Re: [RFC PATCH v9 07/27] Add guard pages around a Shadow Stack.

Kees Cook
2020-02-25T20:11:16Z

On Wed, Feb 05, 2020 at 10:19:15AM -0800, Yu-cheng Yu wrote:
> INCSSPD/INCSSPQ instruction is used to unwind a Shadow Stack (SHSTK). It
> performs 'pop and discard' of the first and last element from SHSTK in the
> range specified in the operand. The maximum value of the operand is 255,
> and the maximum moving distance of the SHSTK pointer is 255 * 4 for
> INCSSPD, 255 * 8 for INCSSPQ.
>
> Since SHSTK has a fixed size, creating a guard page above prevents
> INCSSP/RET from moving beyond. Likewise, creating a guard page below
> prevents CALL from underflowing the SHSTK.

This commit log doesn't really explain why the code changes below are
needed? stack_guard_gap is configurable at boot, etc. This appears to be
limiting it? I don't follow this change...

-Kees

>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
> include/linux/mm.h | 20 ++++++++++++++++----
> 1 file changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index b5145fbe102e..75de07674649 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2464,9 +2464,15 @@ static inline struct vm_area_struct * find_vma_intersection(struct mm_struct * m
> static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
> {
> unsigned long vm_start = vma->vm_start;
> + unsigned long gap = 0;
>
> - if (vma->vm_flags & VM_GROWSDOWN) {
> - vm_start -= stack_guard_gap;
> + if (vma->vm_flags & VM_GROWSDOWN)
> + gap = stack_guard_gap;
> + else if (vma->vm_flags & VM_SHSTK)
> + gap = PAGE_SIZE;
> +
> + if (gap != 0) {
> + vm_start -= gap;
> if (vm_start > vma->vm_start)
> vm_start = 0;
> }
> @@ -2476,9 +2482,15 @@ static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
> static inline unsigned long vm_end_gap(struct vm_area_struct *vma)
> {
> unsigned long vm_end = vma->vm_end;
> + unsigned long gap = 0;
> +
> + if (vma->vm_flags & VM_GROWSUP)
> + gap = stack_guard_gap;
> + else if (vma->vm_flags & VM_SHSTK)
> + gap = PAGE_SIZE;
>
> - if (vma->vm_flags & VM_GROWSUP) {
> - vm_end += stack_guard_gap;
> + if (gap != 0) {
> + vm_end += gap;
> if (vm_end < vma->vm_end)
> vm_end = -PAGE_SIZE;
> }
> --
> 2.21.0
>

--
Kees Cook