Now that all prerequisites are in place:
- Add the prctl command line option
- Default the 'auto' mode to 'prctl'
- When SMT state changes, update the static key which controls the
conditional STIBP evaluation on context switch.
- At init update the static key which controls the conditional IBPB
evaluation on context switch.
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
---
Documentation/admin-guide/kernel-parameters.txt | 5 ++
arch/x86/kernel/cpu/bugs.c | 46 +++++++++++++++++++++---
2 files changed, 45 insertions(+), 6 deletions(-)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4246,7 +4246,10 @@
by spectre_v2=off
auto - Kernel selects the mitigation depending on
the available CPU features and vulnerability.
- Default is off.
+ Default is prctl.
+ prctl - Indirect branch speculation is enabled, but
+ mitigation can be enabled via prctl per
thread.
+ The mitigation control state is inherited on
fork.
Not specifying this option is equivalent to
spectre_v2_app2app=auto.
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -255,11 +255,13 @@ enum spectre_v2_app2app_cmd {
SPECTRE_V2_APP2APP_CMD_NONE,
SPECTRE_V2_APP2APP_CMD_AUTO,
SPECTRE_V2_APP2APP_CMD_FORCE,
+ SPECTRE_V2_APP2APP_CMD_PRCTL,
};
static const char *spectre_v2_app2app_strings[] = {
[SPECTRE_V2_APP2APP_NONE] = "App-App Vulnerable",
[SPECTRE_V2_APP2APP_STRICT] = "App-App Mitigation: STIBP
protection",
+ [SPECTRE_V2_APP2APP_PRCTL] = "App-App Mitigation: STIBP via prctl",
};
static const struct {
@@ -270,6 +272,7 @@ static const struct {
{ "auto", SPECTRE_V2_APP2APP_CMD_AUTO, false },
{ "off", SPECTRE_V2_APP2APP_CMD_NONE, false },
{ "on", SPECTRE_V2_APP2APP_CMD_FORCE, true },
+ { "prctl", SPECTRE_V2_APP2APP_CMD_PRCTL, false },
};
static void __init spec_v2_app_print_cond(const char *reason, bool secure)
@@ -324,12 +327,15 @@ spectre_v2_app2app_select_mitigation(enu
smt_possible = false;
switch (spectre_v2_parse_app2app_cmdline(v2_cmd)) {
- case SPECTRE_V2_APP2APP_CMD_AUTO:
case SPECTRE_V2_APP2APP_CMD_NONE:
goto set_mode;
case SPECTRE_V2_APP2APP_CMD_FORCE:
mode = SPECTRE_V2_APP2APP_STRICT;
break;
+ case SPECTRE_V2_APP2APP_CMD_AUTO:
+ case SPECTRE_V2_APP2APP_CMD_PRCTL:
+ mode = SPECTRE_V2_APP2APP_PRCTL;
+ break;
}
/* Initialize Indirect Branch Prediction Barrier */
@@ -340,6 +346,9 @@ spectre_v2_app2app_select_mitigation(enu
case SPECTRE_V2_APP2APP_STRICT:
static_branch_enable(&switch_to_always_ibpb);
break;
+ case SPECTRE_V2_APP2APP_PRCTL:
+ static_branch_enable(&switch_to_cond_ibpb);
+ break;
default:
break;
}
@@ -352,6 +361,12 @@ spectre_v2_app2app_select_mitigation(enu
if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
return;
+ /*
+ * If STIBP is not available or SMT is not possible clear the STIPB
+ * mode.
+ */
+ if (!smt_possible || !boot_cpu_has(X86_FEATURE_STIBP))
+ mode = SPECTRE_V2_APP2APP_NONE;
set_mode:
spectre_v2_app2app = mode;
/* Only print the STIBP mode when SMT possible */
@@ -552,6 +567,18 @@ static void update_stibp_strict(void)
on_each_cpu(update_stibp_msr, NULL, 1);
}
+/* Update the static key controlling the evaluation of TIF_SPEC_IB */
+static void update_indir_branch_cond(void)
+{
+ if (!IS_ENABLED(CONFIG_SMP))
+ return;
+
+ if (sched_smt_active())
+ static_branch_enable(&switch_to_cond_stibp);
+ else
+ static_branch_disable(&switch_to_cond_stibp);
+}
+
void arch_smt_update(void)
{
/* Enhanced IBRS makes using STIBP unnecessary. No update required. */
@@ -567,6 +594,7 @@ void arch_smt_update(void)
update_stibp_strict();
break;
case SPECTRE_V2_APP2APP_PRCTL:
+ update_indir_branch_cond();
break;
}
@@ -1044,17 +1072,25 @@ static char *stibp_state(void)
case SPECTRE_V2_APP2APP_STRICT:
return ", STIBP: forced";
case SPECTRE_V2_APP2APP_PRCTL:
- return "";
+ return ", STIBP: opt-in";
}
return "";
}
static char *ibpb_state(void)
{
- if (boot_cpu_has(X86_FEATURE_USE_IBPB))
- return ", IBPB";
- else
+ if (!boot_cpu_has(X86_FEATURE_IBPB))
return "";
+
+ switch (spectre_v2_app2app) {
+ case SPECTRE_V2_APP2APP_NONE:
+ return ", IBPB: disabled";
+ case SPECTRE_V2_APP2APP_STRICT:
+ return ", IBPB: forced";
+ case SPECTRE_V2_APP2APP_PRCTL:
+ return ", IBBP: opt-in";
+ }
+ return "";
}
static ssize_t cpu_show_common(struct device *dev, struct device_attribute
*attr,
