x86/srso: Add SRSO_NO support

Add support for the CPUID flag which denotes that the CPU is not
affected by SRSO.
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>

arch/x86/include/asm/cpufeatures.h

@@ -445,7 +445,9 @@
 #define X86_FEATURE_AUTOIBRS		(20*32+ 8) /* "" Automatic IBRS */
 #define X86_FEATURE_NO_SMM_CTL_MSR	(20*32+ 9) /* "" SMM_CTL MSR is not present */
 
+#define X86_FEATURE_SBPB		(20*32+27) /* "" Selective Branch Prediction Barrier */
 #define X86_FEATURE_IBPB_BRTYPE		(20*32+28) /* "" MSR_PRED_CMD[IBPB] flushes all branch type predictions */
+#define X86_FEATURE_SRSO_NO		(20*32+29) /* "" CPU is not affected by SRSO */
 
 /*
  * BUG word(s)

arch/x86/include/asm/msr-index.h

@@ -57,6 +57,7 @@
 
 #define MSR_IA32_PRED_CMD		0x00000049 /* Prediction Command */
 #define PRED_CMD_IBPB			BIT(0)	   /* Indirect Branch Prediction Barrier */
+#define PRED_CMD_SBPB			BIT(7)	   /* Selective Branch Prediction Barrier */
 
 #define MSR_PPIN_CTL			0x0000004e
 #define MSR_PPIN			0x0000004f

arch/x86/include/asm/nospec-branch.h

@@ -492,11 +492,11 @@ void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
 		: "memory");
 }
 
+extern u64 x86_pred_cmd;
+
 static inline void indirect_branch_prediction_barrier(void)
 {
-	u64 val = PRED_CMD_IBPB;
-
-	alternative_msr_write(MSR_IA32_PRED_CMD, val, X86_FEATURE_USE_IBPB);
+	alternative_msr_write(MSR_IA32_PRED_CMD, x86_pred_cmd, X86_FEATURE_USE_IBPB);
 }
 
 /* The Intel SPEC CTRL MSR base value cache */

arch/x86/kernel/cpu/amd.c

@@ -1240,12 +1240,12 @@ bool cpu_has_ibpb_brtype_microcode(void)
 {
 	u8 fam = boot_cpu_data.x86;
 
-	if (fam == 0x17) {
-		/* Zen1/2 IBPB flushes branch type predictions too. */
+	/* Zen1/2 IBPB flushes branch type predictions too. */
+	if (fam == 0x17)
 		return boot_cpu_has(X86_FEATURE_AMD_IBPB);
-	} else if (fam == 0x19) {
+	/* 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
 		return false;
-	}
-
-	return false;
 }

arch/x86/kernel/cpu/bugs.c

@@ -57,6 +57,9 @@ EXPORT_SYMBOL_GPL(x86_spec_ctrl_base);
 DEFINE_PER_CPU(u64, x86_spec_ctrl_current);
 EXPORT_SYMBOL_GPL(x86_spec_ctrl_current);
 
+u64 x86_pred_cmd __ro_after_init = PRED_CMD_IBPB;
+EXPORT_SYMBOL_GPL(x86_pred_cmd);
+
 static DEFINE_MUTEX(spec_ctrl_mutex);
 
 /* Update SPEC_CTRL MSR and its cached copy unconditionally */
@@ -2236,7 +2239,7 @@ static void __init srso_select_mitigation(void)
 	bool has_microcode;
 
 	if (!boot_cpu_has_bug(X86_BUG_SRSO) || cpu_mitigations_off())
-		return;
+		goto pred_cmd;
 
 	/*
 	 * The first check is for the kernel running as a guest in order
@@ -2249,9 +2252,18 @@ static void __init srso_select_mitigation(void)
 	} else {
 		/*
 		 * Enable the synthetic (even if in a real CPUID leaf)
-		 * flag for guests.
+		 * 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))
+			setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
 	}
 
 	switch (srso_cmd) {
@@ -2274,16 +2286,20 @@ static void __init srso_select_mitigation(void)
 			srso_mitigation = SRSO_MITIGATION_SAFE_RET;
 		} else {
 			pr_err("WARNING: kernel not compiled with CPU_SRSO.\n");
-			return;
+			goto pred_cmd;
 		}
 		break;
 
 	default:
 		break;
-
 	}
 
 	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)
+		x86_pred_cmd = PRED_CMD_SBPB;
 }
 
 #undef pr_fmt

arch/x86/kernel/cpu/common.c

@@ -1409,8 +1409,10 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 	if (cpu_matches(cpu_vuln_blacklist, SMT_RSB))
 		setup_force_cpu_bug(X86_BUG_SMT_RSB);
 
-	if (cpu_matches(cpu_vuln_blacklist, SRSO))
-		setup_force_cpu_bug(X86_BUG_SRSO);
+	if (!cpu_has(c, X86_FEATURE_SRSO_NO)) {
+		if (cpu_matches(cpu_vuln_blacklist, SRSO))
+			setup_force_cpu_bug(X86_BUG_SRSO);
+	}
 
 	if (cpu_matches(cpu_vuln_whitelist, NO_MELTDOWN))
 		return;

arch/x86/kvm/cpuid.c

@@ -729,6 +729,9 @@ void kvm_set_cpu_caps(void)
 		F(NULL_SEL_CLR_BASE) | F(AUTOIBRS) | 0 /* PrefetchCtlMsr */
 	);
 
+	if (cpu_feature_enabled(X86_FEATURE_SRSO_NO))
+		kvm_cpu_cap_set(X86_FEATURE_SRSO_NO);
+
 	kvm_cpu_cap_init_kvm_defined(CPUID_8000_0022_EAX,
 		F(PERFMON_V2)
 	);