linux-kernel - [stable:PATCH v4.19.235 17/22] arm64: proton-pack: Report Spectre-BHB vulnerabilities as part of Spectre-v2

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Message-Id: <20220318174842.2321061-18-james.morse@arm.com>
Date:   Fri, 18 Mar 2022 17:48:37 +0000
From:   James Morse <james.morse@....com>
To:     stable@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, james.morse@....com,
        catalin.marinas@....com
Subject: [stable:PATCH v4.19.235 17/22] 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@....com>
[ code move to cpu_errata.c for backport ]
Signed-off-by: James Morse <james.morse@....com>
---
 arch/arm64/include/asm/cpufeature.h |  8 ++++++
 arch/arm64/kernel/cpu_errata.c      | 38 ++++++++++++++++++++++++++---
 2 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index dda6e5056810..e59b9ee43e4a 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -527,6 +527,14 @@ 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);
 #endif /* __ASSEMBLY__ */
 
 #endif
diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
index d191ce8410db..d055ed21eeeb 100644
--- a/arch/arm64/kernel/cpu_errata.c
+++ b/arch/arm64/kernel/cpu_errata.c
@@ -875,14 +875,39 @@ 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)
 {
-	if (__spectrev2_safe)
-		return sprintf(buf, "Not affected\n");
+	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";
+
+	if (__spectrev2_safe) {
+		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";
+	}
 
 	if (__hardenbp_enab)
-		return sprintf(buf, "Mitigation: Branch predictor hardening\n");
+		return sprintf(buf, "Mitigation: %s%s\n", v2_str, bhb_str);
 
 	return sprintf(buf, "Vulnerable\n");
 }
@@ -903,3 +928,10 @@ 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