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Subject: Re: [RFC PATCH v9 01/27] Documentation/x86: Add CET description
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>, x86-patch-review@intel.com
References: <20200205181935.3712-1-yu-cheng.yu@intel.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <9b7ff325-d7cd-9309-d060-ad641486d106@intel.com>
Date:   Mon, 9 Mar 2020 12:35:22 -0700
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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.