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

[Borislav Petkov]

Just typos:

On Mon, Feb 27, 2023 at 02:29:37PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> 
> 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@xxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Acked-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> Co-developed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> 
> ---
> 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.

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