linux-kernel - Re: [PATCH] x86/pti: don't report XenPV as vulnerable

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Date:   Fri, 15 Jun 2018 08:39:03 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Juergen Gross <jgross@...e.com>
cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...e.de>,
        Mike Latimer <mlatimer@...e.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/pti: don't report XenPV as vulnerable

On Fri, 15 Jun 2018, Juergen Gross wrote:

> Why? PTI has to be disabled in PV guests as it can't work there due to 
> missing paravirtualization of the PTI feature (mov to/from %cr3).
> 
> The Xen meltdown mitigation ("XPTI") for 64-bit pv guests is primarily 
> securing the hypervisor against meltdown attacks of the guest. The guest 
> itself can't do anything in this regard in 64-bit mode, as user and 
> kernel code are already using different %cr3 values even without PTI.

That I know. Then I am probably dense today, but could you please again 
explain what you meant by this in your first reply:

	"This is wrong for [ ... ] for 64-bit, too, in case the mitigation is 
	 disabled at hypervisor level."

-- 
Jiri Kosina
SUSE Labs