x86/speculation/taa: Add documentation for TSX Async Abort

Add the documenation for TSX Async Abort. Include the description of
the issue, how to check the mitigation state, control the mitigation,
guidance for system administrators.

[ bp: Add proper SPDX tags, touch ups by Josh and me. ]

Co-developed-by: Antonio Gomez Iglesias <[e-mail]>

Signed-off-by: Pawan Gupta <[e-mail]>
Signed-off-by: Antonio Gomez Iglesias <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Mark Gross <[e-mail]>
Reviewed-by: Tony Luck <[e-mail]>
Reviewed-by: Josh Poimboeuf <[e-mail]>

Documentation/ABI/testing/sysfs-devices-system-cpu

···
 		/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
 Date:		January 2018
 Contact:	Linux kernel mailing list <[e-mail]>
 Description:	Information about CPU vulnerabilities

Documentation/admin-guide/hw-vuln/index.rst

···
    spectre
    l1tf
    mds
+   tsx_async_abort

Documentation/admin-guide/hw-vuln/tsx_async_abort.rst

···
+.. 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".
+
+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).

Documentation/admin-guide/kernel-parameters.txt

···
 					       ssbd=force-off [ARM64]
 					       l1tf=off [X86]
 					       mds=off [X86]
+					       tsx_async_abort=off [X86]
 
 			auto (default)
 				Mitigate all CPU vulnerabilities, but leave SMT
···
 				be fully mitigated, even if it means losing SMT.
 				Equivalent to: l1tf=flush,nosmt [X86]
 					       mds=full,nosmt [X86]
+					       tsx_async_abort=full,nosmt [X86]
 
 	mminit_loglevel=
 			[KNL] When CONFIG_DEBUG_MEMORY_INIT is set, this
···
 
 			See Documentation/admin-guide/hw-vuln/tsx_async_abort.rst
 			for more details.
+
+	tsx_async_abort= [X86,INTEL] Control mitigation for the TSX Async
+			Abort (TAA) vulnerability.
+
+			Similar to Micro-architectural Data Sampling (MDS)
+			certain CPUs that support Transactional
+			Synchronization Extensions (TSX) are vulnerable to an
+			exploit against CPU internal buffers which can forward
+			information to a disclosure gadget under certain
+			conditions.
+
+			In vulnerable processors, the speculatively forwarded
+			data can be used in a cache side channel attack, to
+			access data to which the attacker does not have direct
+			access.
+
+			This parameter controls the TAA mitigation.  The
+			options are:
+
+			full       - Enable TAA mitigation on vulnerable CPUs
+				     if TSX is enabled.
+
+			full,nosmt - Enable TAA mitigation and disable SMT on
+				     vulnerable CPUs. If TSX is disabled, SMT
+				     is not disabled because CPU is not
+				     vulnerable to cross-thread TAA attacks.
+			off        - Unconditionally disable TAA mitigation
+
+			Not specifying this option is equivalent to
+			tsx_async_abort=full.  On CPUs which are MDS affected
+			and deploy MDS mitigation, TAA mitigation is not
+			required and doesn't provide any additional
+			mitigation.
+
+			For details see:
+			Documentation/admin-guide/hw-vuln/tsx_async_abort.rst
 
 	turbografx.map[2|3]=	[HW,JOY]
 			TurboGraFX parallel port interface

Documentation/x86/index.rst

···
    mds
    microcode
    resctrl_ui
+   tsx_async_abort
    usb-legacy-support
    i386/index
    x86_64/index

Documentation/x86/tsx_async_abort.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+TSX Async Abort (TAA) mitigation
+================================
+
+.. _tsx_async_abort:
+
+Overview
+--------
+
+TSX Async Abort (TAA) is a side channel attack on internal buffers in some
+Intel processors similar to Microachitectural Data Sampling (MDS).  In this
+case certain loads may speculatively pass invalid data to dependent operations
+when an asynchronous abort condition is pending in a Transactional
+Synchronization Extensions (TSX) transaction.  This includes loads with no
+fault or assist condition. Such loads may speculatively expose stale data from
+the same uarch data structures as in MDS, with same scope of exposure i.e.
+same-thread and cross-thread. This issue affects all current processors that
+support TSX.
+
+Mitigation strategy
+-------------------
+
+a) TSX disable - one of the mitigations is to disable TSX. A new MSR
+IA32_TSX_CTRL will be available in future and current processors after
+microcode update which can be used to disable TSX. In addition, it
+controls the enumeration of the TSX feature bits (RTM and HLE) in CPUID.
+
+b) Clear CPU buffers - similar to MDS, clearing the CPU buffers mitigates this
+vulnerability. More details on this approach can be found in
+:ref:`Documentation/admin-guide/hw-vuln/mds.rst <mds>`.
+
+Kernel internal mitigation modes
+--------------------------------
+
+ =============    ============================================================
+ off              Mitigation is disabled. Either the CPU is not affected or
+                  tsx_async_abort=off is supplied on the kernel command line.
+
+ tsx disabled     Mitigation is enabled. TSX feature is disabled by default at
+                  bootup on processors that support TSX control.
+
+ verw             Mitigation is enabled. CPU is affected and MD_CLEAR is
+                  advertised in CPUID.
+
+ ucode needed     Mitigation is enabled. CPU is affected and MD_CLEAR is not
+                  advertised in CPUID. That is mainly for virtualization
+                  scenarios where the host has the updated microcode but the
+                  hypervisor does not expose MD_CLEAR in CPUID. It's a best
+                  effort approach without guarantee.
+ =============    ============================================================
+
+If the CPU is affected and the "tsx_async_abort" kernel command line parameter is
+not provided then the kernel selects an appropriate mitigation depending on the
+status of RTM and MD_CLEAR CPUID bits.
+
+Below tables indicate the impact of tsx=on|off|auto cmdline options on state of
+TAA mitigation, VERW behavior and TSX feature for various combinations of
+MSR_IA32_ARCH_CAPABILITIES bits.
+
+1. "tsx=off"
+
+=========  =========  ============  ============  ==============  ===================  ======================
+MSR_IA32_ARCH_CAPABILITIES bits     Result with cmdline tsx=off
+----------------------------------  -------------------------------------------------------------------------
+TAA_NO     MDS_NO     TSX_CTRL_MSR  TSX state     VERW can clear  TAA mitigation       TAA mitigation
+                                    after bootup  CPU buffers     tsx_async_abort=off  tsx_async_abort=full
+=========  =========  ============  ============  ==============  ===================  ======================
+    0          0           0         HW default         Yes           Same as MDS           Same as MDS
+    0          0           1        Invalid case   Invalid case       Invalid case          Invalid case
+    0          1           0         HW default         No         Need ucode update     Need ucode update
+    0          1           1          Disabled          Yes           TSX disabled          TSX disabled
+    1          X           1          Disabled           X             None needed           None needed
+=========  =========  ============  ============  ==============  ===================  ======================
+
+2. "tsx=on"
+
+=========  =========  ============  ============  ==============  ===================  ======================
+MSR_IA32_ARCH_CAPABILITIES bits     Result with cmdline tsx=on
+----------------------------------  -------------------------------------------------------------------------
+TAA_NO     MDS_NO     TSX_CTRL_MSR  TSX state     VERW can clear  TAA mitigation       TAA mitigation
+                                    after bootup  CPU buffers     tsx_async_abort=off  tsx_async_abort=full
+=========  =========  ============  ============  ==============  ===================  ======================
+    0          0           0         HW default        Yes            Same as MDS          Same as MDS
+    0          0           1        Invalid case   Invalid case       Invalid case         Invalid case
+    0          1           0         HW default        No          Need ucode update     Need ucode update
+    0          1           1          Enabled          Yes               None              Same as MDS
+    1          X           1          Enabled          X              None needed          None needed
+=========  =========  ============  ============  ==============  ===================  ======================
+
+3. "tsx=auto"
+
+=========  =========  ============  ============  ==============  ===================  ======================
+MSR_IA32_ARCH_CAPABILITIES bits     Result with cmdline tsx=auto
+----------------------------------  -------------------------------------------------------------------------
+TAA_NO     MDS_NO     TSX_CTRL_MSR  TSX state     VERW can clear  TAA mitigation       TAA mitigation
+                                    after bootup  CPU buffers     tsx_async_abort=off  tsx_async_abort=full
+=========  =========  ============  ============  ==============  ===================  ======================
+    0          0           0         HW default    Yes                Same as MDS           Same as MDS
+    0          0           1        Invalid case  Invalid case        Invalid case          Invalid case
+    0          1           0         HW default    No              Need ucode update     Need ucode update
+    0          1           1          Disabled      Yes               TSX disabled          TSX disabled
+    1          X           1          Enabled       X                 None needed           None needed
+=========  =========  ============  ============  ==============  ===================  ======================
+
+In the tables, TSX_CTRL_MSR is a new bit in MSR_IA32_ARCH_CAPABILITIES that
+indicates whether MSR_IA32_TSX_CTRL is supported.
+
+There are two control bits in IA32_TSX_CTRL MSR:
+
+      Bit 0: When set it disables the Restricted Transactional Memory (RTM)
+             sub-feature of TSX (will force all transactions to abort on the
+             XBEGIN instruction).
+
+      Bit 1: When set it disables the enumeration of the RTM and HLE feature
+             (i.e. it will make CPUID(EAX=7).EBX{bit4} and
+             CPUID(EAX=7).EBX{bit11} read as 0).