Re: [PATCH] x86/retpoline/entry: Disable the entire SYSCALL64 fast path with retpolines on

[Linus Torvalds]

On Thu, Jan 25, 2018 at 1:39 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> If you're rejiggering, can we also put in a mechanism for detecting
> which registers to clear so that userspace can't inject useful values
> into speculation paths?

That actually becomes trivial with just the "no fastpath" patch I sent
out. You can just clear all of them.

Sure, then do_syscall_64() will reload the six first ones, but since
those are the argument registers anyway, and since they are
caller-clobbered, they have very short lifetimes. So it would
effectively not really be an issue.

But yes, SYSCALL_DEFINEx() rejiggery would close even that tiny hole.

                   Linus