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

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Message-ID: <20180711092951.GW2476@hirez.programming.kicks-ass.net>
Date:   Wed, 11 Jul 2018 11:29:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     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>,
        Dave Hansen <dave.hansen@...ux.intel.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>,
        "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 Tue, Jul 10, 2018 at 03:26:28PM -0700, Yu-cheng Yu wrote:
> There are three possible shadow stack PTE settings:
> 
>   Normal SHSTK PTE: (R/O + DIRTY_HW)
>   SHSTK PTE COW'ed: (R/O + DIRTY_HW)
>   SHSTK PTE shared as R/O data: (R/O + DIRTY_SW)

I count _2_ distinct states there.

> Update can_follow_write_pte/pmd for the shadow stack.

So the below disallows can_follow_write when shstk && _PAGE_DIRTY_SW,
but this here Changelog doesn't explain why. Doesn't even get close.

Also, the code is a right mess :/ Can't we try harder to not let this
shadow stack stuff escape arch code.