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

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Message-ID: <nycvar.YFH.7.76.1806152304400.6203@cbobk.fhfr.pm>
Date:   Fri, 15 Jun 2018 23:10:05 +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:

> Like it is possible to switch off PTI in the kernel it is possible to do 
> the same with XPTI in the hypervisor (it is even possible to disable 
> XPTI for dom0 only).
> 
> In case XPTI is disabled for the currently running system it is possible 
> to make use of Meltdown in user programs to read arbitrary physical host 
> memory (i.e. attacking the hypervisor) and this includes the own systems 
> kernel memory.
> 
> So telling a user the system isn't vulnerable regarding Meltdown when
> running as 64-bit pv-guest might not be the truth.

Ok, what a mess.

As I don't think it'd be wise to try to let guest kernel figure out 
whether host has XPTI, I'd suggest at least making the message somehow 
more informative. Something like

+               if (hypervisor_is_type(X86_HYPER_XEN_PV))
+                       return sprintf(buf, "Unknown (XEN PV detected, hypervisor mitigation required\n");

perhaps?

Thanks,

-- 
Jiri Kosina
SUSE Labs