Re: [PATCH v3 05/11] arm: Properly account for stack randomization and stack guard gap

[Kees Cook]

On Wed, Apr 17, 2019 at 12:28 AM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> This commit takes care of stack randomization and stack guard gap when
> computing mmap base address and checks if the task asked for randomization.
> This fixes the problem uncovered and not fixed for arm here:
> https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1429066.html

Please use the official archive instead. This includes headers, linked
patches, etc:
https://lkml.kernel.org/r/20170622200033.25714-1-riel@xxxxxxxxxx

> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> ---
>  arch/arm/mm/mmap.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
> index f866870db749..bff3d00bda5b 100644
> --- a/arch/arm/mm/mmap.c
> +++ b/arch/arm/mm/mmap.c
> @@ -18,8 +18,9 @@
>          (((pgoff)<<PAGE_SHIFT) & (SHMLBA-1)))
>
>  /* gap between mmap and stack */
> -#define MIN_GAP (128*1024*1024UL)
> -#define MAX_GAP ((TASK_SIZE)/6*5)
> +#define MIN_GAP                (128*1024*1024UL)

Might as well fix this up as SIZE_128M

> +#define MAX_GAP                ((TASK_SIZE)/6*5)
> +#define STACK_RND_MASK (0x7ff >> (PAGE_SHIFT - 12))

STACK_RND_MASK is already defined so you don't need to add it here, yes?

>  static int mmap_is_legacy(struct rlimit *rlim_stack)
>  {
> @@ -35,6 +36,15 @@ static int mmap_is_legacy(struct rlimit *rlim_stack)
>  static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
>  {
>         unsigned long gap = rlim_stack->rlim_cur;
> +       unsigned long pad = stack_guard_gap;
> +
> +       /* Account for stack randomization if necessary */
> +       if (current->flags & PF_RANDOMIZE)
> +               pad += (STACK_RND_MASK << PAGE_SHIFT);
> +
> +       /* Values close to RLIM_INFINITY can overflow. */
> +       if (gap + pad > gap)
> +               gap += pad;
>
>         if (gap < MIN_GAP)
>                 gap = MIN_GAP;
> --
> 2.20.1
>

But otherwise, yes:

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

--
Kees Cook