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Date:   Mon, 3 Oct 2022 22:01:07 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Jann Horn <jannh@google.com>
Cc:     Rick Edgecombe <rick.p.edgecombe@intel.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        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>,
        Mike Kravetz <mike.kravetz@oracle.com>,
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        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@intel.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        joao.moreira@intel.com, John Allen <john.allen@amd.com>,
        kcc@google.com, eranian@google.com, rppt@kernel.org,
        jamorris@linux.microsoft.com, dethoma@microsoft.com
Subject: Re: [PATCH v2 00/39] Shadowstacks for userspace
Message-ID: <202210032158.CE0941C4D@keescook>
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
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On Mon, Oct 03, 2022 at 07:25:03PM +0200, Jann Horn wrote:
> On Mon, Oct 3, 2022 at 7:04 PM Kees Cook <keescook@chromium.org> wrote:
> > On Thu, Sep 29, 2022 at 03:28:57PM -0700, Rick Edgecombe wrote:
> > > This is an overdue followup to the “Shadow stacks for userspace” CET series.
> > > Thanks for all the comments on the first version [0]. They drove a decent
> > > amount of changes for v2. Since it has been awhile, I’ll try to summarize the
> > > areas that got major changes since last time. Smaller changes are listed in
> > > each patch.
> >
> > Thanks for the write-up!
> >
> > > [...]
> > >         GUP
> > >         ---
> > >         Shadow stack memory is generally treated as writable by the kernel, but
> > >         it behaves differently then other writable memory with respect to GUP.
> > >         FOLL_WRITE will not GUP shadow stack memory unless FOLL_FORCE is also
> > >         set. Shadow stack memory is writable from the perspective of being
> > >         changeable by userspace, but it is also protected memory from
> > >         userspace’s perspective. So preventing it from being writable via
> > >         FOLL_WRITE help’s make it harder for userspace to arbitrarily write to
> > >         it. However, like read-only memory, FOLL_FORCE can still write through
> > >         it. This means shadow stacks can be written to via things like
> > >         “/proc/self/mem”. Apps that want extra security will have to prevent
> > >         access to kernel features that can write with FOLL_FORCE.
> >
> > This seems like a problem to me -- the point of SS is that there cannot be
> > a way to write to them without specific instruction sequences. The fact
> > that /proc/self/mem bypasses memory protections was an old design mistake
> > that keeps leading to surprising behaviors. It would be much nicer to
> > draw the line somewhere and just say that FOLL_FORCE doesn't work on
> > VM_SHADOW_STACK. Why must FOLL_FORCE be allowed to write to SS?
> 
> But once you have FOLL_FORCE, you can also just write over stuff like
> executable code instead of writing over the stack. I don't think
> allowing FOLL_FORCE writes over shadow stacks from /proc/$pid/mem is
> making things worse in any way, and it's probably helpful for stuff
> like debuggers.
> 
> If you don't want /proc/$pid/mem to be able to do stuff like that,
> then IMO the way to go is to change when /proc/$pid/mem uses
> FOLL_FORCE, or to limit overall write access to /proc/$pid/mem.

Yeah, all reasonable. I just wish we could ditch FOLL_FORCE; it continues
to weird me out how powerful that fd's side-effects are.

-- 
Kees Cook