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1.. SPDX-License-Identifier: GPL-2.02 3SRBDS - Special Register Buffer Data Sampling4=============================================5 6SRBDS is a hardware vulnerability that allows MDS7Documentation/admin-guide/hw-vuln/mds.rst techniques to8infer values returned from special register accesses. Special register9accesses are accesses to off core registers. According to Intel's evaluation,10the special register reads that have a security expectation of privacy are11RDRAND, RDSEED and SGX EGETKEY.12 13When RDRAND, RDSEED and EGETKEY instructions are used, the data is moved14to the core through the special register mechanism that is susceptible15to MDS attacks.16 17Affected processors18-------------------19Core models (desktop, mobile, Xeon-E3) that implement RDRAND and/or RDSEED may20be affected.21 22A processor is affected by SRBDS if its Family_Model and stepping is23in the following list, with the exception of the listed processors24exporting MDS_NO while Intel TSX is available yet not enabled. The25latter class of processors are only affected when Intel TSX is enabled26by software using TSX_CTRL_MSR otherwise they are not affected.27 28 ============= ============ ========29 common name Family_Model Stepping30 ============= ============ ========31 IvyBridge 06_3AH All32 33 Haswell 06_3CH All34 Haswell_L 06_45H All35 Haswell_G 06_46H All36 37 Broadwell_G 06_47H All38 Broadwell 06_3DH All39 40 Skylake_L 06_4EH All41 Skylake 06_5EH All42 43 Kabylake_L 06_8EH <= 0xC44 Kabylake 06_9EH <= 0xD45 ============= ============ ========46 47Related CVEs48------------49 50The following CVE entry is related to this SRBDS issue:51 52 ============== ===== =====================================53 CVE-2020-0543 SRBDS Special Register Buffer Data Sampling54 ============== ===== =====================================55 56Attack scenarios57----------------58An unprivileged user can extract values returned from RDRAND and RDSEED59executed on another core or sibling thread using MDS techniques.60 61 62Mitigation mechanism63--------------------64Intel will release microcode updates that modify the RDRAND, RDSEED, and65EGETKEY instructions to overwrite secret special register data in the shared66staging buffer before the secret data can be accessed by another logical67processor.68 69During execution of the RDRAND, RDSEED, or EGETKEY instructions, off-core70accesses from other logical processors will be delayed until the special71register read is complete and the secret data in the shared staging buffer is72overwritten.73 74This has three effects on performance:75 76#. RDRAND, RDSEED, or EGETKEY instructions have higher latency.77 78#. Executing RDRAND at the same time on multiple logical processors will be79 serialized, resulting in an overall reduction in the maximum RDRAND80 bandwidth.81 82#. Executing RDRAND, RDSEED or EGETKEY will delay memory accesses from other83 logical processors that miss their core caches, with an impact similar to84 legacy locked cache-line-split accesses.85 86The microcode updates provide an opt-out mechanism (RNGDS_MITG_DIS) to disable87the mitigation for RDRAND and RDSEED instructions executed outside of Intel88Software Guard Extensions (Intel SGX) enclaves. On logical processors that89disable the mitigation using this opt-out mechanism, RDRAND and RDSEED do not90take longer to execute and do not impact performance of sibling logical91processors memory accesses. The opt-out mechanism does not affect Intel SGX92enclaves (including execution of RDRAND or RDSEED inside an enclave, as well93as EGETKEY execution).94 95IA32_MCU_OPT_CTRL MSR Definition96--------------------------------97Along with the mitigation for this issue, Intel added a new thread-scope98IA32_MCU_OPT_CTRL MSR, (address 0x123). The presence of this MSR and99RNGDS_MITG_DIS (bit 0) is enumerated by CPUID.(EAX=07H,ECX=0).EDX[SRBDS_CTRL =1009]==1. This MSR is introduced through the microcode update.101 102Setting IA32_MCU_OPT_CTRL[0] (RNGDS_MITG_DIS) to 1 for a logical processor103disables the mitigation for RDRAND and RDSEED executed outside of an Intel SGX104enclave on that logical processor. Opting out of the mitigation for a105particular logical processor does not affect the RDRAND and RDSEED mitigations106for other logical processors.107 108Note that inside of an Intel SGX enclave, the mitigation is applied regardless109of the value of RNGDS_MITG_DS.110 111Mitigation control on the kernel command line112---------------------------------------------113The kernel command line allows control over the SRBDS mitigation at boot time114with the option "srbds=". The option for this is:115 116 ============= =============================================================117 off This option disables SRBDS mitigation for RDRAND and RDSEED on118 affected platforms.119 ============= =============================================================120 121SRBDS System Information122------------------------123The Linux kernel provides vulnerability status information through sysfs. For124SRBDS this can be accessed by the following sysfs file:125/sys/devices/system/cpu/vulnerabilities/srbds126 127The possible values contained in this file are:128 129 ============================== =============================================130 Not affected Processor not vulnerable131 Vulnerable Processor vulnerable and mitigation disabled132 Vulnerable: No microcode Processor vulnerable and microcode is missing133 mitigation134 Mitigation: Microcode Processor is vulnerable and mitigation is in135 effect.136 Mitigation: TSX disabled Processor is only vulnerable when TSX is137 enabled while this system was booted with TSX138 disabled.139 Unknown: Dependent on140 hypervisor status Running on virtual guest processor that is141 affected but with no way to know if host142 processor is mitigated or vulnerable.143 ============================== =============================================144 145SRBDS Default mitigation146------------------------147This new microcode serializes processor access during execution of RDRAND,148RDSEED ensures that the shared buffer is overwritten before it is released for149reuse. Use the "srbds=off" kernel command line to disable the mitigation for150RDRAND and RDSEED.151