MDS - Microarchitectural Data Sampling
======================================

Microarchitectural Data Sampling is a hardware vulnerability which allows
unprivileged speculative access to data which is available in various CPU
internal buffers.

Affected processors
-------------------

This vulnerability affects a wide range of Intel processors. The
vulnerability is not present on:

   - Processors from AMD, Centaur and other non Intel vendors

   - Older processor models, where the CPU family is < 6

   - Some Atoms (Bonnell, Saltwell, Goldmont, GoldmontPlus)

   - Intel processors which have the ARCH_CAP_MDS_NO bit set in the
     IA32_ARCH_CAPABILITIES MSR.

Whether a processor is affected or not can be read out from the MDS
vulnerability file in sysfs. See :ref:`mds_sys_info`.

Not all processors are affected by all variants of MDS, but the mitigation
is identical for all of them so the kernel treats them as a single
vulnerability.

Related CVEs
------------

The following CVE entries are related to the MDS vulnerability:

   ==============  =====  ===================================================
   CVE-2018-12126  MSBDS  Microarchitectural Store Buffer Data Sampling
   CVE-2018-12130  MFBDS  Microarchitectural Fill Buffer Data Sampling
   CVE-2018-12127  MLPDS  Microarchitectural Load Port Data Sampling
   CVE-2019-11091  MDSUM  Microarchitectural Data Sampling Uncacheable Memory
   ==============  =====  ===================================================

Problem
-------

When performing store, load, L1 refill operations, processors write data
into temporary microarchitectural structures (buffers). The data in the
buffer can be forwarded to load operations as an optimization.

Under certain conditions, usually a fault/assist caused by a load
operation, data unrelated to the load memory address can be speculatively
forwarded from the buffers. Because the load operation causes a fault or
assist and its result will be discarded, the forwarded data will not cause
incorrect program execution or state changes. But a malicious operation
may be able to forward this speculative data to a disclosure gadget which
allows in turn to infer the value via a cache side channel attack.

Because the buffers are potentially shared between Hyper-Threads cross
Hyper-Thread attacks are possible.

Deeper technical information is available in the MDS specific x86
architecture section: :ref:`Documentation/x86/mds.rst <mds>`.


Attack scenarios
----------------

Attacks against the MDS vulnerabilities can be mounted from malicious non
priviledged user space applications running on hosts or guest. Malicious
guest OSes can obviously mount attacks as well.

Contrary to other speculation based vulnerabilities the MDS vulnerability
does not allow the attacker to control the memory target address. As a
consequence the attacks are purely sampling based, but as demonstrated with
the TLBleed attack samples can be postprocessed successfully.

Web-Browsers
^^^^^^^^^^^^

  It's unclear whether attacks through Web-Browsers are possible at
  all. The exploitation through Java-Script is considered very unlikely,
  but other widely used web technologies like Webassembly could possibly be
  abused.


.. _mds_sys_info:

MDS system information
-----------------------

The Linux kernel provides a sysfs interface to enumerate the current MDS
status of the system: whether the system is vulnerable, and which
mitigations are active. The relevant sysfs file is:

/sys/devices/system/cpu/vulnerabilities/mds

The possible values in this file are:

  .. list-table::

     * - 'Not affected'
       - The processor is not vulnerable
     * - 'Vulnerable'
       - The processor is vulnerable, but no mitigation enabled
     * - 'Vulnerable: Clear CPU buffers attempted, no microcode'
       - The processor is vulnerable but microcode is not updated.

         The mitigation is enabled on a best effort basis. See :ref:`vmwerv`
     * - 'Mitigation: Clear CPU buffers'
       - The processor is vulnerable and the CPU buffer clearing mitigation is
         enabled.

If the processor is vulnerable then the following information is appended
to the above information:

    ========================  ============================================
    'SMT vulnerable'          SMT is enabled
    'SMT mitigated'           SMT is enabled and mitigated
    'SMT disabled'            SMT is disabled
    'SMT Host state unknown'  Kernel runs in a VM, Host SMT state unknown
    ========================  ============================================

.. _vmwerv:

Best effort mitigation mode
^^^^^^^^^^^^^^^^^^^^^^^^^^^

  If the processor is vulnerable, but the availability of the microcode based
  mitigation mechanism is not advertised via CPUID the kernel selects a best
  effort mitigation mode.  This mode invokes the mitigation instructions
  without a guarantee that they clear the CPU buffers.

  This is done to address virtualization scenarios where the host has the
  microcode update applied, but the hypervisor is not yet updated to expose
  the CPUID to the guest. If the host has updated microcode the protection
  takes effect otherwise a few cpu cycles are wasted pointlessly.

  The state in the mds sysfs file reflects this situation accordingly.


Mitigation mechanism
-------------------------

The kernel detects the affected CPUs and the presence of the microcode
which is required.

If a CPU is affected and the microcode is available, then the kernel
enables the mitigation by default. The mitigation can be controlled at boot
time via a kernel command line option. See
:ref:`mds_mitigation_control_command_line`.

.. _cpu_buffer_clear:

CPU buffer clearing
^^^^^^^^^^^^^^^^^^^

  The mitigation for MDS clears the affected CPU buffers on return to user
  space and when entering a guest.

  If SMT is enabled it also clears the buffers on idle entry when the CPU
  is only affected by MSBDS and not any other MDS variant, because the
  other variants cannot be protected against cross Hyper-Thread attacks.

  For CPUs which are only affected by MSBDS the user space, guest and idle
  transition mitigations are sufficient and SMT is not affected.

.. _virt_mechanism:

Virtualization mitigation
^^^^^^^^^^^^^^^^^^^^^^^^^

  The protection for host to guest transition depends on the L1TF
  vulnerability of the CPU:

  - CPU is affected by L1TF:

    If the L1D flush mitigation is enabled and up to date microcode is
    available, the L1D flush mitigation is automatically protecting the
    guest transition.

    If the L1D flush mitigation is disabled then the MDS mitigation is
    invoked explicit when the host MDS mitigation is enabled.

    For details on L1TF and virtualization see:
    :ref:`Documentation/hw-vuln//l1tf.rst <mitigation_control_kvm>`.

  - CPU is not affected by L1TF:

    CPU buffers are flushed before entering the guest when the host MDS
    mitigation is enabled.

  The resulting MDS protection matrix for the host to guest transition:

  ============ ===== ============= ============ =================
   L1TF         MDS   VMX-L1FLUSH   Host MDS     MDS-State

   Don't care   No    Don't care    N/A          Not affected

   Yes          Yes   Disabled      Off          Vulnerable

   Yes          Yes   Disabled      Full         Mitigated

   Yes          Yes   Enabled       Don't care   Mitigated

   No           Yes   N/A           Off          Vulnerable

   No           Yes   N/A           Full         Mitigated
  ============ ===== ============= ============ =================

  This only covers the host to guest transition, i.e. prevents leakage from
  host to guest, but does not protect the guest internally. Guests need to
  have their own protections.

.. _xeon_phi:

XEON PHI specific considerations
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  The XEON PHI processor family is affected by MSBDS which can be exploited
  cross Hyper-Threads when entering idle states. Some XEON PHI variants allow
  to use MWAIT in user space (Ring 3) which opens an potential attack vector
  for malicious user space. The exposure can be disabled on the kernel
  command line with the 'ring3mwait=disable' command line option.

  XEON PHI is not affected by the other MDS variants and MSBDS is mitigated
  before the CPU enters a idle state. As XEON PHI is not affected by L1TF
  either disabling SMT is not required for full protection.

.. _mds_smt_control:

SMT control
^^^^^^^^^^^

  All MDS variants except MSBDS can be attacked cross Hyper-Threads. That
  means on CPUs which are affected by MFBDS or MLPDS it is necessary to
  disable SMT for full protection. These are most of the affected CPUs; the
  exception is XEON PHI, see :ref:`xeon_phi`.

  Disabling SMT can have a significant performance impact, but the impact
  depends on the type of workloads.

  See the relevant chapter in the L1TF mitigation documentation for details:
  :ref:`Documentation/hw-vuln/l1tf.rst <smt_control>`.


.. _mds_mitigation_control_command_line:

Mitigation control on the kernel command line
---------------------------------------------

The kernel command line allows to control the MDS mitigations at boot
time with the option "mds=". The valid arguments for this option are:

  ============  =============================================================
  full		If the CPU is vulnerable, enable all available mitigations
		for the MDS vulnerability, CPU buffer clearing on exit to
		userspace and when entering a VM. Idle transitions are
		protected as well if SMT is enabled.

		It does not automatically disable SMT.

  off		Disables MDS mitigations completely.

  ============  =============================================================

Not specifying this option is equivalent to "mds=full". For processors
that are affected by both TAA (TSX Asynchronous Abort) and MDS,
specifying just "mds=off" without an accompanying "tsx_async_abort=off"
will have no effect as the same mitigation is used for both
vulnerabilities.

Mitigation selection guide
--------------------------

1. Trusted userspace
^^^^^^^^^^^^^^^^^^^^

   If all userspace applications are from a trusted source and do not
   execute untrusted code which is supplied externally, then the mitigation
   can be disabled.


2. Virtualization with trusted guests
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

   The same considerations as above versus trusted user space apply.

3. Virtualization with untrusted guests
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

   The protection depends on the state of the L1TF mitigations.
   See :ref:`virt_mechanism`.

   If the MDS mitigation is enabled and SMT is disabled, guest to host and
   guest to guest attacks are prevented.

.. _mds_default_mitigations:

Default mitigations
-------------------

  The kernel default mitigations for vulnerable processors are:

  - Enable CPU buffer clearing

  The kernel does not by default enforce the disabling of SMT, which leaves
  SMT systems vulnerable when running untrusted code. The same rationale as
  for L1TF applies.
  See :ref:`Documentation/hw-vuln//l1tf.rst <default_mitigations>`.
