linux-kernel - Re: [RFC PATCH v2 15/27] mm/mprotect: Prevent mprotect from changing shadow stack

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Date:   Wed, 11 Jul 2018 09:22:28 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
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>,
        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 15/27] mm/mprotect: Prevent mprotect from changing
 shadow stack

On 07/11/2018 09:07 AM, Yu-cheng Yu wrote:
>> Why do we need to disallow this? AFAICT the worst that can happen is
>> that a process wrecks itself, so what?
> Agree.  I will remove the patch.

No so quick. :)

We still need to find out a way to handle things that ask for an
mprotect() which is incompatible with shadow stacks.  PROT_READ without
PROT_WRITE comes to mind.  We also need to be careful that we don't
copy-on-write/copy-on-access pages which fault on PROT_NONE.  I *think*
it'll get done correctly but we have to be sure.

BTW, where are all the selftests for this code?  We're slowly building
up a list of pathological things that need to get tested.

I don't think this can or should get merged before we have selftests.