Re: [PATCH 7/9] x86/mm: Shadow stack page fault error checking

Andy Lutomirski
2018-06-07T16:26:57Z

On Thu, Jun 7, 2018 at 7:40 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>
> If a page fault is triggered by a shadow stack access (e.g.
> call/ret) or shadow stack management instructions (e.g.
> wrussq), then bit[6] of the page fault error code is set.
>
> In access_error(), we check if a shadow stack page fault
> is within a shadow stack memory area.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 73bd8c95ac71..2b3b9170109c 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1166,6 +1166,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
> (error_code & X86_PF_INSTR), foreign))
> return 1;
>
> + /*
> + * Verify X86_PF_SHSTK is within a shadow stack VMA.
> + * It is always an error if there is a shadow stack
> + * fault outside a shadow stack VMA.
> + */
> + if (error_code & X86_PF_SHSTK) {
> + if (!(vma->vm_flags & VM_SHSTK))
> + return 1;
> + return 0;
> + }
> +

What, if anything, would go wrong without this change? It seems like
it might be purely an optimization. If so, can you mention that in
the comment?