iTLB multihit
=============

iTLB multihit is an erratum where some processors may incur a machine check
error, possibly resulting in an unrecoverable CPU lockup, when an
instruction fetch hits multiple entries in the instruction TLB. This can
occur when the page size is changed along with either the physical address
or cache type. A malicious guest running on a virtualized system can
exploit this erratum to perform a denial of service attack.


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

Variations of this erratum are present on most Intel Core and Xeon processor
models. The erratum is not present on:

   - non-Intel processors

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

   - Intel processors that have the PSCHANGE_MC_NO bit set in the
     IA32_ARCH_CAPABILITIES MSR.


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

The following CVE entry is related to this issue:

   ==============  =================================================
   CVE-2018-12207  Machine Check Error Avoidance on Page Size Change
   ==============  =================================================


Problem
-------

Privileged software, including OS and virtual machine managers (VMM), are in
charge of memory management. A key component in memory management is the control
of the page tables. Modern processors use virtual memory, a technique that creates
the illusion of a very large memory for processors. This virtual space is split
into pages of a given size. Page tables translate virtual addresses to physical
addresses.

To reduce latency when performing a virtual to physical address translation,
processors include a structure, called TLB, that caches recent translations.
There are separate TLBs for instruction (iTLB) and data (dTLB).

Under this errata, instructions are fetched from a linear address translated
using a 4 KB translation cached in the iTLB. Privileged software modifies the
paging structure so that the same linear address using large page size (2 MB, 4
MB, 1 GB) with a different physical address or memory type.  After the page
structure modification but before the software invalidates any iTLB entries for
the linear address, a code fetch that happens on the same linear address may
cause a machine-check error which can result in a system hang or shutdown.


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

Attacks against the iTLB multihit erratum can be mounted from malicious
guests in a virtualized system.


iTLB multihit system information
--------------------------------

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

/sys/devices/system/cpu/vulnerabilities/itlb_multihit

The possible values in this file are:

.. list-table::

     * - Not affected
       - The processor is not vulnerable.
     * - KVM: Mitigation: Split huge pages
       - Software changes mitigate this issue.
     * - KVM: Mitigation: VMX unsupported
       - KVM is not vulnerable because Virtual Machine Extensions (VMX) is not supported.
     * - KVM: Mitigation: VMX disabled
       - KVM is not vulnerable because Virtual Machine Extensions (VMX) is disabled.
     * - KVM: Vulnerable
       - The processor is vulnerable, but no mitigation enabled


Enumeration of the erratum
--------------------------------

A new bit has been allocated in the IA32_ARCH_CAPABILITIES (PSCHANGE_MC_NO) msr
and will be set on CPU's which are mitigated against this issue.

   =======================================   ===========   ===============================
   IA32_ARCH_CAPABILITIES MSR                Not present   Possibly vulnerable,check model
   IA32_ARCH_CAPABILITIES[PSCHANGE_MC_NO]    '0'           Likely vulnerable,check model
   IA32_ARCH_CAPABILITIES[PSCHANGE_MC_NO]    '1'           Not vulnerable
   =======================================   ===========   ===============================


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

This erratum can be mitigated by restricting the use of large page sizes to
non-executable pages.  This forces all iTLB entries to be 4K, and removes
the possibility of multiple hits.

In order to mitigate the vulnerability, KVM initially marks all huge pages
as non-executable. If the guest attempts to execute in one of those pages,
the page is broken down into 4K pages, which are then marked executable.

If EPT is disabled or not available on the host, KVM is in control of TLB
flushes and the problematic situation cannot happen.  However, the shadow
EPT paging mechanism used by nested virtualization is vulnerable, because
the nested guest can trigger multiple iTLB hits by modifying its own
(non-nested) page tables.  For simplicity, KVM will make large pages
non-executable in all shadow paging modes.

Mitigation control on the kernel command line and KVM - module parameter
------------------------------------------------------------------------

The KVM hypervisor mitigation mechanism for marking huge pages as
non-executable can be controlled with a module parameter "nx_huge_pages=".
The kernel command line allows to control the iTLB multihit mitigations at
boot time with the option "kvm.nx_huge_pages=".

The valid arguments for these options are:

  ==========  ================================================================
  force       Mitigation is enabled. In this case, the mitigation implements
              non-executable huge pages in Linux kernel KVM module. All huge
              pages in the EPT are marked as non-executable.
              If a guest attempts to execute in one of those pages, the page is
              broken down into 4K pages, which are then marked executable.

  off	      Mitigation is disabled.

  auto        Enable mitigation only if the platform is affected and the kernel
              was not booted with the "mitigations=off" command line parameter.
	      This is the default option.
  ==========  ================================================================


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

1. No virtualization in use
^^^^^^^^^^^^^^^^^^^^^^^^^^^

   The system is protected by the kernel unconditionally and no further
   action is required.

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

   If the guest comes from a trusted source, you may assume that the guest will
   not attempt to maliciously exploit these errata and no further action is
   required.

3. Virtualization with untrusted guests
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   If the guest comes from an untrusted source, the guest host kernel will need
   to apply iTLB multihit mitigation via the kernel command line or kvm
   module parameter.