1.. SPDX-License-Identifier: GPL-2.0
2
3SRBDS - Special Register Buffer Data Sampling
4=============================================
5
6SRBDS is a hardware vulnerability that allows MDS :doc:`mds` techniques to
7infer values returned from special register accesses.  Special register
8accesses are accesses to off core registers.  According to Intel's evaluation,
9the special register reads that have a security expectation of privacy are
10RDRAND, RDSEED and SGX EGETKEY.
11
12When RDRAND, RDSEED and EGETKEY instructions are used, the data is moved
13to the core through the special register mechanism that is susceptible
14to MDS attacks.
15
16Affected processors
17-------------------
18Core models (desktop, mobile, Xeon-E3) that implement RDRAND and/or RDSEED may
19be affected.
20
21A processor is affected by SRBDS if its Family_Model and stepping is
22in the following list, with the exception of the listed processors
23exporting MDS_NO while Intel TSX is available yet not enabled. The
24latter class of processors are only affected when Intel TSX is enabled
25by software using TSX_CTRL_MSR otherwise they are not affected.
26
27  =============  ============  ========
28  common name    Family_Model  Stepping
29  =============  ============  ========
30  IvyBridge      06_3AH        All
31
32  Haswell        06_3CH        All
33  Haswell_L      06_45H        All
34  Haswell_G      06_46H        All
35
36  Broadwell_G    06_47H        All
37  Broadwell      06_3DH        All
38
39  Skylake_L      06_4EH        All
40  Skylake        06_5EH        All
41
42  Kabylake_L     06_8EH        <= 0xC
43  Kabylake       06_9EH        <= 0xD
44  =============  ============  ========
45
46Related CVEs
47------------
48
49The following CVE entry is related to this SRBDS issue:
50
51    ==============  =====  =====================================
52    CVE-2020-0543   SRBDS  Special Register Buffer Data Sampling
53    ==============  =====  =====================================
54
55Attack scenarios
56----------------
57An unprivileged user can extract values returned from RDRAND and RDSEED
58executed on another core or sibling thread using MDS techniques.
59
60
61Mitigation mechanism
62--------------------
63Intel will release microcode updates that modify the RDRAND, RDSEED, and
64EGETKEY instructions to overwrite secret special register data in the shared
65staging buffer before the secret data can be accessed by another logical
66processor.
67
68During execution of the RDRAND, RDSEED, or EGETKEY instructions, off-core
69accesses from other logical processors will be delayed until the special
70register read is complete and the secret data in the shared staging buffer is
71overwritten.
72
73This has three effects on performance:
74
75#. RDRAND, RDSEED, or EGETKEY instructions have higher latency.
76
77#. Executing RDRAND at the same time on multiple logical processors will be
78   serialized, resulting in an overall reduction in the maximum RDRAND
79   bandwidth.
80
81#. Executing RDRAND, RDSEED or EGETKEY will delay memory accesses from other
82   logical processors that miss their core caches, with an impact similar to
83   legacy locked cache-line-split accesses.
84
85The microcode updates provide an opt-out mechanism (RNGDS_MITG_DIS) to disable
86the mitigation for RDRAND and RDSEED instructions executed outside of Intel
87Software Guard Extensions (Intel SGX) enclaves. On logical processors that
88disable the mitigation using this opt-out mechanism, RDRAND and RDSEED do not
89take longer to execute and do not impact performance of sibling logical
90processors memory accesses. The opt-out mechanism does not affect Intel SGX
91enclaves (including execution of RDRAND or RDSEED inside an enclave, as well
92as EGETKEY execution).
93
94IA32_MCU_OPT_CTRL MSR Definition
95--------------------------------
96Along with the mitigation for this issue, Intel added a new thread-scope
97IA32_MCU_OPT_CTRL MSR, (address 0x123). The presence of this MSR and
98RNGDS_MITG_DIS (bit 0) is enumerated by CPUID.(EAX=07H,ECX=0).EDX[SRBDS_CTRL =
999]==1. This MSR is introduced through the microcode update.
100
101Setting IA32_MCU_OPT_CTRL[0] (RNGDS_MITG_DIS) to 1 for a logical processor
102disables the mitigation for RDRAND and RDSEED executed outside of an Intel SGX
103enclave on that logical processor. Opting out of the mitigation for a
104particular logical processor does not affect the RDRAND and RDSEED mitigations
105for other logical processors.
106
107Note that inside of an Intel SGX enclave, the mitigation is applied regardless
108of the value of RNGDS_MITG_DS.
109
110Mitigation control on the kernel command line
111---------------------------------------------
112The kernel command line allows control over the SRBDS mitigation at boot time
113with the option "srbds=".  The option for this is:
114
115  ============= =============================================================
116  off           This option disables SRBDS mitigation for RDRAND and RDSEED on
117                affected platforms.
118  ============= =============================================================
119
120SRBDS System Information
121------------------------
122The Linux kernel provides vulnerability status information through sysfs.  For
123SRBDS this can be accessed by the following sysfs file:
124/sys/devices/system/cpu/vulnerabilities/srbds
125
126The possible values contained in this file are:
127
128 ============================== =============================================
129 Not affected                   Processor not vulnerable
130 Vulnerable                     Processor vulnerable and mitigation disabled
131 Vulnerable: No microcode       Processor vulnerable and microcode is missing
132                                mitigation
133 Mitigation: Microcode          Processor is vulnerable and mitigation is in
134                                effect.
135 Mitigation: TSX disabled       Processor is only vulnerable when TSX is
136                                enabled while this system was booted with TSX
137                                disabled.
138 Unknown: Dependent on
139 hypervisor status              Running on virtual guest processor that is
140                                affected but with no way to know if host
141                                processor is mitigated or vulnerable.
142 ============================== =============================================
143
144SRBDS Default mitigation
145------------------------
146This new microcode serializes processor access during execution of RDRAND,
147RDSEED ensures that the shared buffer is overwritten before it is released for
148reuse.  Use the "srbds=off" kernel command line to disable the mitigation for
149RDRAND and RDSEED.
150