[PATCH] rtc: s5m: Remove VLA usage

[PATCH] rtc: s5m: Remove VLA usage

[Gustavo A. R. Silva]

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
+
 /*
  * 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

Re: [PATCH] rtc: s5m: Remove VLA usage

[Kees Cook]

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
>



-- 
Kees Cook
Pixel Security

Re: [PATCH] rtc: s5m: Remove VLA usage

[Gustavo A. R. Silva]

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
>>
> 
> 
> 

Re: [PATCH] rtc: s5m: Remove VLA usage

[Krzysztof Kozlowski]

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

Re: [PATCH] rtc: s5m: Remove VLA usage

[Gustavo A. R. Silva]

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
>