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

[Andy Lutomirski]

> On Mar 9, 2020, at 12:50 PM, H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
> 
> ïOn 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.
> 

Can you explain?

If a program with the magic ELF CET flags missing canât make a thread with IBT and/or SHSTK enabled, then I think weâve made an error and should fix it.

> -- 
> H.J.