Re: [PATCH v4 18/39] mm: Fixup places that call pte_mkwrite() directly

Kees Cook
2022-12-03T02:37:32Z

On Fri, Dec 02, 2022 at 04:35:45PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> 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.
>
> With the introduction of shadow stack memory there are two ways a pte can
> be writable: regular writable memory and shadow stack memory.
>
> In past patches, maybe_mkwrite() has been updated to apply pte_mkwrite()
> or pte_mkwrite_shstk() depending on the VMA flag. This covers most cases
> where a PTE is made writable. However, there are places where pte_mkwrite()
> is called directly and the logic should now also create a shadow stack PTE
> in the case of a shadow stack VMA.
>
> - do_anonymous_page() and migrate_vma_insert_page() check VM_WRITE
> directly and call pte_mkwrite(). Teach it about pte_mkwrite_shstk()
>
> - When userfaultfd is creating a PTE after userspace handles the fault
> it calls pte_mkwrite() directly. Teach it about pte_mkwrite_shstk()
>
> To make the code cleaner, introduce is_shstk_write() which simplifies
> checking for VM_WRITE | VM_SHADOW_STACK together.
>
> In other cases where pte_mkwrite() is called directly, the VMA will not
> be VM_SHADOW_STACK, and so shadow stack memory should not be created.
> - In the case of pte_savedwrite(), shadow stack VMA's are excluded.
> - In the case of the "dirty_accountable" optimization in mprotect(),
> shadow stack VMA's won't be VM_SHARED, so it is not nessary.
>
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook