x86/speculation: Add SRBDS vulnerability and mitigation documentation
Add documentation for the SRBDS vulnerability and its mitigation. [ bp: Massage. jpoimboe: sysfs table strings. ] Signed-off-by: Mark Gross <mgross@linux.intel.com> Signed-off-by: Borislav Petkov <bp@suse.de> Reviewed-by: Tony Luck <tony.luck@intel.com> Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
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Mark Gross
Thomas Gleixner
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@ -14,3 +14,4 @@ are configurable at compile, boot or run time.
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mds
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tsx_async_abort
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multihit.rst
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special-register-buffer-data-sampling.rst
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@ -0,0 +1,148 @@
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.. SPDX-License-Identifier: GPL-2.0
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SRBDS - Special Register Buffer Data Sampling
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=============================================
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SRBDS is a hardware vulnerability that allows MDS :doc:`mds` techniques to
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infer values returned from special register accesses. Special register
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accesses are accesses to off core registers. According to Intel's evaluation,
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the special register reads that have a security expectation of privacy are
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RDRAND, RDSEED and SGX EGETKEY.
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When RDRAND, RDSEED and EGETKEY instructions are used, the data is moved
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to the core through the special register mechanism that is susceptible
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to MDS attacks.
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Affected processors
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--------------------
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Core models (desktop, mobile, Xeon-E3) that implement RDRAND and/or RDSEED may
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be affected.
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A processor is affected by SRBDS if its Family_Model and stepping is
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in the following list, with the exception of the listed processors
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exporting MDS_NO while Intel TSX is available yet not enabled. The
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latter class of processors are only affected when Intel TSX is enabled
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by software using TSX_CTRL_MSR otherwise they are not affected.
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============= ============ ========
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common name Family_Model Stepping
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============= ============ ========
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Haswell 06_3CH All
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Haswell_L 06_45H All
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Haswell_G 06_46H All
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Broadwell_G 06_47H All
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Broadwell 06_3DH All
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Skylake_L 06_4EH All
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Skylake 06_5EH All
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Kabylake_L 06_8EH <=0xC
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Kabylake 06_9EH <=0xD
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============= ============ ========
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Related CVEs
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------------
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The following CVE entry is related to this SRBDS issue:
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============== ===== =====================================
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CVE-2020-0543 SRBDS Special Register Buffer Data Sampling
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============== ===== =====================================
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Attack scenarios
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----------------
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An unprivileged user can extract values returned from RDRAND and RDSEED
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executed on another core or sibling thread using MDS techniques.
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Mitigation mechanism
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-------------------
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Intel will release microcode updates that modify the RDRAND, RDSEED, and
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EGETKEY instructions to overwrite secret special register data in the shared
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staging buffer before the secret data can be accessed by another logical
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processor.
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During execution of the RDRAND, RDSEED, or EGETKEY instructions, off-core
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accesses from other logical processors will be delayed until the special
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register read is complete and the secret data in the shared staging buffer is
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overwritten.
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This has three effects on performance:
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#. RDRAND, RDSEED, or EGETKEY instructions have higher latency.
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#. Executing RDRAND at the same time on multiple logical processors will be
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serialized, resulting in an overall reduction in the maximum RDRAND
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bandwidth.
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#. Executing RDRAND, RDSEED or EGETKEY will delay memory accesses from other
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logical processors that miss their core caches, with an impact similar to
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legacy locked cache-line-split accesses.
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The microcode updates provide an opt-out mechanism (RNGDS_MITG_DIS) to disable
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the mitigation for RDRAND and RDSEED instructions executed outside of Intel
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Software Guard Extensions (Intel SGX) enclaves. On logical processors that
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disable the mitigation using this opt-out mechanism, RDRAND and RDSEED do not
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take longer to execute and do not impact performance of sibling logical
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processors memory accesses. The opt-out mechanism does not affect Intel SGX
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enclaves (including execution of RDRAND or RDSEED inside an enclave, as well
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as EGETKEY execution).
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IA32_MCU_OPT_CTRL MSR Definition
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--------------------------------
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Along with the mitigation for this issue, Intel added a new thread-scope
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IA32_MCU_OPT_CTRL MSR, (address 0x123). The presence of this MSR and
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RNGDS_MITG_DIS (bit 0) is enumerated by CPUID.(EAX=07H,ECX=0).EDX[SRBDS_CTRL =
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9]==1. This MSR is introduced through the microcode update.
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Setting IA32_MCU_OPT_CTRL[0] (RNGDS_MITG_DIS) to 1 for a logical processor
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disables the mitigation for RDRAND and RDSEED executed outside of an Intel SGX
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enclave on that logical processor. Opting out of the mitigation for a
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particular logical processor does not affect the RDRAND and RDSEED mitigations
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for other logical processors.
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Note that inside of an Intel SGX enclave, the mitigation is applied regardless
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of the value of RNGDS_MITG_DS.
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Mitigation control on the kernel command line
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---------------------------------------------
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The kernel command line allows control over the SRBDS mitigation at boot time
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with the option "srbds=". The option for this is:
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============= =============================================================
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off This option disables SRBDS mitigation for RDRAND and RDSEED on
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affected platforms.
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============= =============================================================
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SRBDS System Information
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-----------------------
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The Linux kernel provides vulnerability status information through sysfs. For
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SRBDS this can be accessed by the following sysfs file:
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/sys/devices/system/cpu/vulnerabilities/srbds
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The possible values contained in this file are:
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============================== =============================================
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Not affected Processor not vulnerable
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Vulnerable Processor vulnerable and mitigation disabled
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Vulnerable: No microcode Processor vulnerable and microcode is missing
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mitigation
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Mitigation: Microcode Processor is vulnerable and mitigation is in
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effect.
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Mitigation: TSX disabled Processor is only vulnerable when TSX is
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enabled while this system was booted with TSX
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disabled.
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Unknown: Dependent on
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hypervisor status Running on virtual guest processor that is
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affected but with no way to know if host
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processor is mitigated or vulnerable.
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============================== =============================================
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SRBDS Default mitigation
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------------------------
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This new microcode serializes processor access during execution of RDRAND,
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RDSEED ensures that the shared buffer is overwritten before it is released for
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reuse. Use the "srbds=off" kernel command line to disable the mitigation for
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RDRAND and RDSEED.
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