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

[Kees Cook]

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@xxxxxxxxx>
> ---
>  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