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

[Sean Christopherson <seanjc@google.com>]

On Sat, Jan 18, 2025, Borislav Petkov wrote:
>  static enum srso_mitigation srso_mitigation __ro_after_init = SRSO_MITIGATION_NONE;
> @@ -2663,6 +2665,12 @@ static void __init srso_select_mitigation(void)

Unless I'm missing something, the cpu_mitigations_off() and "srso_cmd == SRSO_CMD_OFF"
cases need to clear the feature

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 9e3ea7f1b3587..3939a8dee27d4 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2581,6 +2581,7 @@ static void __init srso_select_mitigation(void)
            srso_cmd == SRSO_CMD_OFF) {
                if (boot_cpu_has(X86_FEATURE_SBPB))
                        x86_pred_cmd = PRED_CMD_SBPB;
+               setup_clear_cpu_cap(X86_FEATURE_SRSO_BP_SPEC_REDUCE);
                return;
        }

There's also the Zen1/Zen2 ucode+!SMT path, which I assume is irreveleant in
practice:

		if (boot_cpu_data.x86 < 0x19 && !cpu_smt_possible()) {
			setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
			return;
		}

But if we wanted to catch all paths, wrap the guts and clear the feature in the
outer layer?

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 9e3ea7f1b3587..0501e31971421 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2572,7 +2572,7 @@ early_param("spec_rstack_overflow", srso_parse_cmdline);
 
 #define SRSO_NOTICE "WARNING: See https://kernel.org/doc/html/latest/admin-guide/hw-vuln/srso.html for mitigation options."
 
-static void __init srso_select_mitigation(void)
+static void __init __srso_select_mitigation(void)
 {
        bool has_microcode = boot_cpu_has(X86_FEATURE_IBPB_BRTYPE);
 
@@ -2692,11 +2692,15 @@ static void __init srso_select_mitigation(void)
        }
 
 out:
+       pr_info("%s\n", srso_strings[srso_mitigation]);
+}
+
+static void __init srso_select_mitigation(void)
+{
+       __srso_select_mitigation();
 
        if (srso_mitigation != SRSO_MITIGATION_BP_SPEC_REDUCE)
                setup_clear_cpu_cap(X86_FEATURE_SRSO_BP_SPEC_REDUCE);
-
-       pr_info("%s\n", srso_strings[srso_mitigation]);
 }
 
 #undef pr_fmt

>  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_SRSO)) {
>  			if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
>  				setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
> @@ -2684,6 +2692,10 @@ static void __init srso_select_mitigation(void)
>  	}
>  
>  out:
> +

Spurious newlines.

> +	if (srso_mitigation != SRSO_MITIGATION_BP_SPEC_REDUCE)
> +		setup_clear_cpu_cap(X86_FEATURE_SRSO_BP_SPEC_REDUCE);
> +
>  	pr_info("%s\n", srso_strings[srso_mitigation]);
>  }