arm64: proton-pack: Report Spectre-BHB vulnerabilities as part of Spectre-v2

commit dee435be76f4117410bbd90573a881fd33488f37 upstream.

Speculation attacks against some high-performance processors can
make use of branch history to influence future speculation as part of
a spectre-v2 attack. This is not mitigated by CSV2, meaning CPUs that
previously reported 'Not affected' are now moderately mitigated by CSV2.

Update the value in /sys/devices/system/cpu/vulnerabilities/spectre_v2
to also show the state of the BHB mitigation.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm64/include/asm/spectre.h

@@ -29,4 +29,6 @@ bool has_spectre_v4(const struct arm64_cpu_capabilities *cap, int scope);
 void spectre_v4_enable_mitigation(const struct arm64_cpu_capabilities *__unused);
 void spectre_v4_enable_task_mitigation(struct task_struct *tsk);
 
+enum mitigation_state arm64_get_spectre_bhb_state(void);
+
 #endif	/* __ASM_SPECTRE_H */

arch/arm64/kernel/proton-pack.c

@@ -94,14 +94,39 @@ static bool spectre_v2_mitigations_off(void)
 	return ret;
 }
 
+static const char *get_bhb_affected_string(enum mitigation_state bhb_state)
+{
+	switch (bhb_state) {
+	case SPECTRE_UNAFFECTED:
+		return "";
+	default:
+	case SPECTRE_VULNERABLE:
+		return ", but not BHB";
+	case SPECTRE_MITIGATED:
+		return ", BHB";
+	}
+}
+
 ssize_t cpu_show_spectre_v2(struct device *dev, struct device_attribute *attr,
 			    char *buf)
 {
+	enum mitigation_state bhb_state = arm64_get_spectre_bhb_state();
+	const char *bhb_str = get_bhb_affected_string(bhb_state);
+	const char *v2_str = "Branch predictor hardening";
+
 	switch (spectre_v2_state) {
 	case SPECTRE_UNAFFECTED:
+		if (bhb_state == SPECTRE_UNAFFECTED)
 			return sprintf(buf, "Not affected\n");
+
+		/*
+		 * Platforms affected by Spectre-BHB can't report
+		 * "Not affected" for Spectre-v2.
+		 */
+		v2_str = "CSV2";
+		fallthrough;
 	case SPECTRE_MITIGATED:
-		return sprintf(buf, "Mitigation: Branch predictor hardening\n");
+		return sprintf(buf, "Mitigation: %s%s\n", v2_str, bhb_str);
 	case SPECTRE_VULNERABLE:
 		fallthrough;
 	default:
@@ -787,3 +812,10 @@ int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)
 		return -ENODEV;
 	}
 }
+
+static enum mitigation_state spectre_bhb_state;
+
+enum mitigation_state arm64_get_spectre_bhb_state(void)
+{
+	return spectre_bhb_state;
+}