Re: [PATCH v3] qemu-cpu-models: Document -noTSX, mds-no, taa-no, and tsx-ctrl

[Kashyap Chamarthy <kchamart@redhat.com>]

Ping.

On Mon, Jan 27, 2020 at 01:16:24PM +0100, Kashyap Chamarthy wrote:
> - Add the '-noTSX' variants for CascadeLake and SkyLake.
> 
> - Document the three MSR bits: 'mds-no', 'taa-no', and 'tsx-ctrl'
> 
>   Two confusing about 'mds-no' (and the first point applies to the other
>   two MSRs too):
> 
>   (1) The 'mds-no' will _not_ show up in the guest's /proc/cpuinfo.
>       Rather it is used to fill in the guest's sysfs:
> 
>         sys/devices/system/cpu/vulnerabilities/mds:Not affected
> 
>       Paolo confirmed on IRC as such.
> 
>   (2) There are _three_ variants[+] of CascadeLake CPUs, with different
>       stepping levels: 5, 6, and 7.  To quote wikichip.org[*]:
> 
>         "note that while steppings 6 & 7 are fully mitigated, earlier
>         stepping 5 is not protected against MSBDS, MLPDS, nor MDSUM"
> 
>       The above is also indicated in the Intel's document[+], as
>       indicated by "No" under the three columns of MFBDS, MSBDS, and
>       MLPDS.
> 
>       [+] https://software.intel.com/security-software-guidance/insights/processors-affected-microarchitectural-data-sampling
>       [*] https://en.wikichip.org/wiki/intel/microarchitectures/cascade_lake#Key_changes_from_Skylake
> 
> Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
> ---
> v3:
>  - Address feedback from Paolo
>  - Add URL to the deep-dive on Intel's MDS
> v2:
>  - Address feedback from DanPB
>  - Add sections on 'taa-no' and 'tsx-ctrl'
> ---
>  docs/qemu-cpu-models.texi | 65 ++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 64 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/qemu-cpu-models.texi b/docs/qemu-cpu-models.texi
> index f88a1def0d042cc25213259172a648f0a9c514dc..8be3fc34570c5af10557b862c467f5520245a85a 100644
> --- a/docs/qemu-cpu-models.texi
> +++ b/docs/qemu-cpu-models.texi
> @@ -72,14 +72,25 @@ between machines, if live migration compatibility is required, use the newest
>  CPU model that is compatible across all desired hosts.
>  
>  @table @option
> +
> +@item @code{Cascadelake-Server}
> +@item @code{Cascadelake-Server-noTSX}
> +
> +Intel Xeon Processor (Cascade Lake, 2019), with "stepping" levels
> +6 or 7 only.  (The Cascade Lake Xeon processor with @b{stepping 5 is
> +vulnerable to MDS variants}.)
> +
> +
>  @item @code{Skylake-Server}
>  @item @code{Skylake-Server-IBRS}
> +@item @code{Skylake-Server-noTSX-IBRS}
>  
>  Intel Xeon Processor (Skylake, 2016)
>  
>  
>  @item @code{Skylake-Client}
>  @item @code{Skylake-Client-IBRS}
> +@item @code{Skylake-Client-noTSX-IBRS}
>  
>  Intel Core Processor (Skylake, 2015)
>  
> @@ -214,9 +225,61 @@ Must be explicitly turned on for all Intel CPU models.
>  
>  Requires the host CPU microcode to support this feature before it
>  can be used for guest CPUs.
> +
> +@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 Asynchronous Abort (TAA).
> +
> +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 that it can disable the Intel TSX
> +(Transactional Synchronization Extensions) feature; or, if the processor
> +is vulnerable, use the Intel VERW instruction (a processor-level
> +instruction that performs checks on memory access) as a mitigation for
> +the TAA vulnerability.  (For details, refer to
> +@url{https://software.intel.com/security-software-guidance/insights/deep-dive-intel-analysis-microarchitectural-data-sampling,
> +this Intel's deep-dive into MDS}.
> +
> +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 @code{tasx-ctrl} too is an MSR feature, so it does not show up
> +in the Linux @code{/proc/cpuinfo} in the host or guest.
> +
> +To validate that Intel TSX is indeed disabled for the guest, there are
> +two ways: (a) check for the @i{absence} of @code{rtm} in the guest's
> +@code{/proc/cpuinfo}; or (b) the
> +@code{/sys/devices/system/cpu/vulnerabilities/tsx_async_abort} file in
> +the guest should report @code{Mitigation: TSX disabled}.
> +
>  @end table
>  
> -
>  @node preferred_cpu_models_amd_x86
>  @subsubsection Preferred CPU models for AMD x86 hosts
>  
> -- 
> 2.21.0
> 
> 

-- 
/kashyap