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

Dave Hansen
2020-03-09T19:35:37Z

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.