rtc: s5m: Remove VLA usage

From a security viewpoint, the use of Variable Length Arrays can be

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 <[email protected]>

---
 drivers/rtc/rtc-s5m.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

drivers/rtc/rtc-s5m.c +0 -0

index 6deae10..2b5f4f7 100644

drivers/rtc/rtc-s5m.c +7 -7

+
 /*
  * 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

Quoted 21 lines · 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 <[email protected]>

---
  drivers/rtc/rtc-s5m.c | 15 +++++++++------
  1 file changed, 9 insertions(+), 6 deletions(-)

drivers/rtc/rtc-s5m.c +0 -0

index 6deae10..2b5f4f7 100644

drivers/rtc/rtc-s5m.c line 42

Quoted 5 lines

drivers/rtc/rtc-s5m.c +2 -0

@@ -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

Quoted 61 lines

drivers/rtc/rtc-s5m.c +7 -7

+
  /*
   * 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

Quoted 36 lines · Kees Cook

Quoted 22 lines · 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 <[email protected]>

---
  drivers/rtc/rtc-s5m.c | 15 +++++++++------
  1 file changed, 9 insertions(+), 6 deletions(-)

drivers/rtc/rtc-s5m.c +0 -0

index 6deae10..2b5f4f7 100644

drivers/rtc/rtc-s5m.c line 42

Quoted 5 lines

drivers/rtc/rtc-s5m.c +2 -0

@@ -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:

Quoted 5 lines

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.

Quoted 45 lines · Gustavo A. R. Silva

Quoted 41 lines · Kees Cook

Quoted 22 lines · 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 <[email protected]>

---
   drivers/rtc/rtc-s5m.c | 15 +++++++++------
   1 file changed, 9 insertions(+), 6 deletions(-)

drivers/rtc/rtc-s5m.c +0 -0

index 6deae10..2b5f4f7 100644

drivers/rtc/rtc-s5m.c line 42

Quoted 5 lines

drivers/rtc/rtc-s5m.c +2 -0

@@ -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,

?

Quoted 2 lines

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