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

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Date:   Sat, 1 Oct 2022 01:00:48 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@...el.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        joao.moreira@...el.com, John Allen <john.allen@....com>,
        kcc@...gle.com, eranian@...gle.com, rppt@...nel.org,
        jamorris@...ux.microsoft.com, dethoma@...rosoft.com
Subject: Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack memory

On Fri, Sep 30, 2022 at 9:16 PM Dave Hansen <dave.hansen@...el.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?