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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
References: <20180830143904.3168-1-yu-cheng.yu@intel.com>
 <20180830143904.3168-13-yu-cheng.yu@intel.com>
 <CAG48ez0Rca0XsdXJZ07c+iGPyep0Gpxw+sxQuACP5gyPaBgDKA@mail.gmail.com>
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From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <33d45a12-513c-eba2-a2de-3d6b630e928e@linux.intel.com>
Date: Thu, 30 Aug 2018 10:33:34 -0700
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To: Yu-cheng Yu <yu-cheng.yu@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>, 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

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.