linux-kernel - Re: [PATCH v10 01/26] Documentation/x86: Add CET description

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Date:   Fri, 15 May 2020 11:39:14 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v10 01/26] Documentation/x86: Add CET description

On 5/12/20 4:20 PM, Yu-cheng Yu wrote:
> On Wed, 2020-04-29 at 16:02 -0700, Yu-cheng Yu wrote:
>> On Wed, 2020-04-29 at 15:53 -0700, Dave Hansen wrote:
>>> On 4/29/20 3:07 PM, Yu-cheng Yu 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.
>>>
>>> I think Andy and I danced around this last time.  Let me try to say it
>>> more explicitly.
>>>
>>> I want CET kernel enabling to able to be disconnected from the on-disk
>>> binary.  I want a binary compiled with CET to be able to disable it, and
>>> I want a binary not compiled with CET to be able to enable it.  I want
>>> different threads in a process to be able to each have different CET status.
>>
>> The kernel patches we have now can be modified to support this model.  If after
>> discussion this is favorable, I will modify code accordingly.
> 
> To turn on/off and to lock CET are application-level decisions.  The kernel does
> not prevent any of those.  Should there be a need to provide an arch_prctl() to
> turn on CET, it can be added without any conflict to this series.

I spelled out what I wanted pretty clearly.  On your next post, could
you please directly address each of the things I asked for?  Please
directly answer the following questions in your next post with respect
to the code you post:

Can a binary compiled with CET run without CET?
Can a binary compiled without CET run CET-enabled code?
Can different threads in a process have different CET enabling state?

>>> Which JITs was this tested with?  I think as a bare minimum we need to
>>> know that this design can accommodate _a_ modern JIT.  It would be
>>> horrible if the browser javascript engines couldn't use this design, for
>>> instance.
>>
>> JIT work is still in progress.  When that is available I will test it.
> 
> I found CET has been enabled in LLVM JIT, Mesa JIT as well as sljit which is
> used by jit.  So the current model works with JIT.

Great!  I'm glad the model works.  That's not what I asked, though.

Does this *code* work?  Could you please indicate which JITs have been
enabled to use the code in this series?  How much of the new ABI is in use?

Where are the selftests/ for this new ABI?  Were you planning on
submitting any with this series?