278 lines
12 KiB
ReStructuredText
278 lines
12 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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TAA - TSX Asynchronous Abort
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======================================
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TAA is a hardware vulnerability that allows unprivileged speculative access to
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data which is available in various CPU internal buffers by using asynchronous
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aborts within an Intel TSX transactional region.
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Affected processors
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-------------------
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This vulnerability only affects Intel processors that support Intel
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Transactional Synchronization Extensions (TSX) when the TAA_NO bit (bit 8)
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is 0 in the IA32_ARCH_CAPABILITIES MSR. On processors where the MDS_NO bit
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(bit 5) is 0 in the IA32_ARCH_CAPABILITIES MSR, the existing MDS mitigations
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also mitigate against TAA.
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Whether a processor is affected or not can be read out from the TAA
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vulnerability file in sysfs. See :ref:`tsx_async_abort_sys_info`.
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Related CVEs
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------------
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The following CVE entry is related to this TAA issue:
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============== ===== ===================================================
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CVE-2019-11135 TAA TSX Asynchronous Abort (TAA) condition on some
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microprocessors utilizing speculative execution may
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allow an authenticated user to potentially enable
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information disclosure via a side channel with
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local access.
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============== ===== ===================================================
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Problem
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-------
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When performing store, load or L1 refill operations, processors write
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data into temporary microarchitectural structures (buffers). The data in
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those buffers can be forwarded to load operations as an optimization.
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Intel TSX is an extension to the x86 instruction set architecture that adds
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hardware transactional memory support to improve performance of multi-threaded
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software. TSX lets the processor expose and exploit concurrency hidden in an
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application due to dynamically avoiding unnecessary synchronization.
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TSX supports atomic memory transactions that are either committed (success) or
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aborted. During an abort, operations that happened within the transactional region
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are rolled back. An asynchronous abort takes place, among other options, when a
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different thread accesses a cache line that is also used within the transactional
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region when that access might lead to a data race.
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Immediately after an uncompleted asynchronous abort, certain speculatively
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executed loads may read data from those internal buffers and pass it to dependent
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operations. This can be then used to infer the value via a cache side channel
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attack.
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Because the buffers are potentially shared between Hyper-Threads cross
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Hyper-Thread attacks are possible.
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The victim of a malicious actor does not need to make use of TSX. Only the
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attacker needs to begin a TSX transaction and raise an asynchronous abort
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which in turn potentially leaks data stored in the buffers.
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More detailed technical information is available in the TAA specific x86
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architecture section: :ref:`Documentation/x86/tsx_async_abort.rst <tsx_async_abort>`.
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Attack scenarios
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----------------
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Attacks against the TAA vulnerability can be implemented from unprivileged
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applications running on hosts or guests.
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As for MDS, the attacker has no control over the memory addresses that can
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be leaked. Only the victim is responsible for bringing data to the CPU. As
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a result, the malicious actor has to sample as much data as possible and
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then postprocess it to try to infer any useful information from it.
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A potential attacker only has read access to the data. Also, there is no direct
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privilege escalation by using this technique.
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.. _tsx_async_abort_sys_info:
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TAA system information
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-----------------------
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The Linux kernel provides a sysfs interface to enumerate the current TAA status
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of mitigated systems. The relevant sysfs file is:
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/sys/devices/system/cpu/vulnerabilities/tsx_async_abort
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The possible values in this file are:
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.. list-table::
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* - 'Vulnerable'
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- The CPU is affected by this vulnerability and the microcode and kernel mitigation are not applied.
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* - 'Vulnerable: Clear CPU buffers attempted, no microcode'
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- The system tries to clear the buffers but the microcode might not support the operation.
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* - 'Mitigation: Clear CPU buffers'
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- The microcode has been updated to clear the buffers. TSX is still enabled.
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* - 'Mitigation: TSX disabled'
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- TSX is disabled.
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* - 'Not affected'
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- The CPU is not affected by this issue.
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.. _ucode_needed:
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Best effort mitigation mode
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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If the processor is vulnerable, but the availability of the microcode-based
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mitigation mechanism is not advertised via CPUID the kernel selects a best
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effort mitigation mode. This mode invokes the mitigation instructions
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without a guarantee that they clear the CPU buffers.
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This is done to address virtualization scenarios where the host has the
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microcode update applied, but the hypervisor is not yet updated to expose the
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CPUID to the guest. If the host has updated microcode the protection takes
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effect; otherwise a few CPU cycles are wasted pointlessly.
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The state in the tsx_async_abort sysfs file reflects this situation
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accordingly.
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Mitigation mechanism
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--------------------
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The kernel detects the affected CPUs and the presence of the microcode which is
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required. If a CPU is affected and the microcode is available, then the kernel
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enables the mitigation by default.
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The mitigation can be controlled at boot time via a kernel command line option.
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See :ref:`taa_mitigation_control_command_line`.
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Virtualization mitigation
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^^^^^^^^^^^^^^^^^^^^^^^^^
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Affected systems where the host has TAA microcode and TAA is mitigated by
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having disabled TSX previously, are not vulnerable regardless of the status
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of the VMs.
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In all other cases, if the host either does not have the TAA microcode or
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the kernel is not mitigated, the system might be vulnerable.
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.. _taa_mitigation_control_command_line:
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Mitigation control on the kernel command line
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---------------------------------------------
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The kernel command line allows to control the TAA mitigations at boot time with
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the option "tsx_async_abort=". The valid arguments for this option are:
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============ =============================================================
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off This option disables the TAA mitigation on affected platforms.
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If the system has TSX enabled (see next parameter) and the CPU
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is affected, the system is vulnerable.
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full TAA mitigation is enabled. If TSX is enabled, on an affected
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system it will clear CPU buffers on ring transitions. On
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systems which are MDS-affected and deploy MDS mitigation,
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TAA is also mitigated. Specifying this option on those
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systems will have no effect.
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full,nosmt The same as tsx_async_abort=full, with SMT disabled on
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vulnerable CPUs that have TSX enabled. This is the complete
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mitigation. When TSX is disabled, SMT is not disabled because
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CPU is not vulnerable to cross-thread TAA attacks.
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============ =============================================================
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Not specifying this option is equivalent to "tsx_async_abort=full". For
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processors that are affected by both TAA and MDS, specifying just
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"tsx_async_abort=off" without an accompanying "mds=off" will have no
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effect as the same mitigation is used for both vulnerabilities.
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The kernel command line also allows to control the TSX feature using the
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parameter "tsx=" on CPUs which support TSX control. MSR_IA32_TSX_CTRL is used
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to control the TSX feature and the enumeration of the TSX feature bits (RTM
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and HLE) in CPUID.
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The valid options are:
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============ =============================================================
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off Disables TSX on the system.
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Note that this option takes effect only on newer CPUs which are
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not vulnerable to MDS, i.e., have MSR_IA32_ARCH_CAPABILITIES.MDS_NO=1
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and which get the new IA32_TSX_CTRL MSR through a microcode
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update. This new MSR allows for the reliable deactivation of
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the TSX functionality.
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on Enables TSX.
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Although there are mitigations for all known security
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vulnerabilities, TSX has been known to be an accelerator for
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several previous speculation-related CVEs, and so there may be
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unknown security risks associated with leaving it enabled.
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auto Disables TSX if X86_BUG_TAA is present, otherwise enables TSX
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on the system.
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============ =============================================================
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Not specifying this option is equivalent to "tsx=off".
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The following combinations of the "tsx_async_abort" and "tsx" are possible. For
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affected platforms tsx=auto is equivalent to tsx=off and the result will be:
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========= ========================== =========================================
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tsx=on tsx_async_abort=full The system will use VERW to clear CPU
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buffers. Cross-thread attacks are still
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possible on SMT machines.
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tsx=on tsx_async_abort=full,nosmt As above, cross-thread attacks on SMT
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mitigated.
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tsx=on tsx_async_abort=off The system is vulnerable.
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tsx=off tsx_async_abort=full TSX might be disabled if microcode
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provides a TSX control MSR. If so,
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system is not vulnerable.
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tsx=off tsx_async_abort=full,nosmt Ditto
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tsx=off tsx_async_abort=off ditto
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========= ========================== =========================================
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For unaffected platforms "tsx=on" and "tsx_async_abort=full" does not clear CPU
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buffers. For platforms without TSX control (MSR_IA32_ARCH_CAPABILITIES.MDS_NO=0)
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"tsx" command line argument has no effect.
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For the affected platforms below table indicates the mitigation status for the
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combinations of CPUID bit MD_CLEAR and IA32_ARCH_CAPABILITIES MSR bits MDS_NO
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and TSX_CTRL_MSR.
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======= ========= ============= ========================================
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MDS_NO MD_CLEAR TSX_CTRL_MSR Status
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======= ========= ============= ========================================
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0 0 0 Vulnerable (needs microcode)
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0 1 0 MDS and TAA mitigated via VERW
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1 1 0 MDS fixed, TAA vulnerable if TSX enabled
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because MD_CLEAR has no meaning and
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VERW is not guaranteed to clear buffers
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1 X 1 MDS fixed, TAA can be mitigated by
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VERW or TSX_CTRL_MSR
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======= ========= ============= ========================================
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Mitigation selection guide
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--------------------------
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1. Trusted userspace and guests
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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If all user space applications are from a trusted source and do not execute
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untrusted code which is supplied externally, then the mitigation can be
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disabled. The same applies to virtualized environments with trusted guests.
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2. Untrusted userspace and guests
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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If there are untrusted applications or guests on the system, enabling TSX
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might allow a malicious actor to leak data from the host or from other
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processes running on the same physical core.
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If the microcode is available and the TSX is disabled on the host, attacks
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are prevented in a virtualized environment as well, even if the VMs do not
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explicitly enable the mitigation.
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.. _taa_default_mitigations:
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Default mitigations
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-------------------
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The kernel's default action for vulnerable processors is:
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- Deploy TSX disable mitigation (tsx_async_abort=full tsx=off).
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