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@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> > ---
> >   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