598,7 +601,7 @@ static int s5m_rtc_start_alarm(struct s5m_rtc_info *info)
>>   static int s5m_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
>>   {
>>          struct s5m_rtc_info *info = dev_get_drvdata(dev);
>> -       u8 data[info->regs->regs_count];
>> +       u8 data[MAX_NUM_TIME_REGS];
>>          int ret;
>>
>>          switch (info->device_type) {
>> --
>> 2.7.4
>>
> 
> 
>

On Thu, Mar 8, 2018 at 7:03 PM, Gustavo A. R. Silva
<garsilva@embeddedor.com> wrote:
>
>
> On 03/08/2018 11:58 AM, Kees Cook wrote:
>>
>> On Thu, Mar 8, 2018 at 9:20 AM, Gustavo A. R. Silva
>> <gustavo@embeddedor.com> wrote:
>>>
>>> In preparation to enabling -Wvla, remove VLAs and replace them
>>> with fixed-length arrays instead.
>>>
>>>  From a security viewpoint, the use of Variable Length Arrays can be
>>> a vector for stack overflow attacks. Also, in general, as the code
>>> evolves it is easy to lose track of how big a VLA can get. Thus, we
>>> can end up having segfaults that are hard to debug.
>>>
>>> Also, fixed as part of the directive to remove all VLAs from
>>> the kernel: https://lkml.org/lkml/2018/3/7/621
>>>
>>> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
>>> ---
>>>   drivers/rtc/rtc-s5m.c | 15 +++++++++------
>>>   1 file changed, 9 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/rtc/rtc-s5m.c b/drivers/rtc/rtc-s5m.c
>>> index 6deae10..2b5f4f7 100644
>>> --- a/drivers/rtc/rtc-s5m.c
>>> +++ b/drivers/rtc/rtc-s5m.c
>>> @@ -38,6 +38,9 @@
>>>    */
>>>   #define UDR_READ_RETRY_CNT     5
>>>
>>> +/* Maximum number of registers for setting time/alarm0/alarm1 */
>>> +#define MAX_NUM_TIME_REGS      8
>>
>>
>> I would adjust the various const struct s5m_rtc_reg_config's
>> .regs_count to be represented by this new define, so the stack and the
>> structures stay in sync. Something like:
>>
>> static const struct s5m_rtc_reg_config s2mps13_rtc_regs = {
>>          .regs_count             = MAX_NUM_TIME_REGS - 1,
>>
>> ?
>>
>
> Yep. I thought about that and decided to wait for some feedback first. But
> yeah, I think is that'd be a good change.

Define and these assignments should be somehow connected with enum
defining the offsets for data[] (from
include/linux/mfd/samsung/rtc.h). Otherwise we define the same in two
places. The enum could be itself (in separate patch) moved to the
driver because it is meaningless for others.

Best regards,
Krzysztof

Hi Krzysztof,

On 03/09/2018 07:04 AM, Krzysztof Kozlowski wrote:
> On Thu, Mar 8, 2018 at 7:03 PM, Gustavo A. R. Silva
> <garsilva@embeddedor.com> wrote:
>>
>>
>> On 03/08/2018 11:58 AM, Kees Cook wrote:
>>>
>>> On Thu, Mar 8, 2018 at 9:20 AM, Gustavo A. R. Silva
>>> <gustavo@embeddedor.com> wrote:
>>>>
>>>> In preparation to enabling -Wvla, remove VLAs and replace them
>>>> with fixed-length arrays instead.
>>>>
>>>>   From a security viewpoint, the use of Variable Length Arrays can be
>>>> a vector for stack overflow attacks. Also, in general, as the code
>>>> evolves it is easy to lose track of how big a VLA can get. Thus, we
>>>> can end up having segfaults that are hard to debug.
>>>>
>>>> Also, fixed as part of the directive to remove all VLAs from
>>>> the kernel: https://lkml.org/lkml/2018/3/7/621
>>>>
>>>> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
>>>> ---
>>>>    drivers/rtc/rtc-s5m.c | 15 +++++++++------
>>>>    1 file changed, 9 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/drivers/rtc/rtc-s5m.c b/drivers/rtc/rtc-s5m.c
>>>> index 6deae10..2b5f4f7 100644
>>>> --- a/drivers/rtc/rtc-s5m.c
>>>> +++ b/drivers/rtc/rtc-s5m.c
>>>> @@ -38,6 +38,9 @@
>>>>     */
>>>>    #define UDR_READ_RETRY_CNT     5
>>>>
>>>> +/* Maximum number of registers for setting time/alarm0/alarm1 */
>>>> +#define MAX_NUM_TIME_REGS      8
>>>
>>>
>>> I would adjust the various const struct s5m_rtc_reg_config's
>>> .regs_count to be represented by this new define, so the stack and the
>>> structures stay in sync. Something like:
>>>
>>> static const struct s5m_rtc_reg_config s2mps13_rtc_regs = {
>>>           .regs_count             = MAX_NUM_TIME_REGS - 1,
>>>
>>> ?
>>>
>>
>> Yep. I thought about that and decided to wait for some feedback first. But
>> yeah, I think is that'd be a good change.
> 
> Define and these assignments should be somehow connected with enum
> defining the offsets for data[] (from
> include/linux/mfd/samsung/rtc.h). Otherwise we define the same in two
> places. The enum could be itself (in separate patch) moved to the
> driver because it is meaningless for others.
>

I got it.

I'll move the enum to rtc-s5m.c and add RTC_MAX_NUM_TIME_REGS at the end 
of it. I'll send a patch series for this.

Thanks for the feedback.
--
Gustavo

> Best regards,
> Krzysztof
>