Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code

[Andrea Arcangeli]

On Wed, Jan 10, 2018 at 11:58:54AM +0000, David Woodhouse wrote:
> On Wed, 2018-01-10 at 12:54 +0100, Andrea Arcangeli wrote:
> > On Wed, Jan 10, 2018 at 09:27:59AM +0000, David Woodhouse wrote:
> > > I don't know why you're calling that 'IBRS=2'; are you getting
> > confused
> > > by Andrea's distro horridness?
> > 
> > Eh, yes he's got confused. ibrs_enabled 2 simply means to leave IBRS
> > set in SPEC_CTLR 100% of the time, except in guest mode.
> 
> On all current hardware, if you only set IBRS when you exit a guest,
> then you are not protecting yourself from userspace at all. IBRS acts
> as a *barrier* in all current hardware.

Kernel memory is 100% protected if you set only IBRS at vmexit.

Once IBRS is set, there's no way any userland (nor host nor guest) can
attack the kernel memory through spectre variant#2.

What is not protected is host userland from guest userland which is
point 3 in the email I posted earlier and I already provided all
details there on how to fix that purely theoretical issue not part of
the PoC with the provided debugfs tunables, so I won't repeat here.