src/kernel/linux/v4.19/Documentation/admin-guide/hw-vuln/multihit.rst - T800 - Gitiles

 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: 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.
	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: 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.