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

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Date:   Sat, 16 Jun 2018 08:36:31 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jiri Kosina <jikos@...nel.org>
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 15/06/18 23:10, Jiri Kosina wrote:
> 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?

Works for me.


Juergen