x86/bugs: Add a separate config for SRBDS

Currently, the CONFIG_SPECULATION_MITIGATIONS is halfway populated,
where some mitigations have entries in Kconfig, and they could be
modified, while others mitigations do not have Kconfig entries, and
could not be controlled at build time.

Create an entry for the SRBDS CPU mitigation under
CONFIG_SPECULATION_MITIGATIONS. This allow users to enable or disable
it at compilation time.
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20240729164105.554296-8-leitao@debian.org

arch/x86/Kconfig

@@ -2715,6 +2715,20 @@ config MITIGATION_SPECTRE_V1
 	  execution that bypasses conditional branch instructions used for
 	  memory access bounds check.
 	  See also <file:Documentation/admin-guide/hw-vuln/spectre.rst>
+
+config MITIGATION_SRBDS
+	bool "Mitigate Special Register Buffer Data Sampling (SRBDS) hardware bug"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Enable mitigation for Special Register Buffer Data Sampling (SRBDS).
+	  SRBDS is a hardware vulnerability that allows Microarchitectural Data
+	  Sampling (MDS) techniques to infer values returned from special
+	  register accesses. An unprivileged user can extract values returned
+	  from RDRAND and RDSEED executed on another core or sibling thread
+	  using MDS techniques.
+	  See also
+	  <file:Documentation/admin-guide/hw-vuln/special-register-buffer-data-sampling.rst>
 endif
 
 config ARCH_HAS_ADD_PAGES

arch/x86/kernel/cpu/bugs.c

@@ -608,7 +608,8 @@ enum srbds_mitigations {
 	SRBDS_MITIGATION_HYPERVISOR,
 };
 
-static enum srbds_mitigations srbds_mitigation __ro_after_init = SRBDS_MITIGATION_FULL;
+static enum srbds_mitigations srbds_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_MITIGATION_SRBDS) ? SRBDS_MITIGATION_FULL : SRBDS_MITIGATION_OFF;
 
 static const char * const srbds_strings[] = {
 	[SRBDS_MITIGATION_OFF]		= "Vulnerable",