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SubjectRe: [PATCH v3 05/11] arm: Properly account for stack randomization and stack guard gap
On Wed, Apr 17, 2019 at 12:28 AM Alexandre Ghiti <alex@ghiti.fr> 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@vger.kernel.org/msg1429066.html

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

> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> ---
> 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@chromium.org>

--
Kees Cook

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