Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack memory

On Sat, 2022-10-01 at 01:00 +0200, Jann Horn wrote:

On Fri, Sep 30, 2022 at 9:16 PM Dave Hansen [off-list ref]
wrote:

quoted

On 9/29/22 15:29, Rick Edgecombe wrote:

quoted

@@ -1633,6 +1633,9 @@ static inline bool

__pte_access_permitted(unsigned long pteval, bool write)
  {
       unsigned long need_pte_bits = _PAGE_PRESENT|_PAGE_USER;

+     if (write && (pteval & (_PAGE_RW | _PAGE_DIRTY)) ==
_PAGE_DIRTY)
+             return 0;

Do we not have a helper for this?  Seems a bit messy to open-code
these
shadow-stack permissions.  Definitely at least needs a comment.

FWIW, if you look at more context around this diff, the function
looks
like this:

 static inline bool __pte_access_permitted(unsigned long pteval, bool
write)
 {
        unsigned long need_pte_bits = _PAGE_PRESENT|_PAGE_USER;

+       if (write && (pteval & (_PAGE_RW | _PAGE_DIRTY)) ==
_PAGE_DIRTY)
+               return 0;
+
        if (write)
                need_pte_bits |= _PAGE_RW;

        if ((pteval & need_pte_bits) != need_pte_bits)
                return 0;

        return __pkru_allows_pkey(pte_flags_pkey(pteval), write);
 }

So I think this change is actually a no-op - the only thing it does
is
to return 0 if write==1, !_PAGE_RW, and _PAGE_DIRTY. But the check
below will always return 0 if !_PAGE_RW, unless I'm misreading it?
And
this is the only patch in the series that touches this function, so
it's not like this becomes necessary with a later patch in the series
either.

Should this check go in anyway for clarity reasons, or should this
instead be a comment explaining that __pte_access_permitted() behaves
just like the hardware access check, which means shadow pages are
treated as readonly?

Thanks Jann, I was just realizing the same thing. Yes, I think it
doesn't do anything. I can add a comment of why there is no check, but
otherwise the check seems like unnecessary work.