x86/srso: Tie SBPB bit setting to microcode patch detection

The SBPB bit in MSR_IA32_PRED_CMD is supported only after a microcode
patch has been applied so set X86_FEATURE_SBPB only then. Otherwise,
guests would attempt to set that bit and #GP on the MSR write.

While at it, make SMT detection more robust as some guests - depending
on how and what CPUID leafs their report - lead to cpu_smt_control
getting set to CPU_SMT_NOT_SUPPORTED but SRSO_NO should be set for any
guest incarnation where one simply cannot do SMT, for whatever reason.

Fixes: fb3bd91 ("x86/srso: Add a Speculative RAS Overflow mitigation")
Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reported-by: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>

arch/x86/kernel/cpu/amd.c

@@ -1238,14 +1238,19 @@ EXPORT_SYMBOL_GPL(amd_get_highest_perf);
 
 bool cpu_has_ibpb_brtype_microcode(void)
 {
-	u8 fam = boot_cpu_data.x86;
-
+	switch (boot_cpu_data.x86) {
 	/* Zen1/2 IBPB flushes branch type predictions too. */
-	if (fam == 0x17)
+	case 0x17:
 		return boot_cpu_has(X86_FEATURE_AMD_IBPB);
-	/* Poke the MSR bit on Zen3/4 to check its presence. */
-	else if (fam == 0x19)
-		return !wrmsrl_safe(MSR_IA32_PRED_CMD, PRED_CMD_SBPB);
-	else
+	case 0x19:
+		/* Poke the MSR bit on Zen3/4 to check its presence. */
+		if (!wrmsrl_safe(MSR_IA32_PRED_CMD, PRED_CMD_SBPB)) {
+			setup_force_cpu_cap(X86_FEATURE_SBPB);
+			return true;
+		} else {
+			return false;
+		}
+	default:
 		return false;
+	}
 }

arch/x86/kernel/cpu/bugs.c

@@ -2265,14 +2265,13 @@ static void __init srso_select_mitigation(void)
 		 * flags for guests.
 		 */
 		setup_force_cpu_cap(X86_FEATURE_IBPB_BRTYPE);
-		setup_force_cpu_cap(X86_FEATURE_SBPB);
 
 		/*
 		 * Zen1/2 with SMT off aren't vulnerable after the right
 		 * IBPB microcode has been applied.
 		 */
 		if ((boot_cpu_data.x86 < 0x19) &&
-		    (cpu_smt_control == CPU_SMT_DISABLED))
+		    (!cpu_smt_possible() || (cpu_smt_control == CPU_SMT_DISABLED)))
 			setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
 	}
 
@@ -2345,8 +2344,8 @@ static void __init srso_select_mitigation(void)
 	pr_info("%s%s\n", srso_strings[srso_mitigation], (has_microcode ? "" : ", no microcode"));
 
 pred_cmd:
-	if (boot_cpu_has(X86_FEATURE_SRSO_NO) ||
-	    srso_cmd == SRSO_CMD_OFF)
+	if ((boot_cpu_has(X86_FEATURE_SRSO_NO) || srso_cmd == SRSO_CMD_OFF) &&
+	     boot_cpu_has(X86_FEATURE_SBPB))
 		x86_pred_cmd = PRED_CMD_SBPB;
 }