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From:   Jann Horn <jannh@google.com>
Date:   Thu, 30 Aug 2018 19:59:32 +0200
Message-ID: <CAG48ez3ixWROuQc6WZze6qPL6q0e_gCnMU4XF11JUWziePsBJg@mail.gmail.com>
Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and
 pmdp_set_wrprotect for _PAGE_DIRTY_SW
To:     yu-cheng.yu@intel.com
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        linux-doc@vger.kernel.org, Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@amacapital.net>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Florian Weimer <fweimer@redhat.com>, hjl.tools@gmail.com,
        Jonathan Corbet <corbet@lwn.net>, keescook@chromiun.org,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        ravi.v.shankar@intel.com, vedvyas.shanbhogue@intel.com
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On Thu, Aug 30, 2018 at 7:58 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>
> On Thu, 2018-08-30 at 10:33 -0700, Dave Hansen wrote:
> > On 08/30/2018 10:26 AM, Yu-cheng Yu wrote:
> > >
> > > We don't have the guard page now, but there is a shadow stack
> > > token
> > > there, which cannot be used as a return address.
> > The overall concern is that we could overflow into a page that we
> > did
> > not intend.  Either another actual shadow stack or something that a
> > page
> > that the attacker constructed, like the transient scenario Jann
> > described.
> >
>
> A task could go beyond the bottom of its shadow stack by doing either
> 'ret' or 'incssp'.  If it is the 'ret' case, the token prevents it.
>  If it is the 'incssp' case, a guard page cannot prevent it entirely,
> right?

I mean the other direction, on "call".