[PATCH 3/3] x86/bugs: Create an option to disable MMIO vulnerability

[Breno Leitao]

There is no way to disable MMIO Stale data mitigation today at
compilation time. These mitigations are enabled even if
CONFIG_SPECULATION_MITIGATIONS is unset.

Create a new KCONFIG option that allow MMIO mitigation to be disabled in
compilation time.

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 arch/x86/Kconfig           | 10 ++++++++++
 arch/x86/kernel/cpu/bugs.c | 10 ++++++++--
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 140af3b30c45..ba64f7c9b08d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2670,6 +2670,16 @@ config MITIGATE_TAA
 	  which is available in various CPU internal buffers by using
 	  asynchronous aborts within an Intel TSX transactional region.
 
+config MITIGATE_MMIO_STALE_DATA
+	bool "Mitigate MMIO Stale Data hardware bug"
+	depends on CPU_SUP_INTEL && X86_64
+	default y
+	help
+	  Enable mitigation for MMIO Stale Data hardware bugs.  Processor MMIO
+	  Stale Data Vulnerabilities are a class of memory-mapped I/O (MMIO)
+	  vulnerabilities that can expose data. The vulnerabilities require the
+	  attacker to have access to MMIO.
+
 endif
 
 config ARCH_HAS_ADD_PAGES
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 3615bda9573f..b5c171feb05e 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -439,6 +439,7 @@ enum mmio_mitigations {
 /* Default mitigation for Processor MMIO Stale Data vulnerabilities */
 static enum mmio_mitigations mmio_mitigation __ro_after_init = MMIO_MITIGATION_VERW;
 static bool mmio_nosmt __ro_after_init = false;
+#define MMIO_WARN_MSG "WARNING: MMIO Stale Data speculative mitigation disabled!\n"
 
 static const char * const mmio_strings[] = {
 	[MMIO_MITIGATION_OFF]		= "Vulnerable",
@@ -451,12 +452,17 @@ static void __init mmio_select_mitigation(void)
 	u64 ia32_cap;
 
 	if (!boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA) ||
-	     boot_cpu_has_bug(X86_BUG_MMIO_UNKNOWN) ||
-	     cpu_mitigations_off()) {
+	     boot_cpu_has_bug(X86_BUG_MMIO_UNKNOWN)) {
 		mmio_mitigation = MMIO_MITIGATION_OFF;
 		return;
 	}
 
+	if (cpu_mitigations_off() || !IS_ENABLED(CONFIG_MITIGATE_MDS)) {
+		mmio_mitigation = MMIO_MITIGATION_OFF;
+		pr_err(MMIO_WARN_MSG);
+		return;
+	}
+
 	if (mmio_mitigation == MMIO_MITIGATION_OFF)
 		return;
 
-- 
2.34.1