| .. 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`. |
| |
| 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). |
