linux-kernel - Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write

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Date:   Thu, 19 Jul 2018 12:31:43 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, 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>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <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" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for
 shadow stack

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.