From: Jiri Kosina <jkosina@xxxxxxx> 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@xxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: "WoodhouseDavid" <dwmw@xxxxxxxxxxxx> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> Cc: "SchauflerCasey" <casey.schaufler@xxxxxxxxx> Link: https://lkml.kernel.org/r/nycvar.YFH.7.76.1809251438240.15880@xxxxxxxxxxxxx [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@xxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/x86/kernel/cpu/bugs.c | 55 ++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 50 insertions(+), 5 deletions(-) --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -32,12 +32,10 @@ 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. - */ +/* 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 @@ -315,6 +313,46 @@ 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 (IS_ENABLED(CONFIG_SMP)) + 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", + IS_ENABLED(CONFIG_SMP) ? + "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(); @@ -414,6 +452,9 @@ 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 @@ -722,6 +763,8 @@ static void __init l1tf_select_mitigatio 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"); @@ -736,10 +779,12 @@ 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]);
