Re: [PATCH v3 08/11] mips: Properly account for stack randomization and stack guard gap

[Kees Cook]

On Wed, Apr 17, 2019 at 12:31 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 mips here:
> https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg1429066.html

same URL change here please...

>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  arch/mips/mm/mmap.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/arch/mips/mm/mmap.c b/arch/mips/mm/mmap.c
> index 2f616ebeb7e0..3ff82c6f7e24 100644
> --- a/arch/mips/mm/mmap.c
> +++ b/arch/mips/mm/mmap.c
> @@ -21,8 +21,9 @@ unsigned long shm_align_mask = PAGE_SIZE - 1; /* Sane caches */
>  EXPORT_SYMBOL(shm_align_mask);
>
>  /* gap between mmap and stack */
> -#define MIN_GAP (128*1024*1024UL)
> -#define MAX_GAP ((TASK_SIZE)/6*5)
> +#define MIN_GAP                (128*1024*1024UL)
> +#define MAX_GAP                ((TASK_SIZE)/6*5)
> +#define STACK_RND_MASK (0x7ff >> (PAGE_SHIFT - 12))
>
>  static int mmap_is_legacy(struct rlimit *rlim_stack)
>  {
> @@ -38,6 +39,15 @@ 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
>

-- 
Kees Cook