[PATCH 4.19 041/361] x86/speculation: Enable cross-hyperthread spectre v2 STIBP mitigation

Greg Kroah-Hartman
2018-11-12T00:17:32Z

From: Jiri Kosina <jkosina@suse.cz>

commit 53c613fe6349994f023245519265999eed75957f 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>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/nycvar.YFH.7.76.1809251438240.15880@cbobk.fhfr.pm
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/kernel/cpu/bugs.c | 57 ++++++++++++++++++++++++++++++++++++++++-----
kernel/cpu.c | 11 +++++++-
2 files changed, 61 insertions(+), 7 deletions(-)

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -35,12 +35,10 @@ static void __init spectre_v2_select_mit
static void __init ssb_select_mitigation(void);
static void __init l1tf_select_mitigation(void);

-/*
- * Our boot-time value of the SPEC_CTRL MSR. We read it once so that any
- * writes to SPEC_CTRL contain whatever reserved bits have been set.
- */
-u64 __ro_after_init x86_spec_ctrl_base;
+/* The base value of the SPEC_CTRL MSR that always has to be preserved. */
+u64 x86_spec_ctrl_base;
EXPORT_SYMBOL_GPL(x86_spec_ctrl_base);
+static DEFINE_MUTEX(spec_ctrl_mutex);

/*
* The vendor and possibly platform specific bits which can be modified in
@@ -325,6 +323,46 @@ static enum spectre_v2_mitigation_cmd __
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);
+}
+
+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);
+ }
+ mutex_unlock(&spec_ctrl_mutex);
+}
+
static void __init spectre_v2_select_mitigation(void)
{
enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
@@ -424,6 +462,9 @@ specv2_set_mode:
setup_force_cpu_cap(X86_FEATURE_USE_IBRS_FW);
pr_info("Enabling Restricted Speculation for firmware calls\n");
}
+
+ /* Enable STIBP if appropriate */
+ arch_smt_update();
}

#undef pr_fmt
@@ -814,6 +855,8 @@ static ssize_t l1tf_show_state(char *buf
static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr,
char *buf, unsigned int bug)
{
+ int ret;
+
if (!boot_cpu_has_bug(bug))
return sprintf(buf, "Not affected\n");

@@ -831,10 +874,12 @@ static ssize_t cpu_show_common(struct de
return sprintf(buf, "Mitigation: __user pointer sanitization\n");

case X86_BUG_SPECTRE_V2:
- return sprintf(buf, "%s%s%s%s\n", spectre_v2_strings[spectre_v2_enabled],
+ ret = sprintf(buf, "%s%s%s%s%s\n", spectre_v2_strings[spectre_v2_enabled],
boot_cpu_has(X86_FEATURE_USE_IBPB) ? ", IBPB" : "",
boot_cpu_has(X86_FEATURE_USE_IBRS_FW) ? ", IBRS_FW" : "",
+ (x86_spec_ctrl_base & SPEC_CTRL_STIBP) ? ", STIBP" : "",
spectre_v2_module_string());
+ return ret;

case X86_BUG_SPEC_STORE_BYPASS:
return sprintf(buf, "%s\n", ssb_strings[ssb_mode]);
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2026,6 +2026,12 @@ static void cpuhp_online_cpu_device(unsi
kobject_uevent(&dev->kobj, KOBJ_ONLINE);
}

+/*
+ * Architectures that need SMT-specific errata handling during SMT hotplug
+ * should override this.
+ */
+void __weak arch_smt_update(void) { };
+
static int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
{
int cpu, ret = 0;
@@ -2052,8 +2058,10 @@ static int cpuhp_smt_disable(enum cpuhp_
*/
cpuhp_offline_cpu_device(cpu);
}
- if (!ret)
+ if (!ret) {
cpu_smt_control = ctrlval;
+ arch_smt_update();
+ }
cpu_maps_update_done();
return ret;
}
@@ -2064,6 +2072,7 @@ static int cpuhp_smt_enable(void)

cpu_maps_update_begin();
cpu_smt_control = CPU_SMT_ENABLED;
+ arch_smt_update();
for_each_present_cpu(cpu) {
/* Skip online CPUs and CPUs on offline nodes */
if (cpu_online(cpu) || !node_online(cpu_to_node(cpu)))