Messages in this thread | | | Date | Fri, 5 Jan 2018 02:49:44 -0800 | From | Paul Turner <> | Subject | Re: [RFC] Retpoline: Binary mitigation for branch-target-injection (aka "Spectre") |
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On Thu, Jan 04, 2018 at 08:18:57AM -0800, Andy Lutomirski wrote: > 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.
Echoing Andy's thoughts, but from a slightly different angle:
1) BTI is worse than the current classes of return attack. Given this, considered as a binary choice, it's equivalent to the current state of the world (e.g. no CET). 2) CET will not be "free". I suspect in its initial revisions it will be more valuable for protecting end-users then enterprise workloads (cost is not observable for interactive workloads because there's tons of headroom in the first place).
While the potential incompatibility is unfortunate; I'm not sure it makes a significant adoption to the adoption rate of CET.
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