x86/speculation: Enable cross-hyperthread spectre v2 STIBP mitigation

BugLink: https://bugs.launchpad.net/bugs/1830176

commit 53c613fe upstream.

STIBP is a feature provided by certain Intel ucodes / CPUs. This feature
(once enabled) prevents cross-hyperthread control of decisions made by
indirect branch predictors.

Enable this feature if

- the CPU is vulnerable to spectre v2
- the CPU supports SMT and has SMT siblings online
- spectre_v2 mitigation autoselection is enabled (default)

After some previous discussion, this leaves STIBP on all the time, as wrmsr
on crossing kernel boundary is a no-no. This could perhaps later be a bit
more optimized (like disabling it in NOHZ, experiment with disabling it in
idle, etc) if needed.

Note that the synchronization of the mask manipulation via newly added
spec_ctrl_mutex is currently not strictly needed, as the only updater is
already being serialized by cpu_add_remove_lock, but let's make this a
little bit more future-proof.
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc:  "WoodhouseDavid" <dwmw@amazon.co.uk>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc:  "SchauflerCasey" <casey.schaufler@intel.com>
Link: https://lkml.kernel.org/r/nycvar.YFH.7.76.1809251438240.15880@cbobk.fhfr.pm
[bwh: Backported to 4.4:
 - Don't add any calls to arch_smt_update() yet. They will be introduced by
   "x86/speculation: Rework SMT state change".
 - Use IS_ENABLED(CONFIG_SMP) instead of cpu_smt_control for now. This
   will be fixed by "x86/speculation: Rework SMT state change".]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[juergh:
 - Adjusted context.
 - Used cpu_smt_control which is available in Xenial.
 - Adjusted for already present MDS, spec_v2 and SMT code reordering commits.]
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/x86/kernel/cpu/bugs.c

@@ -407,6 +407,22 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 	return cmd;
 }
 
+static bool stibp_needed(void)
+{
+	if (spectre_v2_enabled == SPECTRE_V2_NONE)
+		return false;
+
+	if (!boot_cpu_has(X86_FEATURE_STIBP))
+		return false;
+
+	return true;
+}
+
+static void update_stibp_msr(void *info)
+{
+	wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+}
+
 static void __init spectre_v2_select_mitigation(void)
 {
 	enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
@@ -544,8 +560,27 @@ static void update_mds_branch_idle(void)
 
 void arch_smt_update(void)
 {
+	u64 mask;
+
+	if (!stibp_needed())
+		return;
+
 	mutex_lock(&spec_ctrl_mutex);
 
+	mask = x86_spec_ctrl_base;
+	if (cpu_smt_control == CPU_SMT_ENABLED)
+		mask |= SPEC_CTRL_STIBP;
+	else
+		mask &= ~SPEC_CTRL_STIBP;
+
+	if (mask != x86_spec_ctrl_base) {
+		pr_info("Spectre v2 cross-process SMT mitigation: %s STIBP\n",
+				cpu_smt_control == CPU_SMT_ENABLED ?
+				"Enabling" : "Disabling");
+		x86_spec_ctrl_base = mask;
+		on_each_cpu(update_stibp_msr, NULL, 1);
+	}
+
 	switch (mds_mitigation) {
 	case MDS_MITIGATION_FULL:
 	case MDS_MITIGATION_VMWERV:
@@ -980,17 +1015,17 @@ static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr
 	case X86_BUG_CPU_MELTDOWN:
 		if (boot_cpu_has(X86_FEATURE_KAISER))
 			return sprintf(buf, "Mitigation: PTI\n");
-
 		break;
 
 	case X86_BUG_SPECTRE_V1:
 		return sprintf(buf, "Mitigation: __user pointer sanitization\n");
 
 	case X86_BUG_SPECTRE_V2:
-		return sprintf(buf, "%s%s%s%s%s\n", spectre_v2_strings[spectre_v2_enabled],
+		return sprintf(buf, "%s%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" : "",
+			       (x86_spec_ctrl_base & SPEC_CTRL_STIBP) ? ", STIBP" : "",
 			       spectre_v2_module_string());
 
 	case X86_BUG_SPEC_STORE_BYPASS: