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Date:   Tue, 12 Nov 2019 15:03:27 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Jann Horn <jannh@google.com>
Cc:     "Theodore Y. Ts'o" <tytso@mit.edu>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Florian Weimer <fweimer@redhat.com>,
        Christian Brauner <christian@brauner.io>,
        lkml <linux-kernel@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>,
        Oleg Nesterov <oleg@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        David Howells <dhowells@redhat.com>,
        Pavel Emelyanov <xemul@virtuozzo.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Adrian Reber <adrian@lisas.de>,
        Andrei Vagin <avagin@gmail.com>,
        Linux API <linux-api@vger.kernel.org>,
        Ingo Molnar <mingo@elte.hu>
Subject: Re: For review: documentation of clone3() system call
Message-ID: <201911121502.3CADFBE3@keescook>
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On Mon, Nov 11, 2019 at 09:24:33PM +0100, Jann Horn wrote:
> On Mon, Nov 11, 2019 at 5:58 PM Theodore Y. Ts'o <tytso@mit.edu> wrote:
> > On Mon, Nov 11, 2019 at 03:55:35PM +0100, Jann Horn wrote:
> > > Not on Linux, but on OpenBSD, they do use MAP_STACK now AFAIK; this
> > > was announced here:
> > > <http://openbsd-archive.7691.n7.nabble.com/stack-register-checking-td338238.html>.
> > > Basically they periodically check whether the userspace stack pointer
> > > points into a MAP_STACK region, and if not, they kill the process. So
> > > even if it's a no-op on Linux...
> >
> > Hmm, is that something we should do in Linux?  Even if we only check
> > on syscall entry, which should be pretty inexpensive, it seems like it
> > would be very effective in protecting various ROP techniques.
> 
> I'm not a big fan, especially if that would only happen on syscall
> entry; at the point where you have enough control to perform syscalls,
> it probably isn't too difficult to move your ROP stack over to a
> legitimate stack.

It does have "your process is doing something unexpected" checking,
though, if we want to declare (and enforce) the expected behavior of
userspace stack usage. (i.e. not strictly as a ROP defense.)

-- 
Kees Cook