Re: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on context switch

[Andi Kleen]

> So I was really hoping that in places like context switching etc, we'd
> be able to instead effectively kill off any exploits by clearing
> registers.
> 
> That should make it pretty damn hard to then find a matching "gadget"
> that actually does anything interesting/powerful.
> 
> Together with Spectre already being pretty hard to take advantage of,
> and the eBPF people making those user-proivided gadgets inaccessible,
> it really should be a pretty powerful fix.
> 
> Hmm?

Essentially the RSB are hidden registers, and the only way to clear them
is the FILL_RETURN_BUFFER sequence.  I don't see how clearing anything else
would help?

-Andi