On Thu, Jan 4, 2018 at 1:30 AM, Woodhouse, David <d...@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.
