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Message-ID: <1531868435.3541.18.camel@intel.com>
Subject: Re: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for
 shadow stack
From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     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>,
        Dave Hansen <dave.hansen@linux.intel.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>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>
Date:   Tue, 17 Jul 2018 16:00:35 -0700
In-Reply-To: <20180711092951.GW2476@hirez.programming.kicks-ass.net>
References: <20180710222639.8241-1-yu-cheng.yu@intel.com>
         <20180710222639.8241-17-yu-cheng.yu@intel.com>
         <20180711092951.GW2476@hirez.programming.kicks-ass.net>
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On Wed, 2018-07-11 at 11:29 +0200, Peter Zijlstra wrote:
> 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.

Can we add the following to the log:

When a SHSTK PTE is shared, it is (R/O + DIRTY_SW); otherwise it is
(R/O + DIRTY_HW).

When we (FOLL_WRITE | FOLL_FORCE) on a SHSTK PTE, the following
must be true:

  - It has been COW'ed at least once (FOLL_COW is set);
  - It still is not shared, i.e. PTE is (R/O + DIRTY_HW);

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

We either check here if the VMA is SHSTK mapping or move the logic
to pte_dirty().  The latter would be less obvious.  Or can we
create a can_follow_write_shstk_pte()?

Yu-cheng