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

[Patrick Bellasi <derkling@google.com>]

> 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;
        }