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

Edgecombe, Rick P
2022-09-30T23:04:34Z

On Sat, 2022-10-01 at 01:00 +0200, Jann Horn wrote:
> On Fri, Sep 30, 2022 at 9:16 PM Dave Hansen <dave.hansen@intel.com>
> wrote:
> > On 9/29/22 15:29, Rick Edgecombe wrote:
> > > @@ -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.