Re: [PATCH v2] qemu-cpu-models: Document -noTSX, mds-no, taa-no, and tsx-ctrl

[Paolo Bonzini <pbonzini@redhat.com>]

On 21/01/20 19:49, Kashyap Chamarthy wrote:
> Question: How can a user validate that TSX is indeed disabled for the
>           guest?

Look for rtm in /proc/cpuinfo, or look at the TAA entry in the sysfs
vulnerabilities directory.

> +@item @code{mds-no}
> +
> +Recommended to inform the guest OS that the host is @i{not} vulnerable
> +to any of the MDS variants ([MFBDS] CVE-2018-12130, [MLPDS]
> +CVE-2018-12127, [MSBDS] CVE-2018-12126).
> +
> +This is an MSR (Model-Specific Register) feature rather than a CPUID
> +feature, so it will not appear in the Linux @code{/proc/cpuinfo} in the
> +host or guest.  Instead, the host kernel uses it to populate the MDS
> +vulnerability file in @code{sysfs}.
> +
> +So it should only be enabled for VMs if the host reports @code{Not
> +affected} in the @code{/sys/devices/system/cpu/vulnerabilities/mds}
> +file.
> +
> +@item @code{taa-no}
> +
> +Recommended to inform that the guest that the host is @i{not} vulnerable
> +to CVE-2019-11135, TSX Asyncrnous Abort (TAA).

Asynchronous

> +
> +This too is an MSR feature, so it does not show up in the Linux
> +@code{/proc/cpuinfo} in the host or guest.
> +
> +It should only be enabled for VMs if the host reports @code{Not
> +affected} in the
> +@code{/sys/devices/system/cpu/vulnerabilities/tsx_async_abort} file.
> +
> +@item @code{tsx-ctrl}
> +
> +Recommended to inform the guest to @i{disable} the Intel TSX
> +(Transactional Synchronization Extensions) feature.

Not "to disable" but rather:

Recommended to inform the guest that it can disable the Intel TSX
feature or (if vulnerable) use the VERW instruction as a mitigation for
the TAA vulnerability.

Paolo

> Expose this to the
> +guest OS if and only if: (a) the host has TSX enabled; and (b) the guest
> +has @code{rtm} CPU flag enabled.
> +
> +By disabling TSX, KVM-based guests can avoid paying the price of
> +mitigting TSX-based attacks.
> +
> +Note that too is an MSR feature, so it does not show up in the Linux
> +@code{/proc/cpuinfo} in the host or guest.
> +
>  @end table
>  
> -
>  @node preferred_cpu_models_amd_x86
>  @subsubsection Preferred CPU models for AMD x86 hosts
>  
>