[MODERATED] Re: [PATCH 2/2] v3 more sampling fun 2

Thanks for the valueble feedback.  I'll go over it in more detail tomorrow.
But, one thing I want to point out is that EGETKEY is always mitigate and only
RDRAND and RDSEED can have the mitigation disabled.

This is the first time I've worked with the Documentation.  I did build it but
I only looked to see if it completed :(

--mark

On Wed, Mar 11, 2020 at 09:26:18PM +0100, speck for Thomas Gleixner wrote:
> speck for mark gross <[email protected]> writes:
> > diff --git a/Documentation/admin-guide/hw-vuln/special-register-buffer-data-sampling.rst b/Documentation/admin-guide/hw-vuln/special-register-buffer-data-sampling.rst
> > new file mode 100644
> > index 000000000000..5acc7748f8e9
> > --- /dev/null
> > +++ b/Documentation/admin-guide/hw-vuln/special-register-buffer-data-sampling.rst
> > @@ -0,0 +1,145 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +SRBDS - Special Register Buffer Data Sampling
> > +======================================
> 
> I doubt that this builds w/o warnings. And right:
> 
> Documentation/admin-guide/hw-vuln/special-register-buffer-data-sampling.rst:4: WARNING: Title underline too short.
> 
> SRBDS - Special Register Buffer Data Sampling
> ======================================^
> 
> Documentation/admin-guide/hw-vuln/special-register-buffer-data-sampling.rst:56: WARNING: Title underline too short.
> 
> Attack scenarios
> ---------------^
> 
> Documentation/admin-guide/hw-vuln/special-register-buffer-data-sampling.rst:76: WARNING: Unexpected indentation.
> Documentation/admin-guide/hw-vuln/special-register-buffer-data-sampling.rst:77: WARNING: Block quote ends without a blank line; unexpected unindent.
> 
> Sigh...
> 
> > +SRBDS is a hardware vulnerability that allows MDS techniques to infer values
> 
> lacks a link to the MDS documentation
> 
> > +returned from special register accesses.  Special register accesses are
> > +accesses to off core registers.  According to Intels evaluation, the special
> > +register reads that have a security expectation of privacy are:
> > +RDRAND, RDSEED and SGX EGETKEY.
> 
> And what 
> 
> > +
> > +When RDRAND and RDSEED instructions are used the data is moved to the core
> > +through the special register mechanism.
> 
> This sentence is not providing any new information and can be packed in
> the above where you mention RDRAND, RDSEED already. EGETKEY is no instruction?
> 
> > +Affected processors
> > +--------------------
> > +Core models (desktop, mobile, Xeon-E3) that implement RDRAND and/or RDSEED and
> > +are vulnerable to MFBDS (Micro architectural Fill Buffer Data
> > Sampling) variant
> 
> s/to/to the/
> 
> > +of MDS (Micro architectural Data Sampling) or to TAA (TSX Asynchronous Abort)
> > +when TSX is enabled,
> > +
> > +  =============  ============  ========
> > +  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        <=A
> > +  Kabylake_L     06_8EH        0xB only if TSX is enabled
> > +  Kabylake_L     06_8EH        0xC only if TSX is enabled
> > +
> > +  Kabylake       06_9EH        <=B
> > +  Kabylake       06_9EH        0xC only if TSX is enabled
> > +  Kabylake       06_9EH        0xD only if TSX is enabled
> > +  =============  ============  ========
> 
> > +  =============  ============  ===========================
> 
> ditto at the top of the table.
> 
> > +
> > +Related CVEs
> > +------------
> > +
> > +The following CVE entry is related to this SRBDS issue:
> > +
> > +    ==============  =====  ===================================================
> > +    CVE-2020--0543
> > +    ==============  =====  ===================================================
> 
> Is the void filled at some point?
> 
> > +Attack scenarios
> > +---------------
> > +An unprivileged user can extract returned values from RDRAND and RDSEED
> > +executed on another core or sibling thread using MDS techniques.
> 
> Lacks EGETKEY again.
> 
> > +
> > +
> > +Mitigtion 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 instruction, 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:
> > +1 RDRAND, RDSEED, or EGETKEY instruction have higher latency.
> > +2 Executing RDRAND at the same time on multiple logical processors will be
> > +  serialized, resulting in an overall reduction in the maximum RDRAND bandwidth.
> > +3 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.
> 
> I seriously doubt that you ever looked at the build output of this.
> 
> > +
> > +Because of the performance impact of the mitigation, the microcode updates also
> > +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 of 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 for control over the SRBDS mitigation
> > at boot
> 
> allows control (methinks)
> 
> > +time with the option "srbds=".  The option for this is:
> > +
> > +  ============= ============================================================
> > +  off           This option disables SRBS mitigation for RDRAND and RDSEED on
> > +                affected platforms.
> > +  ============= ============================================================
> > +
> > +SRBS 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/special_register_data_sampling
> > +
> > +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
> > + Mitigated                      Processor is vulnerable and mitigation is in
> > +                                effect.
> > + Not affected (TSX disabled)    Processor is only vulnerable when TSX is
> > +                                enabled while this system was booted with TSX
> > +                                disabled.
> > + Unknown                        Running on virtual guest processor that is
> > +                                affected but with no way to know if host
> > +                                processor is mitigated or vulnerable.
> > + ============================== ===========================================
> > +
> > +Default mitigations
> > +-------------------
> > +This new microcode serializes processor access during execution of RDRAND,
> > +RDSEED ensures that the shared buffer is overwritten before it is released for
> > +reuse.
> 
> Errm. What has this to do with the default chosen by the kernel?
> 
> Thanks,
> 
>         tglx