Re: [RFC PATCH v9 01/27] Documentation/x86: Add CET description

H.J. Lu
2020-03-09T20:54:53Z

On Mon, Mar 9, 2020 at 1:16 PM Andy Lutomirski <luto@amacapital.net> wrote:
>
>
>
> > On Mar 9, 2020, at 12:50 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > ﻿On Mon, Mar 9, 2020 at 12:35 PM Dave Hansen <dave.hansen@intel.com> wrote:
> >>
> >>> On 3/9/20 12:27 PM, Yu-cheng Yu wrote:
> >>> On Mon, 2020-03-09 at 10:21 -0700, Dave Hansen wrote:
> >>>> On 3/9/20 10:00 AM, Yu-cheng Yu wrote:
> >>>>> On Wed, 2020-02-26 at 09:57 -0800, Dave Hansen wrote>>>>> +Note:
> >>>>>>> + There is no CET-enabling arch_prctl function. By design, CET is
> >>>>>>> + enabled automatically if the binary and the system can support it.
> >>>>>>
> >>>>>> This is kinda interesting. It means that a JIT couldn't choose to
> >>>>>> protect the code it generates and have different rules from itself?
> >>>>>
> >>>>> JIT needs to be updated for CET first. Once that is done, it runs with CET
> >>>>> enabled. It can use the NOTRACK prefix, for example.
> >>>>
> >>>> Am I missing something?
> >>>>
> >>>> What's the direct connection between shadow stacks and Indirect Branch
> >>>> Tracking other than Intel marketing umbrellas?
> >>>
> >>> What I meant is that JIT code needs to be updated first; if it skips RETs,
> >>> it needs to unwind the stack, and if it does indirect JMPs somewhere it
> >>> needs to fix up the branch target or use NOTRACK.
> >>
> >> I'm totally lost. I think we have very different models of how a JIT
> >> might generate and run code.
> >>
> >> I can totally see a scenario where a JIT goes and generates a bunch of
> >> code, then forks a new thread to go run that code. The control flow of
> >> the JIT thread itself *NEVER* interacts with the control flow of the
> >> program it writes. They never share a stack and nothing ever jumps or
> >> rets between the two worlds.
> >>
> >> Does anything actually do that? I've got no idea. But, I can clearly
> >> see a world where the entirety of Chrome and Firefox and the entire rust
> >> runtime might not be fully recompiled and CET-enabled for a while. But,
> >> we still want the JIT-generated code to be CET-protected since it has
> >> the most exposed attack surface.
> >>
> >> I don't think that's too far-fetched.
> >
> > CET support is all or nothing. You can mix and match, but you will get
> > no CET protection, similar to NX feature.
> >
>
> Can you explain?

I was talking about creating a program from mixed object files with and without
CET marker.

> If a program with the magic ELF CET flags missing can’t make a thread with IBT and/or SHSTK enabled, then I think we’ve made an error and should fix it.
>

A non-CET program can start a CET program and vice versa.

--
H.J.