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

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Date:   Mon, 3 Oct 2022 21:21:58 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andy Lutomirski <luto@...nel.org>
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>,
        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>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        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 Mon, Oct 03, 2022 at 03:49:18PM -0700, Andy Lutomirski wrote:
> On 10/3/22 11:39, Kees Cook wrote:
> > On Thu, Sep 29, 2022 at 03:29:19PM -0700, Rick Edgecombe wrote:
> > > [...]
> > > Still allow FOLL_FORCE to write through shadow stack protections, as it
> > > does for read-only protections.
> > 
> > As I asked in the cover letter: why do we need to add this for shstk? It
> > was a mistake for general memory. :P
> 
> For debuggers, which use FOLL_FORCE, quite intentionally, to modify text.
> And once a debugger has ptrace write access to a target, shadow stacks
> provide exactly no protection -- ptrace can modify text and all registers.

i.e. via ptrace? Yeah, I grudgingly accept the ptrace need for
FOLL_FORCE.

> But /proc/.../mem may be a different story, and I'd be okay with having
> FOLL_PROC_MEM for legacy compatibility via /proc/.../mem and not allowing
> that to access shadow stacks.  This does seem like it may not be very
> useful, though.

I *really* don't like the /mem use of FOLL_FORCE, though. I think the
rationale has been "using PTRACE_POKE is too slow". Again, I can live
with it, I was just hoping we could avoid expanding that questionable
behavior, especially since it's a bypass of WRSS.

-- 
Kees Cook