Merge commit '1ff01511' into kernel.lnx.4.9.r27-rel

		/sys/devices/system/cpu/vulnerabilities/spec_store_bypass

		/sys/devices/system/cpu/vulnerabilities/l1tf

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

		/sys/devices/system/cpu/vulnerabilities/tsx_async_abort

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

Date:		January 2018

Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>

Description:	Information about CPU vulnerabilities

   l1tf

   mds

   tsx_async_abort

   multihit.rst

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

Not specifying this option is equivalent to "mds=full".

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

--------------------------

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.

.. SPDX-License-Identifier: GPL-2.0

TAA - TSX Asynchronous Abort

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

TAA is a hardware vulnerability that allows unprivileged speculative access to

data which is available in various CPU internal buffers by using asynchronous

aborts within an Intel TSX transactional region.

Affected processors

-------------------

This vulnerability only affects Intel processors that support Intel

Transactional Synchronization Extensions (TSX) when the TAA_NO bit (bit 8)

is 0 in the IA32_ARCH_CAPABILITIES MSR.  On processors where the MDS_NO bit

(bit 5) is 0 in the IA32_ARCH_CAPABILITIES MSR, the existing MDS mitigations

also mitigate against TAA.

Whether a processor is affected or not can be read out from the TAA

vulnerability file in sysfs. See :ref:`tsx_async_abort_sys_info`.

Related CVEs

------------

The following CVE entry is related to this TAA issue:

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

   CVE-2019-11135  TAA    TSX Asynchronous Abort (TAA) condition on some

                          microprocessors utilizing speculative execution may

                          allow an authenticated user to potentially enable

                          information disclosure via a side channel with

                          local access.

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

Problem

-------

When performing store, load or L1 refill operations, processors write

data into temporary microarchitectural structures (buffers). The data in

those buffers can be forwarded to load operations as an optimization.

Intel TSX is an extension to the x86 instruction set architecture that adds

hardware transactional memory support to improve performance of multi-threaded

software. TSX lets the processor expose and exploit concurrency hidden in an

application due to dynamically avoiding unnecessary synchronization.

TSX supports atomic memory transactions that are either committed (success) or

aborted. During an abort, operations that happened within the transactional region

are rolled back. An asynchronous abort takes place, among other options, when a

different thread accesses a cache line that is also used within the transactional

region when that access might lead to a data race.

Immediately after an uncompleted asynchronous abort, certain speculatively

executed loads may read data from those internal buffers and pass it to dependent

operations. This can be then used 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.

The victim of a malicious actor does not need to make use of TSX. Only the

attacker needs to begin a TSX transaction and raise an asynchronous abort

which in turn potenitally leaks data stored in the buffers.

More detailed technical information is available in the TAA specific x86

architecture section: :ref:`Documentation/x86/tsx_async_abort.rst <tsx_async_abort>`.

Attack scenarios

----------------

Attacks against the TAA vulnerability can be implemented from unprivileged

applications running on hosts or guests.

As for MDS, the attacker has no control over the memory addresses that can

be leaked. Only the victim is responsible for bringing data to the CPU. As

a result, the malicious actor has to sample as much data as possible and

then postprocess it to try to infer any useful information from it.

A potential attacker only has read access to the data. Also, there is no direct

privilege escalation by using this technique.

.. _tsx_async_abort_sys_info:

TAA system information

-----------------------

The Linux kernel provides a sysfs interface to enumerate the current TAA status

of mitigated systems. The relevant sysfs file is:

/sys/devices/system/cpu/vulnerabilities/tsx_async_abort

The possible values in this file are:

.. list-table::

   * - 'Vulnerable'

     - The CPU is affected by this vulnerability and the microcode and kernel mitigation are not applied.

   * - 'Vulnerable: Clear CPU buffers attempted, no microcode'

     - The system tries to clear the buffers but the microcode might not support the operation.

   * - 'Mitigation: Clear CPU buffers'

     - The microcode has been updated to clear the buffers. TSX is still enabled.

   * - 'Mitigation: TSX disabled'

     - TSX is disabled.

   * - 'Not affected'

     - The CPU is not affected by this issue.

.. _ucode_needed:

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 tsx_async_abort 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:`taa_mitigation_control_command_line`.

.. _virt_mechanism:

Virtualization mitigation

^^^^^^^^^^^^^^^^^^^^^^^^^

Affected systems where the host has TAA microcode and TAA is mitigated by

having disabled TSX previously, are not vulnerable regardless of the status

of the VMs.

In all other cases, if the host either does not have the TAA microcode or

the kernel is not mitigated, the system might be vulnerable.

.. _taa_mitigation_control_command_line:

Mitigation control on the kernel command line

---------------------------------------------

The kernel command line allows to control the TAA mitigations at boot time with

the option "tsx_async_abort=". The valid arguments for this option are:

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

  off		This option disables the TAA mitigation on affected platforms.

                If the system has TSX enabled (see next parameter) and the CPU

                is affected, the system is vulnerable.

  full	        TAA mitigation is enabled. If TSX is enabled, on an affected

                system it will clear CPU buffers on ring transitions. On

                systems which are MDS-affected and deploy MDS mitigation,

                TAA is also mitigated. Specifying this option on those

                systems will have no effect.

  full,nosmt    The same as tsx_async_abort=full, with SMT disabled on

                vulnerable CPUs that have TSX enabled. This is the complete

                mitigation. When TSX is disabled, SMT is not disabled because

                CPU is not vulnerable to cross-thread TAA attacks.

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

Not specifying this option is equivalent to "tsx_async_abort=full". For

processors that are affected by both TAA and MDS, specifying just

"tsx_async_abort=off" without an accompanying "mds=off" will have no

effect as the same mitigation is used for both vulnerabilities.

The kernel command line also allows to control the TSX feature using the

parameter "tsx=" on CPUs which support TSX control. MSR_IA32_TSX_CTRL is used

to control the TSX feature and the enumeration of the TSX feature bits (RTM

and HLE) in CPUID.

The valid options are:

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

  off		Disables TSX on the system.

                Note that this option takes effect only on newer CPUs which are

                not vulnerable to MDS, i.e., have MSR_IA32_ARCH_CAPABILITIES.MDS_NO=1

                and which get the new IA32_TSX_CTRL MSR through a microcode

                update. This new MSR allows for the reliable deactivation of

                the TSX functionality.

  on		Enables TSX.

                Although there are mitigations for all known security

                vulnerabilities, TSX has been known to be an accelerator for

                several previous speculation-related CVEs, and so there may be

                unknown security risks associated with leaving it enabled.

  auto		Disables TSX if X86_BUG_TAA is present, otherwise enables TSX

                on the system.

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

Not specifying this option is equivalent to "tsx=off".

The following combinations of the "tsx_async_abort" and "tsx" are possible. For

affected platforms tsx=auto is equivalent to tsx=off and the result will be:

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

  tsx=on     tsx_async_abort=full         The system will use VERW to clear CPU

                                          buffers. Cross-thread attacks are still

					  possible on SMT machines.

  tsx=on     tsx_async_abort=full,nosmt   As above, cross-thread attacks on SMT

                                          mitigated.

  tsx=on     tsx_async_abort=off          The system is vulnerable.

  tsx=off    tsx_async_abort=full         TSX might be disabled if microcode

                                          provides a TSX control MSR. If so,

					  system is not vulnerable.

  tsx=off    tsx_async_abort=full,nosmt   Ditto

  tsx=off    tsx_async_abort=off          ditto

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

For unaffected platforms "tsx=on" and "tsx_async_abort=full" does not clear CPU

buffers.  For platforms without TSX control (MSR_IA32_ARCH_CAPABILITIES.MDS_NO=0)

"tsx" command line argument has no effect.

For the affected platforms below table indicates the mitigation status for the

combinations of CPUID bit MD_CLEAR and IA32_ARCH_CAPABILITIES MSR bits MDS_NO

and TSX_CTRL_MSR.

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

  MDS_NO   MD_CLEAR   TSX_CTRL_MSR   Status

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

    0          0            0        Vulnerable (needs microcode)

    0          1            0        MDS and TAA mitigated via VERW

    1          1            0        MDS fixed, TAA vulnerable if TSX enabled

                                     because MD_CLEAR has no meaning and

                                     VERW is not guaranteed to clear buffers

    1          X            1        MDS fixed, TAA can be mitigated by

                                     VERW or TSX_CTRL_MSR

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

Mitigation selection guide

--------------------------

1. Trusted userspace and guests

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If all user space applications are from a trusted source and do not execute

untrusted code which is supplied externally, then the mitigation can be

disabled. The same applies to virtualized environments with trusted guests.

2. Untrusted userspace and guests

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If there are untrusted applications or guests on the system, enabling TSX

might allow a malicious actor to leak data from the host or from other

processes running on the same physical core.

If the microcode is available and the TSX is disabled on the host, attacks

are prevented in a virtualized environment as well, even if the VMs do not

explicitly enable the mitigation.

.. _taa_default_mitigations:

Default mitigations

-------------------

The kernel's default action for vulnerable processors is:

  - Deploy TSX disable mitigation (tsx_async_abort=full tsx=off).