[PATCH 1/3] x86/bugs: Create an option to disable MDS

[Breno Leitao]

There is no way to disable MDS mitigation today at compilation time. MDS
is enabled even if CONFIG_SPECULATION_MITIGATIONS is unset.

Create a new KCONFIG option that allow MDS mitigations to be disabled in
compilation time.

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 arch/x86/Kconfig           | 11 +++++++++++
 arch/x86/kernel/cpu/bugs.c |  9 ++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 53bab123a8ee..d25132b2d54f 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2649,6 +2649,17 @@ config SLS
 	  against straight line speculation. The kernel image might be slightly
 	  larger.
 
+config MITIGATE_MDS
+	bool "Mitigate Microarchitectural Data Sampling (MDS) hardware bug"
+	depends on CPU_SUP_INTEL && X86_64
+	default y
+	help
+	  Enable mitigation for Microarchitectural Data Sampling (MDS). MDS is
+	  a hardware vulnerability which allows unprivileged speculative access
+	  to data which is available in various CPU internal buffer. Deeper
+	  technical information is available in the MDS specific x86 architecture
+	  section: Documentation/arch/x86/mds.rst.
+
 endif
 
 config ARCH_HAS_ADD_PAGES
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 182af64387d0..50f12829dce9 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -269,6 +269,7 @@ static void x86_amd_ssb_disable(void)
 /* Default mitigation for MDS-affected CPUs */
 static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;
 static bool mds_nosmt __ro_after_init = false;
+#define MDS_WARN_MSG "WARNING: Microarchitectural Data Sampling (MDS) speculative mitigation disabled!\n"
 
 static const char * const mds_strings[] = {
 	[MDS_MITIGATION_OFF]	= "Vulnerable",
@@ -278,11 +279,17 @@ static const char * const mds_strings[] = {
 
 static void __init mds_select_mitigation(void)
 {
-	if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
+	if (!boot_cpu_has_bug(X86_BUG_MDS)) {
 		mds_mitigation = MDS_MITIGATION_OFF;
 		return;
 	}
 
+	if (cpu_mitigations_off() || !IS_ENABLED(CONFIG_MITIGATE_MDS)) {
+		mds_mitigation = MDS_MITIGATION_OFF;
+		pr_err(MDS_WARN_MSG);
+		return;
+	}
+
 	if (mds_mitigation == MDS_MITIGATION_FULL) {
 		if (!boot_cpu_has(X86_FEATURE_MD_CLEAR))
 			mds_mitigation = MDS_MITIGATION_VMWERV;
-- 
2.34.1