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

[H.J. Lu]

On Mon, Mar 9, 2020 at 12:35 PM Dave Hansen <dave.hansen@xxxxxxxxx> 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.

-- 
H.J.