x86/speculation: Add Special Register Buffer Data Sampling (SRBDS) mitigation
commit 7e5b3c267d256822407a22fdce6afdf9cd13f9fb upstream
SRBDS is an MDS-like speculative side channel that can leak bits from the
random number generator (RNG) across cores and threads. New microcode
serializes the processor access during the execution of RDRAND and
RDSEED. This ensures that the shared buffer is overwritten before it is
released for reuse.
While it is present on all affected CPU models, the microcode mitigation
is not needed on models that enumerate ARCH_CAPABILITIES[MDS_NO] in the
cases where TSX is not supported or has been disabled with TSX_CTRL.
The mitigation is activated by default on affected processors and it
increases latency for RDRAND and RDSEED instructions. Among other
effects this will reduce throughput from /dev/urandom.
* Enable administrator to configure the mitigation off when desired using
either mitigations=off or srbds=off.
* Export vulnerability status via sysfs
* Rename file-scoped macros to apply for non-whitelist table initializations.
[ bp: Massage,
- s/VULNBL_INTEL_STEPPING/VULNBL_INTEL_STEPPINGS/g,
- do not read arch cap MSR a second time in tsx_fused_off() - just pass it in,
- flip check in cpu_set_bug_bits() to save an indentation level,
- reflow comments.
jpoimboe: s/Mitigated/Mitigation/ in user-visible strings
tglx: Dropped the fused off magic for now
]
Signed-off-by: Mark Gross <mgross@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Neelima Krishnan <neelima.krishnan@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index e05d65d..4060251 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -4262,6 +4262,26 @@
spia_pedr=
spia_peddr=
+ srbds= [X86,INTEL]
+ Control the Special Register Buffer Data Sampling
+ (SRBDS) mitigation.
+
+ Certain CPUs are vulnerable to an MDS-like
+ exploit which can leak bits from the random
+ number generator.
+
+ By default, this issue is mitigated by
+ microcode. However, the microcode fix can cause
+ the RDRAND and RDSEED instructions to become
+ much slower. Among other effects, this will
+ result in reduced throughput from /dev/urandom.
+
+ The microcode mitigation can be disabled with
+ the following option:
+
+ off: Disable mitigation and remove
+ performance impact to RDRAND and RDSEED
+
ssbd= [ARM64,HW]
Speculative Store Bypass Disable control