Re: [PATCH final?] x86/bugs: KVM: Add support for SRSO_MSR_FIX

Patrick Bellasi
2025-02-18T14:43:15Z

> in the interest of finally making some progress here I'd like to commit this
> below (will test it one more time just in case but it should work :-P). It is
> simple and straight-forward and doesn't need an IBPB when the bit gets
> cleared.

That's indeed simple and straight-forward for the time being.

Maybe a small improvement we could add on top is to have a separate and
dedicated cmdline option?

Indeed, with `X86_FEATURE_SRSO_USER_KERNEL_NO` we are not effectively using an
IBPB on VM-Exit anymore. Something like the diff down below?

Best,
Patrick

---
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 1d7afc40f2272..7609d80eda123 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2531,6 +2531,7 @@ enum srso_mitigation_cmd {
SRSO_CMD_SAFE_RET,
SRSO_CMD_IBPB,
SRSO_CMD_IBPB_ON_VMEXIT,
+ SRSO_CMD_BP_SPEC_REDUCE,
};

static const char * const srso_strings[] = {
@@ -2562,6 +2563,8 @@ static int __init srso_parse_cmdline(char *str)
srso_cmd = SRSO_CMD_IBPB;
else if (!strcmp(str, "ibpb-vmexit"))
srso_cmd = SRSO_CMD_IBPB_ON_VMEXIT;
+ else if (!strcmp(str, "spec-reduce"))
+ srso_cmd = SRSO_CMD_BP_SPEC_REDUCE;
else
pr_err("Ignoring unknown SRSO option (%s).", str);

@@ -2617,7 +2620,7 @@ static void __init srso_select_mitigation(void)

case SRSO_CMD_SAFE_RET:
if (boot_cpu_has(X86_FEATURE_SRSO_USER_KERNEL_NO))
- goto ibpb_on_vmexit;
+ goto spec_reduce;

if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
/*
@@ -2670,14 +2673,7 @@ static void __init srso_select_mitigation(void)
}
break;

-ibpb_on_vmexit:
case SRSO_CMD_IBPB_ON_VMEXIT:
- if (boot_cpu_has(X86_FEATURE_SRSO_BP_SPEC_REDUCE)) {
- pr_notice("Reducing speculation to address VM/HV SRSO attack vector.\n");
- srso_mitigation = SRSO_MITIGATION_BP_SPEC_REDUCE;
- break;
- }
-
if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
if (has_microcode) {
setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
@@ -2694,6 +2690,14 @@ static void __init srso_select_mitigation(void)
pr_err("WARNING: kernel not compiled with MITIGATION_IBPB_ENTRY.\n");
}
break;
+
+spec_reduce:
+ case SRSO_CMD_BP_SPEC_REDUCE:
+ if (boot_cpu_has(X86_FEATURE_SRSO_BP_SPEC_REDUCE)) {
+ pr_notice("Reducing speculation to address VM/HV SRSO attack vector.\n");
+ srso_mitigation = SRSO_MITIGATION_BP_SPEC_REDUCE;
+ break;
+ }
default:
break;
}