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Subject[PATCH] x86/speculation: Enable cross-hyperthread spectre v2 STIBP mitigation
From: Jiri Kosina <jkosina@suse.cz>

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@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
---

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
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