Re: [RFC PATCH v2 12/27] x86/mm: Shadow stack page fault error checking

[Dave Hansen]

On 07/10/2018 03:26 PM, Yu-cheng Yu wrote:
> +	/*
> +	 * 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;
> +	}

It turns out that a X86_PF_SHSTK just means that the processor faulted
while doing access to something it thinks should be a shadow-stack
virtual address.

But, we *can* have faults on shadow stack accesses for non-shadow-stack
reasons.

I think you need to remove the 'return 0' and let it fall through to the
other access checks that we might be failing.  If it's a shadow stack
access, it has to be a shadow stack VMA.  But, a shadow-stack access
fault to a shadow stack VMA isn't _necessarily_ OK.