[stable:PATCH v5.4.184 18/22] arm64: proton-pack: Report Spectre-BHB vulnerabilities as part of Spectre-v2

[James Morse]

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 at arm.com>
[ code move to cpu_errata.c for backport ]
Signed-off-by: James Morse <james.morse at arm.com>
---
 arch/arm64/include/asm/cpufeature.h |  9 +++++++
 arch/arm64/kernel/cpu_errata.c      | 41 ++++++++++++++++++++++++++---
 2 files changed, 46 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index ccae05da98a7..a798443ed76f 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -639,6 +639,15 @@ static inline int arm64_get_ssbd_state(void)
 
 void arm64_set_ssbd_mitigation(bool state);
 
+/* Watch out, ordering is important here. */
+enum mitigation_state {
+	SPECTRE_UNAFFECTED,
+	SPECTRE_MITIGATED,
+	SPECTRE_VULNERABLE,
+};
+
+enum mitigation_state arm64_get_spectre_bhb_state(void);
+
 extern int do_emulate_mrs(struct pt_regs *regs, u32 sys_reg, u32 rt);
 
 static inline u32 id_aa64mmfr0_parange_to_phys_shift(int parange)
diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
index 1e16c4e00e77..182305000de3 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -989,15 +989,41 @@ ssize_t cpu_show_spectre_v1(struct device *dev, struct device_attribute *attr,
 	return sprintf(buf, "Mitigation: __user pointer sanitization\n");
 }
 
+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 (get_spectre_v2_workaround_state()) {
 	case ARM64_BP_HARDEN_NOT_REQUIRED:
-		return sprintf(buf, "Not affected\n");
-        case ARM64_BP_HARDEN_WA_NEEDED:
-		return sprintf(buf, "Mitigation: Branch predictor hardening\n");
-        case ARM64_BP_HARDEN_UNKNOWN:
+		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 ARM64_BP_HARDEN_WA_NEEDED:
+		return sprintf(buf, "Mitigation: %s%s\n", v2_str, bhb_str);
+	case ARM64_BP_HARDEN_UNKNOWN:
+		fallthrough;
 	default:
 		return sprintf(buf, "Vulnerable\n");
 	}
@@ -1019,3 +1045,10 @@ ssize_t cpu_show_spec_store_bypass(struct device *dev,
 
 	return sprintf(buf, "Vulnerable\n");
 }
+
+static enum mitigation_state spectre_bhb_state;
+
+enum mitigation_state arm64_get_spectre_bhb_state(void)
+{
+	return spectre_bhb_state;
+}
-- 
2.30.2