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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
From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     Dave Hansen <dave.hansen@linux.intel.com>,
        Jann Horn <jannh@google.com>
Cc:     the arch/x86 maintainers <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
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        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
Date:   Thu, 30 Aug 2018 10:54:26 -0700
In-Reply-To: <33d45a12-513c-eba2-a2de-3d6b630e928e@linux.intel.com>
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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?

Yu-cheng