x86/bugs: Fix BHI handling of RRSBA

The ARCH_CAP_RRSBA check isn't correct: RRSBA may have already been
disabled by the Spectre v2 mitigation (or can otherwise be disabled by
the BHI mitigation itself if needed).  In that case retpolines are fine.

Fixes: ec9404e4 ("x86/bhi: Add BHI mitigation knob")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/6f56f13da34a0834b69163467449be7f58f253dc.1712813475.git.jpoimboe@kernel.org

arch/x86/kernel/cpu/bugs.c

@@ -1538,20 +1538,25 @@ static enum spectre_v2_mitigation __init spectre_v2_select_retpoline(void)
 	return SPECTRE_V2_RETPOLINE;
 }
 
+static bool __ro_after_init rrsba_disabled;
+
 /* Disable in-kernel use of non-RSB RET predictors */
 static void __init spec_ctrl_disable_kernel_rrsba(void)
 {
-	u64 x86_arch_cap_msr;
+	if (rrsba_disabled)
+		return;
 
-	if (!boot_cpu_has(X86_FEATURE_RRSBA_CTRL))
+	if (!(x86_arch_cap_msr & ARCH_CAP_RRSBA)) {
+		rrsba_disabled = true;
 		return;
+	}
 
-	x86_arch_cap_msr = x86_read_arch_cap_msr();
+	if (!boot_cpu_has(X86_FEATURE_RRSBA_CTRL))
+		return;
 
-	if (x86_arch_cap_msr & ARCH_CAP_RRSBA) {
 	x86_spec_ctrl_base |= SPEC_CTRL_RRSBA_DIS_S;
 	update_spec_ctrl(x86_spec_ctrl_base);
-	}
+	rrsba_disabled = true;
 }
 
 static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_mitigation mode)
@@ -1652,9 +1657,11 @@ static void __init bhi_select_mitigation(void)
 		return;
 
 	/* Retpoline mitigates against BHI unless the CPU has RRSBA behavior */
-	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE) &&
-	    !(x86_read_arch_cap_msr() & ARCH_CAP_RRSBA))
+	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {
+		spec_ctrl_disable_kernel_rrsba();
+		if (rrsba_disabled)
 			return;
+	}
 
 	if (spec_ctrl_bhi_dis())
 		return;
@@ -2809,8 +2816,7 @@ static const char *spectre_bhi_state(void)
 		return "; BHI: BHI_DIS_S";
 	else if  (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP))
 		return "; BHI: SW loop, KVM: SW loop";
-	else if (boot_cpu_has(X86_FEATURE_RETPOLINE) &&
-		 !(x86_arch_cap_msr & ARCH_CAP_RRSBA))
+	else if (boot_cpu_has(X86_FEATURE_RETPOLINE) && rrsba_disabled)
 		return "; BHI: Retpoline";
 	else if  (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT))
 		return "; BHI: Syscall hardening, KVM: SW loop";