[PATCH v2] x86/bugs: Break down mitigations configurations

[Breno Leitao]

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

Create a new KCONFIG option for each mitigation under
CONFIG_SPECULATION_MITIGATIONS that allows these
mitigations to be disabled by default at compilation time.

It is still possible to enable these mitigations by passing kernel
parameters, even if they are disabled by default.

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
V1->V2: Change the default value of the global mitigation flag, other
	than disabling at the function itself, so, it could be
	re-enabled/overwritten if a parameter is passed.

---
 arch/x86/Kconfig           | 31 +++++++++++++++++++++++++++++++
 arch/x86/kernel/cpu/bugs.c | 12 ++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 53bab123a8ee..ba64f7c9b08d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2649,6 +2649,37 @@ 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.
+
+config MITIGATE_TAA
+	bool "Mitigate TSX Asynchronous Abort (TAA) hardware bug"
+	depends on CPU_SUP_INTEL && X86_64
+	default y
+	help
+	  Enable mitigation for TSX Asynchronous Abort (TAA). TAA is a hardware
+	  vulnerability that allows unprivileged speculative access to data
+	  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 182af64387d0..ba653830796f 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -267,7 +267,11 @@ static void x86_amd_ssb_disable(void)
 #define pr_fmt(fmt)	"MDS: " fmt
 
 /* Default mitigation for MDS-affected CPUs */
+#if IS_ENABLED(CONFIG_MITIGATE_MDS)
 static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;
+#else
+static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_OFF;
+#endif
 static bool mds_nosmt __ro_after_init = false;
 
 static const char * const mds_strings[] = {
@@ -327,7 +331,11 @@ enum taa_mitigations {
 };
 
 /* Default mitigation for TAA-affected CPUs */
+#if IS_ENABLED(CONFIG_MITIGATE_TAA)
 static enum taa_mitigations taa_mitigation __ro_after_init = TAA_MITIGATION_VERW;
+#else
+static enum taa_mitigations taa_mitigation __ro_after_init = TAA_MITIGATION_OFF;
+#endif
 static bool taa_nosmt __ro_after_init;
 
 static const char * const taa_strings[] = {
@@ -428,7 +436,11 @@ enum mmio_mitigations {
 };
 
 /* Default mitigation for Processor MMIO Stale Data vulnerabilities */
+#if IS_ENABLED(CONFIG_MITIGATE_MMIO_STALE_DATA)
 static enum mmio_mitigations mmio_mitigation __ro_after_init = MMIO_MITIGATION_VERW;
+#else
+static enum mmio_mitigations mmio_mitigation __ro_after_init = MMIO_MITIGATION_OFF;
+#endif
 static bool mmio_nosmt __ro_after_init = false;
 
 static const char * const mmio_strings[] = {
-- 
2.34.1