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

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Message-ID: <44314145f644ab822ac36cc8c78520d5f34d5bd8.camel@intel.com>
Date:   Fri, 30 Sep 2022 20:30:47 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack
 memory

On Fri, 2022-09-30 at 12:16 -0700, Dave Hansen 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.

It's because pteval is an unsigned long. We could create a pte_t, and
use the helpers, but then we would be using pte_foo() on pmd's, etc. So
probably comment is the better option?