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

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