Two x86 fixes related to TSX:

- Use either MSR_TSX_FORCE_ABORT or MSR_IA32_TSX_CTRL to disable TSX to
     cover all CPUs which allow to disable it.
 
   - Disable TSX development mode at boot so that a microcode update which
     provides TSX development mode does not suddenly make the system
     vulnerable to TSX Asynchronous Abort.
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Merge tag 'x86-urgent-2022-04-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fixes from Thomas Gleixner:
 "Two x86 fixes related to TSX:

   - Use either MSR_TSX_FORCE_ABORT or MSR_IA32_TSX_CTRL to disable TSX
     to cover all CPUs which allow to disable it.

   - Disable TSX development mode at boot so that a microcode update
     which provides TSX development mode does not suddenly make the
     system vulnerable to TSX Asynchronous Abort"

* tag 'x86-urgent-2022-04-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/tsx: Disable TSX development mode at boot
  x86/tsx: Use MSR_TSX_CTRL to clear CPUID bits

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

@@ -717,13 +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)
-		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;
 
@@ -58,7 +58,7 @@ void tsx_enable(void)
 	wrmsrl(MSR_IA32_TSX_CTRL, tsx);
 }
 
-static bool __init tsx_ctrl_is_supported(void)
+static bool tsx_ctrl_is_supported(void)
 {
 	u64 ia32_cap = x86_read_arch_cap_msr();
 
@@ -84,7 +84,45 @@ static enum tsx_ctrl_states x86_get_tsx_auto_mode(void)
 	return TSX_CTRL_ENABLE;
 }
 
-void tsx_clear_cpuid(void)
+/*
+ * Disabling TSX is not a trivial business.
+ *
+ * First of all, there's a CPUID bit: X86_FEATURE_RTM_ALWAYS_ABORT
+ * which says that TSX is practically disabled (all transactions are
+ * aborted by default). When that bit is set, the kernel unconditionally
+ * disables TSX.
+ *
+ * In order to do that, however, it needs to dance a bit:
+ *
+ * 1. The first method to disable it is through MSR_TSX_FORCE_ABORT and
+ * the MSR is present only when *two* CPUID bits are set:
+ *
+ * - X86_FEATURE_RTM_ALWAYS_ABORT
+ * - X86_FEATURE_TSX_FORCE_ABORT
+ *
+ * 2. The second method is for CPUs which do not have the above-mentioned
+ * MSR: those use a different MSR - MSR_IA32_TSX_CTRL and disable TSX
+ * through that one. Those CPUs can also have the initially mentioned
+ * CPUID bit X86_FEATURE_RTM_ALWAYS_ABORT set and for those the same strategy
+ * applies: TSX gets disabled unconditionally.
+ *
+ * When either of the two methods are present, the kernel disables TSX and
+ * clears the respective RTM and HLE feature flags.
+ *
+ * An additional twist in the whole thing presents late microcode loading
+ * which, when done, may cause for the X86_FEATURE_RTM_ALWAYS_ABORT CPUID
+ * bit to be set after the update.
+ *
+ * A subsequent hotplug operation on any logical CPU except the BSP will
+ * cause for the supported CPUID feature bits to get re-detected and, if
+ * RTM and HLE get cleared all of a sudden, but, userspace did consult
+ * them before the update, then funny explosions will happen. Long story
+ * short: the kernel doesn't modify CPUID feature bits after booting.
+ *
+ * That's why, this function's call in init_intel() doesn't clear the
+ * feature flags.
+ */
+static void tsx_clear_cpuid(void)
 {
 	u64 msr;
 
@@ -97,6 +135,39 @@ void tsx_clear_cpuid(void)
 		rdmsrl(MSR_TSX_FORCE_ABORT, msr);
 		msr |= MSR_TFA_TSX_CPUID_CLEAR;
 		wrmsrl(MSR_TSX_FORCE_ABORT, msr);
+	} else if (tsx_ctrl_is_supported()) {
+		rdmsrl(MSR_IA32_TSX_CTRL, msr);
+		msr |= TSX_CTRL_CPUID_CLEAR;
+		wrmsrl(MSR_IA32_TSX_CTRL, msr);
+	}
+}
+
+/*
+ * 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);
 	}
 }
 
@@ -105,14 +176,14 @@ void __init tsx_init(void)
 	char arg[5] = {};
 	int ret;
 
+	tsx_dev_mode_disable();
+
 	/*
-	 * Hardware will always abort a TSX transaction if both CPUID bits
-	 * RTM_ALWAYS_ABORT and TSX_FORCE_ABORT are set. In this case, it is
-	 * better not to enumerate CPUID.RTM and CPUID.HLE bits. Clear them
-	 * here.
+	 * 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
+	 * CPUID.RTM and CPUID.HLE bits. Clear them here.
 	 */
-	if (boot_cpu_has(X86_FEATURE_RTM_ALWAYS_ABORT) &&
-	    boot_cpu_has(X86_FEATURE_TSX_FORCE_ABORT)) {
+	if (boot_cpu_has(X86_FEATURE_RTM_ALWAYS_ABORT)) {
 		tsx_ctrl_state = TSX_CTRL_RTM_ALWAYS_ABORT;
 		tsx_clear_cpuid();
 		setup_clear_cpu_cap(X86_FEATURE_RTM);
@@ -175,3 +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