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