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

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Date:   Tue, 19 Jun 2018 15:38:57 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Kees Cook <keescook@...omium.org>
Cc:     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 Tue, 2018-06-19 at 13:47 -0700, Andy Lutomirski wrote:
> > 
> > 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@...capita
> > > l.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).

We have the arch_prctl now and only need to add ptrace lock/unlock.

Back to the dlopen() "relaxed" mode. Would the following work?

If the lib being loaded does not use setjmp/getcontext families (the
loader knows?), then the loader leaves shstk on.  Otherwise, if the
system-wide setting is "relaxed", the loader turns off shstk and issues
a warning.  In addition, if (dlopen == relaxed), then cet is not locked
in any time.

The system-wide setting (somewhere in /etc?) can be:

	dlopen=force|relaxed /* controls dlopen of non-cet libs */
	exec=force|relaxed /* controls exec of non-cet apps */

--
Yu-cheng