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

Andy Lutomirski
2020-03-09T20:16:19Z

> 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?

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.

> --
> H.J.