Re: [PATCH v7 21/41] mm: Add guard pages around a shadow stack.

[Deepak Gupta <debug@rivosinc.com>]

On Mon, Feb 27, 2023 at 2:31 PM Rick Edgecombe
<rick.p.edgecombe@intel.com> wrote:
>
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> The x86 Control-flow Enforcement Technology (CET) feature includes a new
> type of memory called shadow stack. This shadow stack memory has some
> unusual properties, which requires some core mm changes to function
> properly.
>
> The architecture of shadow stack constrains the ability of userspace to
> move the shadow stack pointer (SSP) in order to  prevent corrupting or
> switching to other shadow stacks. The RSTORSSP can move the ssp to
> different shadow stacks, but it requires a specially placed token in order
> to do this. However, the architecture does not prevent incrementing the
> stack pointer to wander onto an adjacent shadow stack. To prevent this in
> software, enforce guard pages at the beginning of shadow stack vmas, such
> that there will always be a gap between adjacent shadow stacks.
>
> Make the gap big enough so that no userspace SSP changing operations
> (besides RSTORSSP), can move the SSP from one stack to the next. The
> SSP can increment or decrement by CALL, RET  and INCSSP. CALL and RET
> can move the SSP by a maximum of 8 bytes, at which point the shadow
> stack would be accessed.
>
> The INCSSP instruction can also increment the shadow stack pointer. It
> is the shadow stack analog of an instruction like:
>
>         addq    $0x80, %rsp
>
> However, there is one important difference between an ADD on %rsp and
> INCSSP. In addition to modifying SSP, INCSSP also reads from the memory
> of the first and last elements that were "popped". It can be thought of
> as acting like this:
>
> READ_ONCE(ssp);       // read+discard top element on stack
> ssp += nr_to_pop * 8; // move the shadow stack
> READ_ONCE(ssp-8);     // read+discard last popped stack element
>
> The maximum distance INCSSP can move the SSP is 2040 bytes, before it
> would read the memory. Therefore a single page gap will be enough to
> prevent any operation from shifting the SSP to an adjacent stack, since
> it would have to land in the gap at least once, causing a fault.
>
> This could be accomplished by using VM_GROWSDOWN, but this has a
> downside. The behavior would allow shadow stack's to grow, which is
> unneeded and adds a strange difference to how most regular stacks work.
>
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Tested-by: Kees Cook <keescook@chromium.org>
> Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Cc: Kees Cook <keescook@chromium.org>
>
> ---
> v5:
>  - Fix typo in commit log
>
> v4:
>  - Drop references to 32 bit instructions
>  - Switch to generic code to drop __weak (Peterz)
>
> v2:
>  - Use __weak instead of #ifdef (Dave Hansen)
>  - Only have start gap on shadow stack (Andy Luto)
>  - Create stack_guard_start_gap() to not duplicate code
>    in an arch version of vm_start_gap() (Dave Hansen)
>  - Improve commit log partly with verbiage from (Dave Hansen)
>
> Yu-cheng v25:
>  - Move SHADOW_STACK_GUARD_GAP to arch/x86/mm/mmap.c.
> ---
>  include/linux/mm.h | 31 ++++++++++++++++++++++++++-----
>  1 file changed, 26 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 097544afb1aa..6a093daced88 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3107,15 +3107,36 @@ struct vm_area_struct *vma_lookup(struct mm_struct *mm, unsigned long addr)
>         return mtree_load(&mm->mm_mt, addr);
>  }
>
> +static inline 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 INCSSPQ.
> +        * INCSSPQ moves shadow stack pointer up to 255 * 8 = ~2 KB
> +        * 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
> +        */
> +       if (vma->vm_flags & VM_SHADOW_STACK)
> +               return PAGE_SIZE;

This is an arch agnostic header file. Can we remove `VM_SHADOW_STACK`
from here? and instead
have `arch_is_shadow_stack` which consumes vma flags and returns true or false.
This allows different architectures to choose their own encoding of
vma flags to represent a shadow stack.

> +
> +       return 0;
> +}
> +
>  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;
>  }
>
> --
> 2.17.1
>