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Date: Thu, 8 Mar 2018 09:58:57 -0800
From: Kees Cook <keescook@...omium.org>
To: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc: Sangbeom Kim <sbkim73@...sung.com>, Krzysztof Kozlowski <krzk@...nel.org>, 
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>, Alessandro Zummo <a.zummo@...ertech.it>, 
	Alexandre Belloni <alexandre.belloni@...tlin.com>, LKML <linux-kernel@...r.kernel.org>, 
	linux-samsung-soc@...r.kernel.org, linux-rtc@...r.kernel.org, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>, 
	"Gustavo A. R. Silva" <garsilva@...eddedor.com>
Subject: Re: [PATCH] rtc: s5m: Remove VLA usage

On Thu, Mar 8, 2018 at 9:20 AM, Gustavo A. R. Silva
<gustavo@...eddedor.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@...eddedor.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

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