Re: [PATCH v3 06/11] arm: Use STACK_TOP when computing mmap base address

[Kees Cook <keescook@chromium.org>]

On Wed, Apr 17, 2019 at 12:29 AM Alexandre Ghiti <alex@ghiti.fr> wrote:
>
> mmap base address must be computed wrt stack top address, using TASK_SIZE
> is wrong since STACK_TOP and TASK_SIZE are not equivalent.
>
> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> ---
>  arch/arm/mm/mmap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
> index bff3d00bda5b..0b94b674aa91 100644
> --- a/arch/arm/mm/mmap.c
> +++ b/arch/arm/mm/mmap.c
> @@ -19,7 +19,7 @@
>
>  /* gap between mmap and stack */
>  #define MIN_GAP                (128*1024*1024UL)
> -#define MAX_GAP                ((TASK_SIZE)/6*5)
> +#define MAX_GAP                ((STACK_TOP)/6*5)

Parens around STACK_TOP aren't needed, but you'll be removing it
entirely, so I can't complain. ;)

>  #define STACK_RND_MASK (0x7ff >> (PAGE_SHIFT - 12))
>
>  static int mmap_is_legacy(struct rlimit *rlim_stack)
> @@ -51,7 +51,7 @@ static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
>         else if (gap > MAX_GAP)
>                 gap = MAX_GAP;
>
> -       return PAGE_ALIGN(TASK_SIZE - gap - rnd);
> +       return PAGE_ALIGN(STACK_TOP - gap - rnd);
>  }
>
>  /*
> --
> 2.20.1
>

Acked-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook