Re: [RFC] Retpoline: Binary mitigation for branch-target-injection (aka "Spectre")

[Andy Lutomirski <luto@kernel.org>]

On Thu, Jan 4, 2018 at 1:30 AM, Woodhouse, David <dwmw@amazon.co.uk> wrote:
> On Thu, 2018-01-04 at 01:10 -0800, Paul Turner wrote:
>> Apologies for the discombobulation around today's disclosure.  Obviously the
>> original goal was to communicate this a little more coherently, but the
>> unscheduled advances in the disclosure disrupted the efforts to pull this
>> together more cleanly.
>>
>> I wanted to open discussion the "retpoline" approach and and define its
>> requirements so that we can separate the core
>> details from questions regarding any particular implementation thereof.
>>
>> As a starting point, a full write-up describing the approach is available at:
>>   https://support.google.com/faqs/answer/7625886
>
> Note that (ab)using 'ret' in this way is incompatible with CET on
> upcoming processors. HJ added a -mno-indirect-branch-register option to
> the latest round of GCC patches, which puts the branch target in a
> register instead of on the stack. My kernel patches (which I'm about to
> reconcile with Andi's tweaks and post) do the same.
>
> That means that in the cases where at runtime we want to ALTERNATIVE
> out the retpoline, it just turns back into a bare 'jmp *\reg'.
>
>

I hate to say this, but I think Intel should postpone CET until the
dust settles.  Intel should also consider a hardware-protected stack
that is only accessible with PUSH, POP, CALL, RET, and a new MOVSTACK
instruction.  That, by itself, would give considerable protection.
But we still need JMP_NO_SPECULATE.  Or, better yet, get the CPU to
stop leaking data during speculative execution.