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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 <yu-cheng.yu@intel.com>, Jann Horn <jannh@google.com>
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Cc:     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>,
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        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@amacapital.net>,
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        Cyrill Gorcunov <gorcunov@gmail.com>,
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        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
From:   Dave Hansen <dave.hansen@linux.intel.com>
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Message-ID: <33d45a12-513c-eba2-a2de-3d6b630e928e@linux.intel.com>
Date:   Thu, 30 Aug 2018 10:33:34 -0700
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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.