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Date:   Mon, 3 Oct 2022 21:21:58 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Rick Edgecombe <rick.p.edgecombe@intel.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>,
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        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        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>, Jann Horn <jannh@google.com>,
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        jamorris@linux.microsoft.com, dethoma@microsoft.com
Subject: Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack
 memory
Message-ID: <202210032119.EF573F9E@keescook>
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
 <20220929222936.14584-23-rick.p.edgecombe@intel.com>
 <202210031134.B0B6B37@keescook>
 <bcfca48f-e02a-fc43-fb92-9cc119e2d28f@kernel.org>
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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