x86/tsx: Disable TSX development mode at boot

A microcode update on some Intel processors causes all TSX transactions
to always abort by default[*]. Microcode also added functionality to
re-enable TSX for development purposes. With this microcode loaded, if
tsx=on was passed on the cmdline, and TSX development mode was already
enabled before the kernel boot, it may make the system vulnerable to TSX
Asynchronous Abort (TAA).

To be on safer side, unconditionally disable TSX development mode during
boot. If a viable use case appears, this can be revisited later.

  [*]: Intel TSX Disable Update for Selected Processors, doc ID: 643557

  [ bp: Drop unstable web link, massage heavily. ]

Suggested-by: Andrew Cooper <[email protected]>
Suggested-by: Borislav Petkov <[email protected]>
Signed-off-by: Pawan Gupta <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Tested-by: Neelima Krishnan <[email protected]>
Cc: <[email protected]>
Link: https://lore.kernel.org/r/347bd844da3a333a9793c6687d4e4eb3b2419a3e.1646943780.git.pawan.kumar.gupta@linux.intel.com

Author: pa1gupta

arch/x86/include/asm/msr-index.h

@@ -128,9 +128,9 @@
 #define TSX_CTRL_RTM_DISABLE		BIT(0)	/* Disable RTM feature */
 #define TSX_CTRL_CPUID_CLEAR		BIT(1)	/* Disable TSX enumeration */
 
-/* SRBDS support */
 #define MSR_IA32_MCU_OPT_CTRL		0x00000123
-#define RNGDS_MITG_DIS			BIT(0)
+#define RNGDS_MITG_DIS			BIT(0)	/* SRBDS support */
+#define RTM_ALLOW			BIT(1)	/* TSX development mode */
 
 #define MSR_IA32_SYSENTER_CS		0x00000174
 #define MSR_IA32_SYSENTER_ESP		0x00000175

arch/x86/kernel/cpu/common.c

@@ -1855,6 +1855,8 @@ void identify_secondary_cpu(struct cpuinfo_x86 *c)
 	validate_apic_and_package_id(c);
 	x86_spec_ctrl_setup_ap();
 	update_srbds_msr();
+
+	tsx_ap_init();
 }
 
 static __init int setup_noclflush(char *arg)

arch/x86/kernel/cpu/cpu.h

@@ -55,11 +55,10 @@ enum tsx_ctrl_states {
 extern __ro_after_init enum tsx_ctrl_states tsx_ctrl_state;
 
 extern void __init tsx_init(void);
-extern void tsx_enable(void);
-extern void tsx_disable(void);
-extern void tsx_clear_cpuid(void);
+void tsx_ap_init(void);
 #else
 static inline void tsx_init(void) { }
+static inline void tsx_ap_init(void) { }
 #endif /* CONFIG_CPU_SUP_INTEL */
 
 extern void get_cpu_cap(struct cpuinfo_x86 *c);

arch/x86/kernel/cpu/intel.c

@@ -717,14 +717,6 @@ static void init_intel(struct cpuinfo_x86 *c)
 
 	init_intel_misc_features(c);
 
-	if (tsx_ctrl_state == TSX_CTRL_ENABLE)
-		tsx_enable();
-	else if (tsx_ctrl_state == TSX_CTRL_DISABLE)
-		tsx_disable();
-	else if (tsx_ctrl_state == TSX_CTRL_RTM_ALWAYS_ABORT)
-		/* See comment over that function for more details. */
-		tsx_clear_cpuid();
-
 	split_lock_init();
 	bus_lock_init();
 

arch/x86/kernel/cpu/tsx.c

@@ -19,7 +19,7 @@
 
 enum tsx_ctrl_states tsx_ctrl_state __ro_after_init = TSX_CTRL_NOT_SUPPORTED;
 
-void tsx_disable(void)
+static void tsx_disable(void)
 {
 	u64 tsx;
 
@@ -39,7 +39,7 @@ void tsx_disable(void)
 	wrmsrl(MSR_IA32_TSX_CTRL, tsx);
 }
 
-void tsx_enable(void)
+static void tsx_enable(void)
 {
 	u64 tsx;
 
@@ -122,7 +122,7 @@ static enum tsx_ctrl_states x86_get_tsx_auto_mode(void)
  * That's why, this function's call in init_intel() doesn't clear the
  * feature flags.
  */
-void tsx_clear_cpuid(void)
+static void tsx_clear_cpuid(void)
 {
 	u64 msr;
 
@@ -142,11 +142,42 @@ void tsx_clear_cpuid(void)
 	}
 }
 
+/*
+ * Disable TSX development mode
+ *
+ * When the microcode released in Feb 2022 is applied, TSX will be disabled by
+ * default on some processors. MSR 0x122 (TSX_CTRL) and MSR 0x123
+ * (IA32_MCU_OPT_CTRL) can be used to re-enable TSX for development, doing so is
+ * not recommended for production deployments. In particular, applying MD_CLEAR
+ * flows for mitigation of the Intel TSX Asynchronous Abort (TAA) transient
+ * execution attack may not be effective on these processors when Intel TSX is
+ * enabled with updated microcode.
+ */
+static void tsx_dev_mode_disable(void)
+{
+	u64 mcu_opt_ctrl;
+
+	/* Check if RTM_ALLOW exists */
+	if (!boot_cpu_has_bug(X86_BUG_TAA) || !tsx_ctrl_is_supported() ||
+	    !cpu_feature_enabled(X86_FEATURE_SRBDS_CTRL))
+		return;
+
+	rdmsrl(MSR_IA32_MCU_OPT_CTRL, mcu_opt_ctrl);
+
+	if (mcu_opt_ctrl & RTM_ALLOW) {
+		mcu_opt_ctrl &= ~RTM_ALLOW;
+		wrmsrl(MSR_IA32_MCU_OPT_CTRL, mcu_opt_ctrl);
+		setup_force_cpu_cap(X86_FEATURE_RTM_ALWAYS_ABORT);
+	}
+}
+
 void __init tsx_init(void)
 {
 	char arg[5] = {};
 	int ret;
 
+	tsx_dev_mode_disable();
+
 	/*
 	 * Hardware will always abort a TSX transaction when the CPUID bit
 	 * RTM_ALWAYS_ABORT is set. In this case, it is better not to enumerate
@@ -215,3 +246,16 @@ void __init tsx_init(void)
 		setup_force_cpu_cap(X86_FEATURE_HLE);
 	}
 }
+
+void tsx_ap_init(void)
+{
+	tsx_dev_mode_disable();
+
+	if (tsx_ctrl_state == TSX_CTRL_ENABLE)
+		tsx_enable();
+	else if (tsx_ctrl_state == TSX_CTRL_DISABLE)
+		tsx_disable();
+	else if (tsx_ctrl_state == TSX_CTRL_RTM_ALWAYS_ABORT)
+		/* See comment over that function for more details. */
+		tsx_clear_cpuid();
+}

tools/arch/x86/include/asm/msr-index.h

@@ -128,9 +128,9 @@
 #define TSX_CTRL_RTM_DISABLE		BIT(0)	/* Disable RTM feature */
 #define TSX_CTRL_CPUID_CLEAR		BIT(1)	/* Disable TSX enumeration */
 
-/* SRBDS support */
 #define MSR_IA32_MCU_OPT_CTRL		0x00000123
-#define RNGDS_MITG_DIS			BIT(0)
+#define RNGDS_MITG_DIS			BIT(0)	/* SRBDS support */
+#define RTM_ALLOW			BIT(1)	/* TSX development mode */
 
 #define MSR_IA32_SYSENTER_CS		0x00000174
 #define MSR_IA32_SYSENTER_ESP		0x00000175