1================================== 2Register File Data Sampling (RFDS) 3================================== 4 5Register File Data Sampling (RFDS) is a microarchitectural vulnerability that 6only affects Intel Atom parts(also branded as E-cores). RFDS may allow 7a malicious actor to infer data values previously used in floating point 8registers, vector registers, or integer registers. RFDS does not provide the 9ability to choose which data is inferred. CVE-2023-28746 is assigned to RFDS. 10 11Affected Processors 12=================== 13Below is the list of affected Intel processors [#f1]_: 14 15 =================== ============ 16 Common name Family_Model 17 =================== ============ 18 ATOM_GOLDMONT 06_5CH 19 ATOM_GOLDMONT_D 06_5FH 20 ATOM_GOLDMONT_PLUS 06_7AH 21 ATOM_TREMONT_D 06_86H 22 ATOM_TREMONT 06_96H 23 ALDERLAKE 06_97H 24 ALDERLAKE_L 06_9AH 25 ATOM_TREMONT_L 06_9CH 26 RAPTORLAKE 06_B7H 27 RAPTORLAKE_P 06_BAH 28 ATOM_GRACEMONT 06_BEH 29 RAPTORLAKE_S 06_BFH 30 =================== ============ 31 32As an exception to this table, Intel Xeon E family parts ALDERLAKE(06_97H) and 33RAPTORLAKE(06_B7H) codenamed Catlow are not affected. They are reported as 34vulnerable in Linux because they share the same family/model with an affected 35part. Unlike their affected counterparts, they do not enumerate RFDS_CLEAR or 36CPUID.HYBRID. This information could be used to distinguish between the 37affected and unaffected parts, but it is deemed not worth adding complexity as 38the reporting is fixed automatically when these parts enumerate RFDS_NO. 39 40Mitigation 41========== 42Intel released a microcode update that enables software to clear sensitive 43information using the VERW instruction. Like MDS, RFDS deploys the same 44mitigation strategy to force the CPU to clear the affected buffers before an 45attacker can extract the secrets. This is achieved by using the otherwise 46unused and obsolete VERW instruction in combination with a microcode update. 47The microcode clears the affected CPU buffers when the VERW instruction is 48executed. 49 50Mitigation points 51----------------- 52VERW is executed by the kernel before returning to user space, and by KVM 53before VMentry. None of the affected cores support SMT, so VERW is not required 54at C-state transitions. 55 56New bits in IA32_ARCH_CAPABILITIES 57---------------------------------- 58Newer processors and microcode update on existing affected processors added new 59bits to IA32_ARCH_CAPABILITIES MSR. These bits can be used to enumerate 60vulnerability and mitigation capability: 61 62- Bit 27 - RFDS_NO - When set, processor is not affected by RFDS. 63- Bit 28 - RFDS_CLEAR - When set, processor is affected by RFDS, and has the 64 microcode that clears the affected buffers on VERW execution. 65 66Mitigation control on the kernel command line 67--------------------------------------------- 68The kernel command line allows to control RFDS mitigation at boot time with the 69parameter "reg_file_data_sampling=". The valid arguments are: 70 71 ========== ================================================================= 72 on If the CPU is vulnerable, enable mitigation; CPU buffer clearing 73 on exit to userspace and before entering a VM. 74 off Disables mitigation. 75 ========== ================================================================= 76 77Mitigation default is selected by CONFIG_MITIGATION_RFDS. 78 79Mitigation status information 80----------------------------- 81The Linux kernel provides a sysfs interface to enumerate the current 82vulnerability status of the system: whether the system is vulnerable, and 83which mitigations are active. The relevant sysfs file is: 84 85 /sys/devices/system/cpu/vulnerabilities/reg_file_data_sampling 86 87The possible values in this file are: 88 89 .. list-table:: 90 91 * - 'Not affected' 92 - The processor is not vulnerable 93 * - 'Vulnerable' 94 - The processor is vulnerable, but no mitigation enabled 95 * - 'Vulnerable: No microcode' 96 - The processor is vulnerable but microcode is not updated. 97 * - 'Mitigation: Clear Register File' 98 - The processor is vulnerable and the CPU buffer clearing mitigation is 99 enabled. 100 101References 102---------- 103.. [#f1] Affected Processors 104 https://www.intel.com/content/www/us/en/developer/topic-technology/software-security-guidance/processors-affected-consolidated-product-cpu-model.html 105