x86/bugs: Add attack vector controls for MDS

Use attack vector controls to determine if MDS mitigation is required.
The global mitigations=off command now simply disables all attack vectors
so explicit checking of mitigations=off is no longer needed.

If cross-thread attack mitigations are required, disable SMT.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250707183316.1349127-6-david.kaplan@amd.com

Author: dkaplan2

arch/x86/kernel/cpu/bugs.c

@@ -441,13 +441,17 @@ static bool verw_clear_cpu_buf_mitigation_selected __ro_after_init;
 
 static void __init mds_select_mitigation(void)
 {
-	if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
+	if (!boot_cpu_has_bug(X86_BUG_MDS)) {
 		mds_mitigation = MDS_MITIGATION_OFF;
 		return;
 	}
 
-	if (mds_mitigation == MDS_MITIGATION_AUTO)
-		mds_mitigation = MDS_MITIGATION_FULL;
+	if (mds_mitigation == MDS_MITIGATION_AUTO) {
+		if (should_mitigate_vuln(X86_BUG_MDS))
+			mds_mitigation = MDS_MITIGATION_FULL;
+		else
+			mds_mitigation = MDS_MITIGATION_OFF;
+	}
 
 	if (mds_mitigation == MDS_MITIGATION_OFF)
 		return;
@@ -457,7 +461,7 @@ static void __init mds_select_mitigation(void)
 
 static void __init mds_update_mitigation(void)
 {
-	if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off())
+	if (!boot_cpu_has_bug(X86_BUG_MDS))
 		return;
 
 	/* If TAA, MMIO, or RFDS are being mitigated, MDS gets mitigated too. */
@@ -478,7 +482,7 @@ static void __init mds_apply_mitigation(void)
 	    mds_mitigation == MDS_MITIGATION_VMWERV) {
 		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
 		if (!boot_cpu_has(X86_BUG_MSBDS_ONLY) &&
-		    (mds_nosmt || cpu_mitigations_auto_nosmt()))
+		    (mds_nosmt || smt_mitigations == SMT_MITIGATIONS_ON))
 			cpu_smt_disable(false);
 	}
 }