1.. SPDX-License-Identifier: GPL-2.0 2 3TAA - TSX Asynchronous Abort 4====================================== 5 6TAA is a hardware vulnerability that allows unprivileged speculative access to 7data which is available in various CPU internal buffers by using asynchronous 8aborts within an Intel TSX transactional region. 9 10Affected processors 11------------------- 12 13This vulnerability only affects Intel processors that support Intel 14Transactional Synchronization Extensions (TSX) when the TAA_NO bit (bit 8) 15is 0 in the IA32_ARCH_CAPABILITIES MSR. On processors where the MDS_NO bit 16(bit 5) is 0 in the IA32_ARCH_CAPABILITIES MSR, the existing MDS mitigations 17also mitigate against TAA. 18 19Whether a processor is affected or not can be read out from the TAA 20vulnerability file in sysfs. See :ref:`tsx_async_abort_sys_info`. 21 22Related CVEs 23------------ 24 25The following CVE entry is related to this TAA issue: 26 27 ============== ===== =================================================== 28 CVE-2019-11135 TAA TSX Asynchronous Abort (TAA) condition on some 29 microprocessors utilizing speculative execution may 30 allow an authenticated user to potentially enable 31 information disclosure via a side channel with 32 local access. 33 ============== ===== =================================================== 34 35Problem 36------- 37 38When performing store, load or L1 refill operations, processors write 39data into temporary microarchitectural structures (buffers). The data in 40those buffers can be forwarded to load operations as an optimization. 41 42Intel TSX is an extension to the x86 instruction set architecture that adds 43hardware transactional memory support to improve performance of multi-threaded 44software. TSX lets the processor expose and exploit concurrency hidden in an 45application due to dynamically avoiding unnecessary synchronization. 46 47TSX supports atomic memory transactions that are either committed (success) or 48aborted. During an abort, operations that happened within the transactional region 49are rolled back. An asynchronous abort takes place, among other options, when a 50different thread accesses a cache line that is also used within the transactional 51region when that access might lead to a data race. 52 53Immediately after an uncompleted asynchronous abort, certain speculatively 54executed loads may read data from those internal buffers and pass it to dependent 55operations. This can be then used to infer the value via a cache side channel 56attack. 57 58Because the buffers are potentially shared between Hyper-Threads cross 59Hyper-Thread attacks are possible. 60 61The victim of a malicious actor does not need to make use of TSX. Only the 62attacker needs to begin a TSX transaction and raise an asynchronous abort 63which in turn potenitally leaks data stored in the buffers. 64 65More detailed technical information is available in the TAA specific x86 66architecture section: :ref:`Documentation/x86/tsx_async_abort.rst <tsx_async_abort>`. 67 68 69Attack scenarios 70---------------- 71 72Attacks against the TAA vulnerability can be implemented from unprivileged 73applications running on hosts or guests. 74 75As for MDS, the attacker has no control over the memory addresses that can 76be leaked. Only the victim is responsible for bringing data to the CPU. As 77a result, the malicious actor has to sample as much data as possible and 78then postprocess it to try to infer any useful information from it. 79 80A potential attacker only has read access to the data. Also, there is no direct 81privilege escalation by using this technique. 82 83 84.. _tsx_async_abort_sys_info: 85 86TAA system information 87----------------------- 88 89The Linux kernel provides a sysfs interface to enumerate the current TAA status 90of mitigated systems. The relevant sysfs file is: 91 92/sys/devices/system/cpu/vulnerabilities/tsx_async_abort 93 94The possible values in this file are: 95 96.. list-table:: 97 98 * - 'Vulnerable' 99 - The CPU is affected by this vulnerability and the microcode and kernel mitigation are not applied. 100 * - 'Vulnerable: Clear CPU buffers attempted, no microcode' 101 - The system tries to clear the buffers but the microcode might not support the operation. 102 * - 'Mitigation: Clear CPU buffers' 103 - The microcode has been updated to clear the buffers. TSX is still enabled. 104 * - 'Mitigation: TSX disabled' 105 - TSX is disabled. 106 * - 'Not affected' 107 - The CPU is not affected by this issue. 108 109.. _ucode_needed: 110 111Best effort mitigation mode 112^^^^^^^^^^^^^^^^^^^^^^^^^^^ 113 114If the processor is vulnerable, but the availability of the microcode-based 115mitigation mechanism is not advertised via CPUID the kernel selects a best 116effort mitigation mode. This mode invokes the mitigation instructions 117without a guarantee that they clear the CPU buffers. 118 119This is done to address virtualization scenarios where the host has the 120microcode update applied, but the hypervisor is not yet updated to expose the 121CPUID to the guest. If the host has updated microcode the protection takes 122effect; otherwise a few CPU cycles are wasted pointlessly. 123 124The state in the tsx_async_abort sysfs file reflects this situation 125accordingly. 126 127 128Mitigation mechanism 129-------------------- 130 131The kernel detects the affected CPUs and the presence of the microcode which is 132required. If a CPU is affected and the microcode is available, then the kernel 133enables the mitigation by default. 134 135 136The mitigation can be controlled at boot time via a kernel command line option. 137See :ref:`taa_mitigation_control_command_line`. 138 139Virtualization mitigation 140^^^^^^^^^^^^^^^^^^^^^^^^^ 141 142Affected systems where the host has TAA microcode and TAA is mitigated by 143having disabled TSX previously, are not vulnerable regardless of the status 144of the VMs. 145 146In all other cases, if the host either does not have the TAA microcode or 147the kernel is not mitigated, the system might be vulnerable. 148 149 150.. _taa_mitigation_control_command_line: 151 152Mitigation control on the kernel command line 153--------------------------------------------- 154 155The kernel command line allows to control the TAA mitigations at boot time with 156the option "tsx_async_abort=". The valid arguments for this option are: 157 158 ============ ============================================================= 159 off This option disables the TAA mitigation on affected platforms. 160 If the system has TSX enabled (see next parameter) and the CPU 161 is affected, the system is vulnerable. 162 163 full TAA mitigation is enabled. If TSX is enabled, on an affected 164 system it will clear CPU buffers on ring transitions. On 165 systems which are MDS-affected and deploy MDS mitigation, 166 TAA is also mitigated. Specifying this option on those 167 systems will have no effect. 168 169 full,nosmt The same as tsx_async_abort=full, with SMT disabled on 170 vulnerable CPUs that have TSX enabled. This is the complete 171 mitigation. When TSX is disabled, SMT is not disabled because 172 CPU is not vulnerable to cross-thread TAA attacks. 173 ============ ============================================================= 174 175Not specifying this option is equivalent to "tsx_async_abort=full". For 176processors that are affected by both TAA and MDS, specifying just 177"tsx_async_abort=off" without an accompanying "mds=off" will have no 178effect as the same mitigation is used for both vulnerabilities. 179 180The kernel command line also allows to control the TSX feature using the 181parameter "tsx=" on CPUs which support TSX control. MSR_IA32_TSX_CTRL is used 182to control the TSX feature and the enumeration of the TSX feature bits (RTM 183and HLE) in CPUID. 184 185The valid options are: 186 187 ============ ============================================================= 188 off Disables TSX on the system. 189 190 Note that this option takes effect only on newer CPUs which are 191 not vulnerable to MDS, i.e., have MSR_IA32_ARCH_CAPABILITIES.MDS_NO=1 192 and which get the new IA32_TSX_CTRL MSR through a microcode 193 update. This new MSR allows for the reliable deactivation of 194 the TSX functionality. 195 196 on Enables TSX. 197 198 Although there are mitigations for all known security 199 vulnerabilities, TSX has been known to be an accelerator for 200 several previous speculation-related CVEs, and so there may be 201 unknown security risks associated with leaving it enabled. 202 203 auto Disables TSX if X86_BUG_TAA is present, otherwise enables TSX 204 on the system. 205 ============ ============================================================= 206 207Not specifying this option is equivalent to "tsx=off". 208 209The following combinations of the "tsx_async_abort" and "tsx" are possible. For 210affected platforms tsx=auto is equivalent to tsx=off and the result will be: 211 212 ========= ========================== ========================================= 213 tsx=on tsx_async_abort=full The system will use VERW to clear CPU 214 buffers. Cross-thread attacks are still 215 possible on SMT machines. 216 tsx=on tsx_async_abort=full,nosmt As above, cross-thread attacks on SMT 217 mitigated. 218 tsx=on tsx_async_abort=off The system is vulnerable. 219 tsx=off tsx_async_abort=full TSX might be disabled if microcode 220 provides a TSX control MSR. If so, 221 system is not vulnerable. 222 tsx=off tsx_async_abort=full,nosmt Ditto 223 tsx=off tsx_async_abort=off ditto 224 ========= ========================== ========================================= 225 226 227For unaffected platforms "tsx=on" and "tsx_async_abort=full" does not clear CPU 228buffers. For platforms without TSX control (MSR_IA32_ARCH_CAPABILITIES.MDS_NO=0) 229"tsx" command line argument has no effect. 230 231For the affected platforms below table indicates the mitigation status for the 232combinations of CPUID bit MD_CLEAR and IA32_ARCH_CAPABILITIES MSR bits MDS_NO 233and TSX_CTRL_MSR. 234 235 ======= ========= ============= ======================================== 236 MDS_NO MD_CLEAR TSX_CTRL_MSR Status 237 ======= ========= ============= ======================================== 238 0 0 0 Vulnerable (needs microcode) 239 0 1 0 MDS and TAA mitigated via VERW 240 1 1 0 MDS fixed, TAA vulnerable if TSX enabled 241 because MD_CLEAR has no meaning and 242 VERW is not guaranteed to clear buffers 243 1 X 1 MDS fixed, TAA can be mitigated by 244 VERW or TSX_CTRL_MSR 245 ======= ========= ============= ======================================== 246 247Mitigation selection guide 248-------------------------- 249 2501. Trusted userspace and guests 251^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 252 253If all user space applications are from a trusted source and do not execute 254untrusted code which is supplied externally, then the mitigation can be 255disabled. The same applies to virtualized environments with trusted guests. 256 257 2582. Untrusted userspace and guests 259^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 260 261If there are untrusted applications or guests on the system, enabling TSX 262might allow a malicious actor to leak data from the host or from other 263processes running on the same physical core. 264 265If the microcode is available and the TSX is disabled on the host, attacks 266are prevented in a virtualized environment as well, even if the VMs do not 267explicitly enable the mitigation. 268 269 270.. _taa_default_mitigations: 271 272Default mitigations 273------------------- 274 275The kernel's default action for vulnerable processors is: 276 277 - Deploy TSX disable mitigation (tsx_async_abort=full tsx=off). 278