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Date:   Thu, 30 Aug 2018 23:01:26 +0200
From:   Jann Horn <jannh@...gle.com>
To:     yu-cheng.yu@...el.com
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Florian Weimer <fweimer@...hat.com>, hjl.tools@...il.com,
        Jonathan Corbet <corbet@....net>, keescook@...omiun.org,
        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>,
        ravi.v.shankar@...el.com, vedvyas.shanbhogue@...el.com
Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and
 pmdp_set_wrprotect for _PAGE_DIRTY_SW

On Thu, Aug 30, 2018 at 10:57 PM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
>
> On Thu, 2018-08-30 at 22:44 +0200, Jann Horn wrote:
> > On Thu, Aug 30, 2018 at 10:25 PM Yu-cheng Yu <yu-cheng.yu@...el.com>
> > wrote:
> ...
> > > In the flow you described, if C writes to the overflow page before
> > > B
> > > gets in with a 'call', the return address is still correct for
> > > B.  To
> > > make an attack, C needs to write again before the TLB flush.  I
> > > agree
> > > that is possible.
> > >
> > > Assume we have a guard page, can someone in the short window do
> > > recursive calls in B, move ssp to the end of the guard page, and
> > > trigger the same again?  He can simply take the incssp route.
> > I don't understand what you're saying. If the shadow stack is
> > between
> > guard pages, you should never be able to move SSP past that area's
> > guard pages without an appropriate shadow stack token (not even with
> > INCSSP, since that has a maximum range of PAGE_SIZE/2), and
> > therefore,
> > it shouldn't matter whether memory outside that range is incorrectly
> > marked as shadow stack. Am I missing something?
>
> INCSSP has a range of 256, but we can do multiple of that.
> But I realize the key is not to have the transient SHSTK page at all.
> The guard page is !pte_write() and even we have flaws in
> ptep_set_wrprotect(), there will not be any transient SHSTK pages. I
> will add guard pages to both ends.
>
> Still thinking how to fix ptep_set_wrprotect().

cmpxchg loop? Or is that slow?

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