x86/bugs: Add attack vector controls for retbleed

Use attack vector controls to determine if retbleed mitigation is
required.

Disable SMT if cross-thread protection is desired and STIBP is not
available.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250707183316.1349127-13-david.kaplan@amd.com

Author: dkaplan2

arch/x86/kernel/cpu/bugs.c

@@ -1313,7 +1313,7 @@ early_param("retbleed", retbleed_parse_cmdline);
 
 static void __init retbleed_select_mitigation(void)
 {
-	if (!boot_cpu_has_bug(X86_BUG_RETBLEED) || cpu_mitigations_off()) {
+	if (!boot_cpu_has_bug(X86_BUG_RETBLEED)) {
 		retbleed_mitigation = RETBLEED_MITIGATION_NONE;
 		return;
 	}
@@ -1350,6 +1350,11 @@ static void __init retbleed_select_mitigation(void)
 	if (retbleed_mitigation != RETBLEED_MITIGATION_AUTO)
 		return;
 
+	if (!should_mitigate_vuln(X86_BUG_RETBLEED)) {
+		retbleed_mitigation = RETBLEED_MITIGATION_NONE;
+		return;
+	}
+
 	/* Intel mitigation selected in retbleed_update_mitigation() */
 	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
 	    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON) {
@@ -1373,7 +1378,7 @@ static void __init retbleed_select_mitigation(void)
 
 static void __init retbleed_update_mitigation(void)
 {
-	if (!boot_cpu_has_bug(X86_BUG_RETBLEED) || cpu_mitigations_off())
+	if (!boot_cpu_has_bug(X86_BUG_RETBLEED))
 		return;
 
 	 /* ITS can also enable stuffing */
@@ -1468,7 +1473,7 @@ static void __init retbleed_apply_mitigation(void)
 	}
 
 	if (mitigate_smt && !boot_cpu_has(X86_FEATURE_STIBP) &&
-	    (retbleed_nosmt || cpu_mitigations_auto_nosmt()))
+	    (retbleed_nosmt || smt_mitigations == SMT_MITIGATIONS_ON))
 		cpu_smt_disable(false);
 }