.. SPDX-License-Identifier: GPL-2.0

SRBDS - Special Register Buffer Data Sampling
=============================================

SRBDS is a hardware vulnerability that allows MDS :doc:`mds` techniques to
infer values returned from special register accesses.  Special register
accesses are accesses to off core registers.  According to Intel's evaluation,
the special register reads that have a security expectation of privacy are
RDRAND, RDSEED and SGX EGETKEY.

When RDRAND, RDSEED and EGETKEY instructions are used, the data is moved
to the core through the special register mechanism that is susceptible
to MDS attacks.

Affected processors
--------------------
Core models (desktop, mobile, Xeon-E3) that implement RDRAND and/or RDSEED may
be affected.

A processor is affected by SRBDS if its Family_Model and stepping is
in the following list, with the exception of the listed processors
exporting MDS_NO while Intel TSX is available yet not enabled. The
latter class of processors are only affected when Intel TSX is enabled
by software using TSX_CTRL_MSR otherwise they are not affected.

  =============  ============  ========
  common name    Family_Model  Stepping
  =============  ============  ========
  IvyBridge      06_3AH        All

  Haswell        06_3CH        All
  Haswell_L      06_45H        All
  Haswell_G      06_46H        All

  Broadwell_G    06_47H        All
  Broadwell      06_3DH        All

  Skylake_L      06_4EH        All
  Skylake        06_5EH        All

  Kabylake_L     06_8EH        <= 0xC
  Kabylake       06_9EH        <= 0xD
  =============  ============  ========

Related CVEs
------------

The following CVE entry is related to this SRBDS issue:

    ==============  =====  =====================================
    CVE-2020-0543   SRBDS  Special Register Buffer Data Sampling
    ==============  =====  =====================================

Attack scenarios
----------------
An unprivileged user can extract values returned from RDRAND and RDSEED
executed on another core or sibling thread using MDS techniques.


Mitigation mechanism
-------------------
Intel will release microcode updates that modify the RDRAND, RDSEED, and
EGETKEY instructions to overwrite secret special register data in the shared
staging buffer before the secret data can be accessed by another logical
processor.

During execution of the RDRAND, RDSEED, or EGETKEY instructions, off-core
accesses from other logical processors will be delayed until the special
register read is complete and the secret data in the shared staging buffer is
overwritten.

This has three effects on performance:

#. RDRAND, RDSEED, or EGETKEY instructions have higher latency.

#. Executing RDRAND at the same time on multiple logical processors will be
   serialized, resulting in an overall reduction in the maximum RDRAND
   bandwidth.

#. Executing RDRAND, RDSEED or EGETKEY will delay memory accesses from other
   logical processors that miss their core caches, with an impact similar to
   legacy locked cache-line-split accesses.

The microcode updates provide an opt-out mechanism (RNGDS_MITG_DIS) to disable
the mitigation for RDRAND and RDSEED instructions executed outside of Intel
Software Guard Extensions (Intel SGX) enclaves. On logical processors that
disable the mitigation using this opt-out mechanism, RDRAND and RDSEED do not
take longer to execute and do not impact performance of sibling logical
processors memory accesses. The opt-out mechanism does not affect Intel SGX
enclaves (including execution of RDRAND or RDSEED inside an enclave, as well
as EGETKEY execution).

IA32_MCU_OPT_CTRL MSR Definition
--------------------------------
Along with the mitigation for this issue, Intel added a new thread-scope
IA32_MCU_OPT_CTRL MSR, (address 0x123). The presence of this MSR and
RNGDS_MITG_DIS (bit 0) is enumerated by CPUID.(EAX=07H,ECX=0).EDX[SRBDS_CTRL =
9]==1. This MSR is introduced through the microcode update.

Setting IA32_MCU_OPT_CTRL[0] (RNGDS_MITG_DIS) to 1 for a logical processor
disables the mitigation for RDRAND and RDSEED executed outside of an Intel SGX
enclave on that logical processor. Opting out of the mitigation for a
particular logical processor does not affect the RDRAND and RDSEED mitigations
for other logical processors.

Note that inside of an Intel SGX enclave, the mitigation is applied regardless
of the value of RNGDS_MITG_DS.

Mitigation control on the kernel command line
---------------------------------------------
The kernel command line allows control over the SRBDS mitigation at boot time
with the option "srbds=".  The option for this is:

  ============= =============================================================
  off           This option disables SRBDS mitigation for RDRAND and RDSEED on
                affected platforms.
  ============= =============================================================

SRBDS System Information
-----------------------
The Linux kernel provides vulnerability status information through sysfs.  For
SRBDS this can be accessed by the following sysfs file:
/sys/devices/system/cpu/vulnerabilities/srbds

The possible values contained in this file are:

 ============================== =============================================
 Not affected                   Processor not vulnerable
 Vulnerable                     Processor vulnerable and mitigation disabled
 Vulnerable: No microcode       Processor vulnerable and microcode is missing
                                mitigation
 Mitigation: Microcode          Processor is vulnerable and mitigation is in
                                effect.
 Mitigation: TSX disabled       Processor is only vulnerable when TSX is
                                enabled while this system was booted with TSX
                                disabled.
 Unknown: Dependent on
 hypervisor status              Running on virtual guest processor that is
                                affected but with no way to know if host
                                processor is mitigated or vulnerable.
 ============================== =============================================

SRBDS Default mitigation
------------------------
This new microcode serializes processor access during execution of RDRAND,
RDSEED ensures that the shared buffer is overwritten before it is released for
reuse.  Use the "srbds=off" kernel command line to disable the mitigation for
RDRAND and RDSEED.