Transient execution CPU vulnerability

Transient execution CPU vulnerabilities are vulnerabilities in a computer system in which a speculative execution optimization implemented in a microprocessor is exploited to leak secret data to an unauthorized party. The classic example is Spectre that gave its name to this kind of side-channel attack, but since January 2018 many different vulnerabilities have been identified.

Main articles: Meltdown (security vulnerability), Spectre (security vulnerability), Foreshadow (security vulnerability), Speculative Store Bypass, Lazy FP state restore, Bounds Check Bypass Store, Microarchitectural Data Sampling, and Load value injection

Overview

Modern computers are highly parallel devices, composed of components with very different performance characteristics. If an operation (such as a branch) cannot yet be performed because some earlier slow operation (such as a memory read) has not yet completed, a microprocessor may attempt to predict the result of the earlier operation and execute the later operation speculatively, acting as if the prediction was correct. The prediction may be based on recent behavior of the system. When the earlier, slower operation completes, the microprocessor determines whether prediction was correct or incorrect. If it was correct then execution proceeds uninterrupted; if it was incorrect then the microprocessor rolls back the speculatively executed operations and repeats the original instruction with the real result of the slow operation. Specifically, a transient instruction[1] refers to an instruction processed by error by the processor (incriminating the branch predictor in the case of Spectre) which can affect the micro-architectural state of the processor, leaving the architectural state without any trace of its execution.

In terms of the directly visible behavior of the computer it is as if the speculatively executed code "never happened". However, this speculative execution may affect the state of certain components of the microprocessor, such as the cache, and this effect may be discovered by careful monitoring of the timing of subsequent operations.

If an attacker can arrange that the speculatively executed code (which may be directly written by the attacker, or may be a suitable gadget that they have found in the targeted system) operates on secret data that they are unauthorized to access, and has a different effect on the cache for different values of the secret data, they may be able to discover the value of the secret data.

Starting in 2017, multiple examples of such vulnerabilities were identified, with publication starting in early 2018.

Vulnerabilities and mitigations summary
Mitigation Type Comprehensiveness Effectiveness Performance Impact
Hardware Full Full None…Small
Firmware Microcode Update Partial Partial…Full None…Large
OS/VMM Partial Partial…Full Small…Large
Software Recompilation Poor Partial…Full Medium…Large

Hardware mitigations require change to the CPU design and thus a new iteration of hardware, but impose close to zero performance loss. Microcode updates alter the software that the CPU runs on, requiring patches to be released and integrated into every operating system and for each CPU. OS/VMM mitigations are applied at the operating system or virtual machine level and (depending on workload) often incur quite a significant performance loss. Software recompilation requires recompiling every piece of software and usually incur a severe performance hit.

Vulnerability Name

(aliases)

 CVE Affected CPU architectures and mitigations
Intel[2] AMD[3]
Ice Lake[4] Cascade Lake,
Comet Lake		 Whiskey Lake,
Amber Lake		 Coffee Lake
(9th gen)[5]		 Coffee Lake
(8th gen)*		 Zen 1 / Zen 1+ Zen 2[6]
Spectre v1
Bounds Check Bypass		 2017-5753 Software Recompilation Software Recompilation[7]
Spectre v2
Branch Target Injection		 2017-5715 Hardware + OS Microcode + OS Microcode + OS Microcode + OS/VMM Hardware + OS/VMM
SpectreRSB[8]/ret2spec[9] Return Mispredict 2018-15572 OS[10]
Meltdown
Rogue Data Cache Load		 2017-5754 Not affected Microcode Not affected
Spectre-NG v3a 2018-3640 Not affected[11] Microcode
Spectre-NG v4
Speculative Store Bypass		 2018-3639 Hardware + OS/VMM[11] Microcode + OS OS/VMM Hardware + OS/VMM
Foreshadow
L1 Terminal Fault, L1TF		 2018-3615 Not affected Microcode Not affected
Spectre-NG
Lazy FP State Restore		 2018-3665 OS/VMM[12]
Spectre-NG v1.1
Bounds Check Bypass Store		 2018-3693 OS/VMM[13]
Spectre-NG v1.2
Read-only Protection Bypass (RPB)		 No CVE and has never been confirmed by Intel Not affected[3]
Foreshadow-OS
L1 Terminal Fault (L1TF)		 2018-3620 Not affected Microcode + OS Not affected
Foreshadow-VMM
L1 Terminal Fault (L1TF)		 2018-3646
RIDL/ZombieLoad
Microarchitectural Fill Buffer Data Sampling (MFBDS)		 2018-12130
RIDL
Microarchitectural Load Port Data Sampling (MLPDS)		 2018-12127 Not affected Not affected Not affected Microcode + OS[14]
RIDL
Microarchitectural Data Sampling Uncacheable Memory (MDSUM)		 2019-11091 Not affected Microcode + OS
Fallout
Microarchitectural Store Buffer Data Sampling (MSBDS)		 2018-12126 Microcode[15][16] Not affected Not affected Microcode + OS
Spectre SWAPGS[17][18][19] 2019-1125 Same as Spectre 1
RIDL/ZombieLoad v2
Transactional Asynchronous Abort (TAA)[20][21][22]		 2019-11135 Not Affected[23] Microcode + OS
RIDL/CacheOut
L1D Eviction Sampling (L1DES)[24][25][26]		 2020-0549 Not Affected
RIDL
Vector Register Sampling (VRS)[24][25]		 2020-0548
Load Value Injection (LVI)[27][28][29][30] 2020-0551 Software recompilation
Take a Way[31][32] Not affected Not fixed yet (disputed[33])[34]
CROSSTalk
Special Register Buffer Data Sampling (SRBDS)[35][36][37]		 2020-0543 Not affected Microcode Not affected
Blindside[38][39] Affected, not fixed yet

The 8th generation Coffee Lake architecture in this table also applies to a wide range of previously released Intel CPUs, not limited to the architectures based on Intel Core, Pentium 4 and Intel Atom starting with Silvermont.[40][41] Various CPU microarchitectures not included above are also affected, among them are IBM Power, ARM, MIPS and others.[42][43][44][45]

Future

Spectre class vulnerabilities will remain unfixed because otherwise CPU designers will have to disable OoOE which will entail a massive performance loss.

Notes
1.^ Stepping 5 of the 2nd Generation Intel® Xeon® Scalable Processors based on Cascade Lake microarchitecture is affected by both MSBDS and MLPDS.

References
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  10. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fdf82a7856b32d905c39afc85e34364491e46346
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  21. at 18:02, Shaun Nichols in San Francisco 12 Nov 2019. "True to its name, Intel CPU flaw ZombieLoad comes shuffling back with new variant". www.theregister.co.uk. Retrieved 2019-11-12.
  22. Cimpanu, Catalin. "Intel's Cascade Lake CPUs impacted by new Zombieload v2 attack". ZDNet. Retrieved 2019-11-12.
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  27. at 17:00, Thomas Claburn in San Francisco 10 Mar 2020. "You only LVI twice: Meltdown The Sequel strikes Intel chips – and full mitigation against data-meddling flaw will cost you 50%+ of performance". www.theregister.co.uk. Retrieved 2020-03-10.
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  1. Vulnerabilities associated with CPU speculative execution
  2. A systematic evaluation of transient execution attacks and defenses
  3. A dynamic tree of transient execution vulnerabilities for Intel, AMD and ARM CPUs
  4. Transient Execution Attacks by Daniel Gruss, June 20, 2019
  5. CPU Bugs
  6. Intel: Refined Speculative Execution Terminology
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