Re: [PATCH v6 09/14] mips: Properly account for stack randomization and stack guard gap

[Sergei Shtylyov]

Hello!

On 08.08.2019 9:17, Alexandre Ghiti 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://lkml.kernel.org/r/20170622200033.25714-1-riel@xxxxxxxxxx
>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Acked-by: Paul Burton <paul.burton@xxxxxxxx>
> Reviewed-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> ---
>   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 d79f2b432318..f5c778113384 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)

   Could add spaces around *, while touching this anyway? And parens
around TASK_SIZE shouldn't be needed...

> +#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);

   Parens not needed here.

> +
> +	/* Values close to RLIM_INFINITY can overflow. */
> +	if (gap + pad > gap)
> +		gap += pad;
>   
>   	if (gap < MIN_GAP)
>   		gap = MIN_GAP;