Re: [PATCH v2 18/39] mm: Add guard pages around a shadow stack.

Kees Cook
2022-10-03T18:31:10Z

On Thu, Sep 29, 2022 at 03:29:15PM -0700, Rick Edgecombe wrote:
> [...]
> +unsigned long stack_guard_start_gap(struct vm_area_struct *vma)
> +{
> + if (vma->vm_flags & VM_GROWSDOWN)
> + return stack_guard_gap;
> +
> + /*
> + * Shadow stack pointer is moved by CALL, RET, and INCSSP(Q/D).
> + * INCSSPQ moves shadow stack pointer up to 255 * 8 = ~2 KB
> + * (~1KB for INCSSPD) and touches the first and the last element
> + * in the range, which triggers a page fault if the range is not
> + * in a shadow stack. Because of this, creating 4-KB guard pages
> + * around a shadow stack prevents these instructions from going
> + * beyond.
> + *
> + * Creation of VM_SHADOW_STACK is tightly controlled, so a vma
> + * can't be both VM_GROWSDOWN and VM_SHADOW_STACK
> + */

Thank you for the details on how the size choice is made here! :)

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index fef14ab3abcb..09458e77bf52 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2775,15 +2775,16 @@ struct vm_area_struct *vma_lookup(struct mm_struct *mm, unsigned long addr)
> return vma;
> }
>
> +unsigned long stack_guard_start_gap(struct vm_area_struct *vma);
> +
> static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
> {
> + unsigned long gap = stack_guard_start_gap(vma);
> unsigned long vm_start = vma->vm_start;
>
> - if (vma->vm_flags & VM_GROWSDOWN) {
> - vm_start -= stack_guard_gap;
> - if (vm_start > vma->vm_start)
> - vm_start = 0;
> - }
> + vm_start -= gap;
> + if (vm_start > vma->vm_start)
> + vm_start = 0;
> return vm_start;
> }
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 9d780f415be3..f0d2e9143bd0 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -247,6 +247,13 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
> return origbrk;
> }
>

I feel like something could be done with this definitions to make them
inline, instead of __weak:

#ifndef stack_guard_start_gap
> +unsigned long __weak stack_guard_start_gap(struct vm_area_struct *vma)
> +{
> + if (vma->vm_flags & VM_GROWSDOWN)
> + return stack_guard_gap;
> + return 0;
> +}
#endif

And then move the x86 stack_guard_start_gap to a header?

It's not exactly fast-path, but it feels a little weird. Regardlesss:

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook