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

Browse Source 
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>
This commit is contained in:
Pawan Gupta 2019-10-23 12:32:55 +02:00 committed by Thomas Gleixner
parent 7531a3596e
commit a7a248c593
6 changed files with 434 additions and 0 deletions
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1
Documentation/ABI/testing/sysfs-devices-system-cpu
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@ -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

1
Documentation/admin-guide/hw-vuln/index.rst
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@ -12,3 +12,4 @@ are configurable at compile, boot or run time.
spectre
l1tf
mds
tsx_async_abort

276
Documentation/admin-guide/hw-vuln/tsx_async_abort.rst Normal file
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@ -0,0 +1,276 @@
.. 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).

38
Documentation/admin-guide/kernel-parameters.txt
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@ -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:

1
Documentation/x86/index.rst
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@ -27,6 +27,7 @@ x86-specific Documentation
mds
microcode
resctrl_ui
tsx_async_abort
usb-legacy-support
i386/index
x86_64/index

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Documentation/x86/tsx_async_abort.rst Normal file
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@ -0,0 +1,117 @@
.. 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).