Re: [PATCH v2 16/39] x86/mm: Update maybe_mkwrite() for shadow stack

From: Kees Cook
Date: Mon Oct 03 2022 - 14:23:05 EST

On Thu, Sep 29, 2022 at 03:29:13PM -0700, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> When serving a page fault, maybe_mkwrite() makes a PTE writable if there is
> a write access to it, and its vma has VM_WRITE. Shadow stack accesses to
> shadow stack vma's are also treated as write accesses by the fault handler.
> This is because setting shadow stack memory makes it writable via some
> instructions, so COW has to happen even for shadow stack reads.
> So maybe_mkwrite() should continue to set VM_WRITE vma's as normally
> writable, but also set VM_WRITE|VM_SHADOW_STACK vma's as shadow stack.
> Do this by adding a pte_mkwrite_shstk() and a cross-arch stub. Check for
> VM_SHADOW_STACK in maybe_mkwrite() and call pte_mkwrite_shstk()
> accordingly.
> Apply the same changes to maybe_pmd_mkwrite().
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

Kees Cook