linux-kernel - Re: [PATCH 06/10] x86/cet: Add arch

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Date:   Tue, 19 Jun 2018 13:47:32 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Kees Cook <keescook@...omium.org>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. J. Lu" <hjl.tools@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        mike.kravetz@...cle.com, Florian Weimer <fweimer@...hat.com>
Subject: Re: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack


> On Jun 19, 2018, at 1:12 PM, Kees Cook <keescook@...omium.org> wrote:
> 
>> On Tue, Jun 19, 2018 at 10:20 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> 
>>> On Jun 19, 2018, at 10:07 AM, Kees Cook <keescook@...omium.org> wrote:
>>> 
>>> Does it provide anything beyond what PR_DUMPABLE does?
>> 
>> What do you mean?
> 
> I was just going by the name of it. I wasn't sure what "ptrace CET
> lock" meant, so I was trying to understand if it was another "you
> can't ptrace me" toggle, and if so, wouldn't it be redundant with
> PR_SET_DUMPABLE = 0, etc.
> 

No, other way around. The valid CET states are on/unlocked, off/unlocked, on/locked, off/locked. arch_prctl can freely the state unless locked. ptrace can change it no matter what.  The lock is to prevent the existence of a gadget to disable CET (unless the gadget involves ptrace, but I don’t think that’s a real concern).