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Subject: Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for
 shadow stack
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, 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>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <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" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>
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From:   Dave Hansen <dave.hansen@linux.intel.com>
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Message-ID: <b3cafb94-39c1-3e03-a4f0-295e506da0a6@linux.intel.com>
Date:   Thu, 19 Jul 2018 12:31:43 -0700
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On 07/19/2018 10:06 AM, Yu-cheng Yu wrote:
> Which pte_write() do you think is right?

There isn't one that's right.

The problem is that the behavior right now is ambiguous.  Some callers
of pte_write() need to know about _PAGE_RW alone and others want to know
if (_PAGE_RW || is_shstk()).

The point is that you need both, plus a big audit of all the pte_write()
users to ensure they use the right one.

For instance, see spurious_fault_check().  We can get a shadowstack
fault that also has X86_PF_WRITE, but pte_write()==0.  That might make a
shadowstack write fault falsely appear spurious.