UBUNTU: SAUCE: x86/speculation: Cleanup IBRS runtime control handling

In Ubuntu, we have runtime control for enabling/disabling IBRS via the
commandline ("noibrs") and through the proc interface
/proc/sys/kernel/ibrs_enabled. This commit simplifies the current
(probably broken) implementation by merging it with all the IBRS-related
upstream changes from previous commits.

What we have now is the upstream implementation for detecting the presence
of IBRS support. This commit adds a global state variable 'ibrs_enabled'
which is set to 1 if the CPU supports IBRS but can be overridden via the
commandline "noibrs" switch or by writting 0, 1 or 2 to
/proc/sys/kernel/ibrs_enabled at runtime.

Note that the runtime controls are disabled if the CPU runs in Enhanced
IBRS mode.

CVE-2017-5715
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/x86/include/asm/mwait.h

@@ -106,15 +106,13 @@ static inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx)
 			mb();
 		}
 
-		if (ibrs_inuse)
-			native_wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+		ubuntu_restrict_branch_speculation_end();
 
 		__monitor((void *)&current_thread_info()->flags, 0, 0);
 		if (!need_resched())
 			__mwait(eax, ecx);
 
-		if (ibrs_inuse)
-			native_wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base | SPEC_CTRL_IBRS);
+		ubuntu_restrict_branch_speculation_start();
 	}
 	current_clr_polling();
 }

arch/x86/include/asm/nospec-branch.h

@@ -188,6 +188,10 @@
 extern unsigned int ibpb_enabled;
 int set_ibpb_enabled(unsigned int);
 
+/* The IBRS runtime control knob */
+extern unsigned int ibrs_enabled;
+int set_ibrs_enabled(unsigned int);
+
 /* The Spectre V2 mitigation variants */
 enum spectre_v2_mitigation {
 	SPECTRE_V2_NONE,
@@ -275,6 +279,22 @@ do {									\
 	preempt_enable();						\
 } while (0)
 
+#define ubuntu_restrict_branch_speculation_start()			\
+do {									\
+	u64 val = x86_spec_ctrl_base | SPEC_CTRL_IBRS;			\
+									\
+	if (ibrs_enabled)						\
+		native_wrmsrl(MSR_IA32_SPEC_CTRL, val);			\
+} while (0)
+
+#define ubuntu_restrict_branch_speculation_end()			\
+do {									\
+	u64 val = x86_spec_ctrl_base;					\
+									\
+	if (ibrs_enabled)						\
+		native_wrmsrl(MSR_IA32_SPEC_CTRL, val);			\
+ } while (0)
+
 #endif /* __ASSEMBLY__ */
 
 /*

arch/x86/include/asm/spec_ctrl.h

@@ -8,7 +8,7 @@
 
 #ifdef __ASSEMBLY__
 
-.extern use_ibrs
+.extern ibrs_enabled
 
 #define __ASM_ENABLE_IBRS			\
 	pushq %rax;				\
@@ -21,11 +21,13 @@
 	popq %rdx;				\
 	popq %rcx;				\
 	popq %rax
+
 #define __ASM_ENABLE_IBRS_CLOBBER		\
 	movl $MSR_IA32_SPEC_CTRL, %ecx;		\
 	movl $0, %edx;				\
 	movl $SPEC_CTRL_IBRS, %eax;		\
 	wrmsr;
+
 #define __ASM_DISABLE_IBRS			\
 	pushq %rax;				\
 	pushq %rcx;				\
@@ -39,7 +41,7 @@
 	popq %rax
 
 .macro ENABLE_IBRS
-	testl	$1, use_ibrs
+	testl	$1, ibrs_enabled
 	jz	10f
 	__ASM_ENABLE_IBRS
 	jmp 20f
@@ -49,7 +51,7 @@
 .endm
 
 .macro ENABLE_IBRS_CLOBBER
-	testl	$1, use_ibrs
+	testl	$1, ibrs_enabled
 	jz	11f
 	__ASM_ENABLE_IBRS_CLOBBER
 	jmp 21f
@@ -59,7 +61,7 @@
 .endm
 
 .macro DISABLE_IBRS
-	testl	$1, use_ibrs
+	testl	$1, ibrs_enabled
 	jz	9f
 	__ASM_DISABLE_IBRS
 9:

arch/x86/kernel/cpu/bugs.c

@@ -40,6 +40,16 @@ static int __init noibpb_handler(char *str)
 
 early_param("noibpb", noibpb_handler);
 
+unsigned int noibrs = 0;
+
+static int __init noibrs_handler(char *str)
+{
+	noibrs = 1;
+	return 0;
+}
+
+early_param("noibrs", noibrs_handler);
+
 static void __init spectre_v2_select_mitigation(void);
 static void __init ssb_select_mitigation(void);
 static void __init l1tf_select_mitigation(void);
@@ -410,18 +420,6 @@ static void __init spectre_v2_select_mitigation(void)
 			set_ibpb_enabled(1);   /* Enable IBPB */
 	}
 
-	/* Initialize Indirect Branch Restricted Speculation if supported */
-	if (boot_cpu_has(X86_FEATURE_IBRS)) {
-		pr_info("Spectre v2 mitigation: Enabling Indirect Branch Restricted Speculation\n");
-
-		set_ibrs_supported();
-		if (ibrs_inuse)
-			sysctl_ibrs_enabled = 1;
-        }
-
-	pr_info("Spectre v2 mitigation: Speculation control IBRS %s",
-	        ibrs_supported ? "supported" : "not-supported");
-
 	/*
 	 * If spectre v2 protection has been enabled, unconditionally fill
 	 * RSB during a context switch; this protects against two independent
@@ -433,21 +431,6 @@ static void __init spectre_v2_select_mitigation(void)
 	setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
 	pr_info("Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch\n");
 
-	/*
-	 * If we have a full retpoline mode and then disable IBPB in kernel mode
-	 * we do not require both.
-	 */
-	if (mode == SPECTRE_V2_RETPOLINE_AMD ||
-	    mode == SPECTRE_V2_RETPOLINE_GENERIC)
-	{
-		if (ibrs_supported) {
-			pr_info("Retpoline compiled kernel.  Defaulting IBRS to disabled");
-			set_ibrs_disabled();
-			if (!ibrs_inuse)
-				sysctl_ibrs_enabled = 0;
-		}
-	}
-
 	/*
 	 * Retpoline means the kernel is safe because it has no indirect
 	 * branches. Enhanced IBRS protects firmware too, so, enable restricted
@@ -462,6 +445,11 @@ static void __init spectre_v2_select_mitigation(void)
 	if (boot_cpu_has(X86_FEATURE_IBRS) && mode != SPECTRE_V2_IBRS_ENHANCED) {
 		setup_force_cpu_cap(X86_FEATURE_USE_IBRS_FW);
 		pr_info("Enabling Restricted Speculation for firmware calls\n");
+
+		if (!noibrs &&
+		    mode != SPECTRE_V2_RETPOLINE_GENERIC &&
+		    mode != SPECTRE_V2_RETPOLINE_AMD)
+			set_ibrs_enabled(1);   /* Enable IBRS */
 	}
 }
 
@@ -871,8 +859,9 @@ static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr
 		break;
 
 	case X86_BUG_SPECTRE_V2:
-		return sprintf(buf, "%s%s%s%s\n", spectre_v2_strings[spectre_v2_enabled],
+		return sprintf(buf, "%s%s%s%s%s\n", spectre_v2_strings[spectre_v2_enabled],
 			       ibpb_enabled ? ", IBPB" : "",
+			       ibrs_enabled == 2 ? ", IBRS (user space)" : ibrs_enabled ? ", IBRS" : "",
 			       boot_cpu_has(X86_FEATURE_USE_IBRS_FW) ? ", IBRS_FW" : "",
 			       spectre_v2_module_string());
 

arch/x86/kernel/cpu/microcode/core.c

@@ -439,17 +439,6 @@ static ssize_t reload_store(struct device *dev,
 	if (!ret)
 		perf_check_microcode();
 
-	/* Initialize Indirect Branch Restricted Speculation if supported */
-	if (boot_cpu_has(X86_FEATURE_IBRS)) {
-		pr_info("Enabling Indirect Branch Restricted Speculation\n");
-
-		mutex_lock(&spec_ctrl_mutex);
-		set_ibrs_supported();
-		if (ibrs_inuse)
-			sysctl_ibrs_enabled = 1;
-		mutex_unlock(&spec_ctrl_mutex);
-	}
-
 	mutex_unlock(&microcode_mutex);
 	put_online_cpus();
 

arch/x86/kernel/process.c

@@ -566,17 +566,15 @@ static void mwait_idle(void)
 			smp_mb(); /* quirk */
 		}
 
-		if (ibrs_inuse)
-                        native_wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+		ubuntu_restrict_branch_speculation_end();
 
 		__monitor((void *)&current_thread_info()->flags, 0, 0);
+
 		if (!need_resched()) {
 			__sti_mwait(0, 0);
-			if (ibrs_inuse)
-				native_wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base | SPEC_CTRL_IBRS);
+			ubuntu_restrict_branch_speculation_start();
 		} else {
-			if (ibrs_inuse)
-				native_wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base | SPEC_CTRL_IBRS);
+			ubuntu_restrict_branch_speculation_start();
 			local_irq_enable();
 		}
 		trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());

arch/x86/kernel/smpboot.c

@@ -1697,15 +1697,13 @@ void native_play_dead(void)
 	play_dead_common();
 	tboot_shutdown(TB_SHUTDOWN_WFS);
 
-	if (ibrs_inuse)
-		native_wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+	ubuntu_restrict_branch_speculation_end();
 
 	mwait_play_dead();	/* Only returns on failure */
 	if (cpuidle_play_dead())
 		hlt_play_dead();
 
-	if (ibrs_inuse)
-		native_wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base | SPEC_CTRL_IBRS);
+	ubuntu_restrict_branch_speculation_start();
 }
 
 #else /* ... !CONFIG_HOTPLUG_CPU */

arch/x86/lib/delay.c

@@ -107,8 +107,8 @@ static void delay_mwaitx(unsigned long __loops)
 	for (;;) {
 		delay = min_t(u64, MWAITX_MAX_LOOPS, loops);
 
-		if (ibrs_inuse && (delay > IBRS_DISABLE_THRESHOLD))
-			native_wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+		if (delay > IBRS_DISABLE_THRESHOLD)
+			ubuntu_restrict_branch_speculation_end();
 
 		/*
 		 * Use cpu_tss as a cacheline-aligned, seldomly
@@ -123,8 +123,8 @@ static void delay_mwaitx(unsigned long __loops)
 		 */
 		__mwaitx(MWAITX_DISABLE_CSTATES, delay, MWAITX_ECX_TIMER_ENABLE);
 
-		if (ibrs_inuse && (delay > IBRS_DISABLE_THRESHOLD))
-			native_wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base | SPEC_CTRL_IBRS);
+		if (delay > IBRS_DISABLE_THRESHOLD)
+			ubuntu_restrict_branch_speculation_start();
 
 		end = rdtsc_ordered();
 

include/linux/smp.h

@@ -50,52 +50,6 @@ void on_each_cpu_cond(bool (*cond_func)(int cpu, void *info),
 
 int smp_call_function_single_async(int cpu, struct call_single_data *csd);
 
-#ifdef CONFIG_X86
-
-#define IBxx_INUSE 	(1 << 0)
-#define IBxx_SUPPORTED 	(1 << 1)
-#define IBxx_DISABLED 	(1 << 2)
-
-/* indicate usage of IBRS to control execution speculation */
-extern int use_ibrs;
-extern u32 sysctl_ibrs_enabled;
-extern struct mutex spec_ctrl_mutex;
-
-static inline int __check_ibrs_inuse(void)
-{
-	if (use_ibrs & IBxx_INUSE)
-		return 1;
-	else
-		/* rmb to prevent wrong speculation for security */
-		rmb();
-	return 0;
-}
-
-#define ibrs_inuse 	(__check_ibrs_inuse())
-#define ibrs_supported	(use_ibrs & IBxx_SUPPORTED)
-#define ibrs_disabled	(use_ibrs & IBxx_DISABLED)
-
-static inline void set_ibrs_supported(void)
-{
-	use_ibrs |= IBxx_SUPPORTED;
-	if (!ibrs_disabled)
-		use_ibrs |= IBxx_INUSE;
-}
-static inline void set_ibrs_disabled(void)
-{
-	use_ibrs |= IBxx_DISABLED;
-	if (ibrs_inuse)
-		use_ibrs &= ~IBxx_INUSE;
-}
-static inline void clear_ibrs_disabled(void)
-{
-	use_ibrs &= ~IBxx_DISABLED;
-	if (ibrs_supported)
-		use_ibrs |= IBxx_INUSE;
-}
-
-#endif /* CONFIG_X86 */
-
 #ifdef CONFIG_SMP
 
 #include <linux/preempt.h>

kernel/smp.c

@@ -499,22 +499,6 @@ EXPORT_SYMBOL(smp_call_function);
 unsigned int setup_max_cpus = NR_CPUS;
 EXPORT_SYMBOL(setup_max_cpus);
 
-#ifdef CONFIG_X86
-/*
- * use IBRS
- * bit 0 = indicate if ibrs is currently in use
- * bit 1 = indicate if system supports ibrs
- * bit 2 = indicate if admin disables ibrs
-*/
-
-int use_ibrs;
-EXPORT_SYMBOL(use_ibrs);
-#endif
-
-/* mutex to serialize IBRS & IBPB control changes */
-DEFINE_MUTEX(spec_ctrl_mutex);
-EXPORT_SYMBOL(spec_ctrl_mutex);
-
 /*
  * Setup routine for controlling SMP activation
  *
@@ -538,18 +522,6 @@ static int __init nosmp(char *str)
 
 early_param("nosmp", nosmp);
 
-#ifdef CONFIG_X86
-static int __init noibrs(char *str)
-{
-	set_ibrs_disabled();
-
-	return 0;
-}
-
-early_param("noibrs", noibrs);
-#endif
-
-
 /* this is hard limit */
 static int __init nrcpus(char *str)
 {

kernel/sysctl.c

@@ -202,8 +202,8 @@ static int proc_dostring_coredump(struct ctl_table *table, int write,
 #endif
 
 #ifdef CONFIG_X86
-int proc_dointvec_ibrs_ctrl(struct ctl_table *table, int write,
-                 void __user *buffer, size_t *lenp, loff_t *ppos);
+/* Mutex to serialize IBPB and IBRS runtime control changes */
+DEFINE_MUTEX(spec_ctrl_mutex);
 
 unsigned int ibpb_enabled = 0;
 EXPORT_SYMBOL(ibpb_enabled);   /* Required in some modules */
@@ -252,6 +252,70 @@ static int ibpb_enabled_handler(struct ctl_table *table, int write,
 
 	return set_ibpb_enabled(__ibpb_enabled);
 }
+
+unsigned int ibrs_enabled = 0;
+EXPORT_SYMBOL(ibrs_enabled);   /* Required in some modules */
+
+static unsigned int __ibrs_enabled = 0;   /* procfs shadow variable */
+
+int set_ibrs_enabled(unsigned int val)
+{
+	int error = 0;
+	unsigned int cpu;
+	unsigned int prev = ibrs_enabled;
+
+	mutex_lock(&spec_ctrl_mutex);
+
+	/* Only enable/disable IBRS if the CPU supports it */
+	if (boot_cpu_has(X86_FEATURE_USE_IBRS_FW)) {
+		ibrs_enabled = val;
+		if (ibrs_enabled != prev)
+			pr_info("Spectre V2 : Spectre v2 mitigation: %s "
+				"Indirect Branch Restricted Speculation%s\n",
+				ibrs_enabled ? "Enabling" : "Disabling",
+				ibrs_enabled == 2 ? " (user space)" : "");
+
+		if (ibrs_enabled == 0) {
+			/* Always disable IBRS */
+			u64 val = x86_spec_ctrl_base;
+
+			for_each_online_cpu(cpu)
+				wrmsrl_on_cpu(cpu, MSR_IA32_SPEC_CTRL, val);
+		} else if (ibrs_enabled == 2) {
+			/* Always enable IBRS, even in user space */
+			u64 val = x86_spec_ctrl_base | SPEC_CTRL_IBRS;
+
+			for_each_online_cpu(cpu)
+				wrmsrl_on_cpu(cpu, MSR_IA32_SPEC_CTRL, val);
+		}
+	} else {
+		ibrs_enabled = 0;
+		if (val) {
+			/* IBRS is not supported but we try to turn it on */
+			error = -EINVAL;
+		}
+	}
+
+	/* Update the shadow variable */
+	__ibrs_enabled = ibrs_enabled;
+
+	mutex_unlock(&spec_ctrl_mutex);
+
+	return error;
+}
+
+static int ibrs_enabled_handler(struct ctl_table *table, int write,
+                               void __user *buffer, size_t *lenp,
+                               loff_t *ppos)
+{
+	int error;
+
+	error = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+	if (error)
+		return error;
+
+	return set_ibrs_enabled(__ibrs_enabled);
+}
 #endif
 
 #ifdef CONFIG_MAGIC_SYSRQ
@@ -290,9 +354,6 @@ extern struct ctl_table epoll_table[];
 int sysctl_legacy_va_layout;
 #endif
 
-u32 sysctl_ibrs_enabled = 0;
-EXPORT_SYMBOL(sysctl_ibrs_enabled);
-
 /* The default sysctl tables: */
 
 static struct ctl_table sysctl_base_table[] = {
@@ -1281,10 +1342,10 @@ static struct ctl_table kern_table[] = {
 #ifdef CONFIG_X86
 	{
 		.procname       = "ibrs_enabled",
-		.data           = &sysctl_ibrs_enabled,
+		.data           = &__ibrs_enabled,
 		.maxlen         = sizeof(unsigned int),
 		.mode           = 0644,
-		.proc_handler   = proc_dointvec_ibrs_ctrl,
+		.proc_handler   = ibrs_enabled_handler,
 		.extra1         = &zero,
 		.extra2         = &two,
 	},
@@ -2473,46 +2534,6 @@ int proc_dointvec_minmax(struct ctl_table *table, int write,
 				do_proc_dointvec_minmax_conv, &param);
 }
 
-#ifdef CONFIG_X86
-int proc_dointvec_ibrs_ctrl(struct ctl_table *table, int write,
-	void __user *buffer, size_t *lenp, loff_t *ppos)
-{
-	int ret;
-	unsigned int cpu;
-
-	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
-	pr_debug("sysctl_ibrs_enabled = %u\n", sysctl_ibrs_enabled);
-	pr_debug("before:use_ibrs = %d\n", use_ibrs);
-	mutex_lock(&spec_ctrl_mutex);
-	if (sysctl_ibrs_enabled == 0) {
-		/* always set IBRS off */
-		set_ibrs_disabled();
-		if (ibrs_supported) {
-			for_each_online_cpu(cpu)
-				wrmsrl_on_cpu(cpu, MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
-		}
-	} else if (sysctl_ibrs_enabled == 2) {
-		/* always set IBRS on, even in user space */
-		clear_ibrs_disabled();
-		if (ibrs_supported) {
-			for_each_online_cpu(cpu)
-				wrmsrl_on_cpu(cpu, MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base | SPEC_CTRL_IBRS);
-		} else {
-			sysctl_ibrs_enabled = 0;
-		}
-	} else if (sysctl_ibrs_enabled == 1) {
-		/* use IBRS in kernel */
-		clear_ibrs_disabled();
-		if (!ibrs_inuse)
-			/* platform don't support ibrs */
-			sysctl_ibrs_enabled = 0;
-	}
-	mutex_unlock(&spec_ctrl_mutex);
-	pr_debug("after:use_ibrs = %d\n", use_ibrs);
-	return ret;
-}
-#endif
-
 static void validate_coredump_safety(void)
 {
 #ifdef CONFIG_COREDUMP