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

Borislav Petkov
2023-03-06T08:08:17Z

Just typos:

On Mon, Feb 27, 2023 at 02:29:37PM -0800, Rick Edgecombe 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
^
instruction

s/ssp/SSP/g

> 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

VMAs

> 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 be incremented or decremented"

> 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

s/stack's/stacks/

> 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.

I'd prefer the equivalant explanation above from the commit message - it
is more precise.

> + *
> + * 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;
> +
> + return 0;
> +}

--
Regards/Gruss,
Boris.