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

[Jiri Kosina]

From: Jiri Kosina <jkosina@xxxxxxx>

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
- spectre_v2 mitigation autoselection is enabled (default)

After some previous discussion, this patch 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.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
---

Let's add the most basic STIBP support, as it has been kind of lost in all 
the previous noise.

 arch/x86/kernel/cpu/bugs.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 4c2313d0b9ca..02edf8a6ced7 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -325,6 +325,12 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
 	return cmd;
 }
 
+static bool __init stibp_needed(void)
+{
+	return (cpu_smt_control != CPU_SMT_NOT_SUPPORTED &&
+			boot_cpu_has(X86_FEATURE_STIBP));
+}
+
 static void __init spectre_v2_select_mitigation(void)
 {
 	enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
@@ -344,6 +350,12 @@ static void __init spectre_v2_select_mitigation(void)
 
 	case SPECTRE_V2_CMD_FORCE:
 	case SPECTRE_V2_CMD_AUTO:
+		if (stibp_needed()) {
+			/* Enable STIBP on SMT-capable systems */
+			pr_info("Spectre v2 cross-process SMT mitigation: Enabling STIBP\n");
+			x86_spec_ctrl_base |= SPEC_CTRL_STIBP;
+			wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+		}
 		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
 			mode = SPECTRE_V2_IBRS_ENHANCED;
 			/* Force it so VMEXIT will restore correctly */
-- 
Jiri Kosina
SUSE Labs