Re: Detecting whether dom0 is in a VM

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.07.2023 11:52, George Dunlap wrote:
> On Fri, Jul 7, 2023 at 9:00 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
> 
>> On 06.07.2023 17:35, zithro wrote:
>>> On 06 Jul 2023 09:02, Jan Beulich wrote:
>>>> On 05.07.2023 18:20, zithro wrote:
>>>>> So I'm wondering, isn't that path enough for correct detection ?
>>>>> I mean, if "/sys/class/dmi/id/sys_vendor" reports Xen (or KVM, or any
>>>>> other known hypervisor), it's nested, otherwise it's on hardware ?
>>>>>
>>>>> Is that really mandatory to use CPUID leaves ?
>>>>
>>>> Let me ask the other way around: In user mode code under a non-nested
>>>> vs nested Xen, what would you be able to derive from CPUID? The
>>>> "hypervisor" bit is going to be set in both cases. (All assuming you
>>>> run on new enough hardware+Xen such that CPUID would be intercepted
>>>> even for PV.)
>>>
>>> I'm a bit clueless about CPUID stuff, but if I understand correctly,
>>> you're essentially saying that using CPUID may not be the perfect way ?
>>> Also, I don't get why the cpuid command returns two different values,
>>> depending on the -k switch :
>>> # cpuid -l 0x40000000
>>> hypervisor_id (0x40000000) = "\0\0\0\0\0\0\0\0\0\0\0\0"
>>> # cpuid -k -l 0x40000000
>>> hypervisor_id (0x40000000) = "XenVMMXenVMM"
>>
>> I'm afraid I can't comment on this without knowing what tool you're
>> taking about. Neither of the two systems I checked have one of this
>> name.
>>
>>>> Yet relying on DMI is fragile, too: Along the lines of
>>>> https://lists.xen.org/archives/html/xen-devel/2022-01/msg00604.html
>>>> basically any value in there could be "inherited" from the host (i.e.
>>>> from the layer below, to be precise).
>>>
>>> So using "/sys/class/dmi/id/sys_vendor", or simply doing "dmesg | grep
>>> DMI:" is also not perfect, as values can be inherited/spoofed by
>>> underneath hypervisor ?
>>
>> That's my understanding, yes.
>>
>>>> The only way to be reasonably
>>>> certain is to ask Xen about its view. The raw or host featuresets
>>>> should give you this information, in the "mirror" of said respective
>>>> CPUID leave's "hypervisor" bit.
>>>
>>> As said above, I'm clueless, can you expand please ?
>>
>> Xen's public interface offers access to the featuresets known / found /
>> used by the hypervisor. See XEN_SYSCTL_get_cpu_featureset, accessible
>> via xc_get_cpu_featureset().
>>
> 
> Are any of these exposed in dom0 via sysctl, or hypfs?

sysctl - yes (as the quoted name also says). hypfs no, afaict.

>  SYSCTLs are
> unfortunately not stable interfaces, correct?  So it wouldn't be practical
> for systemd to use them.

Indeed, neither sysctl-s nor the libxc interfaces are stable.

Jan