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From:   Jann Horn <jannh@google.com>
Date:   Sat, 1 Oct 2022 01:00:48 +0200
Message-ID: <CAG48ez3-dgcrLxKNAs4_K++FXn-9qL=6kjVY=2Cn-AxoML33Vg@mail.gmail.com>
Subject: Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack memory
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@intel.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H . J . Lu" <hjl.tools@gmail.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
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        Weijiang Yang <weijiang.yang@intel.com>,
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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?