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 <antonio.gomez.iglesias@intel.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Antonio Gomez Iglesias <antonio.gomez.iglesias@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mark Gross <mgross@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>

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

@@ -486,6 +486,7 @@ What:		/sys/devices/system/cpu/vulnerabilities
 		/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 <linux-kernel@vger.kernel.org>
 Description:	Information about CPU vulnerabilities

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

@@ -12,3 +12,4 @@ are configurable at compile, boot or run time.
    spectre
    l1tf
    mds
+   tsx_async_abort

Documentation/admin-guide/kernel-parameters.txt

@@ -2636,6 +2636,7 @@
 					       ssbd=force-off [ARM64]
 					       l1tf=off [X86]
 					       mds=off [X86]
+					       tsx_async_abort=off [X86]
 
 			auto (default)
 				Mitigate all CPU vulnerabilities, but leave SMT
@@ -2651,6 +2652,7 @@
 				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
@@ -4877,6 +4879,42 @@
 			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
 			Format:

Documentation/x86/index.rst

@@ -27,6 +27,7 @@ x86-specific Documentation
    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).