[PATCH 4.19 39/57] arm64: proton-pack: Report Spectre-BHB vulnerabilities as part of Spectre-v2

Greg Kroah-Hartman
2022-03-21T14:05:38Z

From: James Morse <james.morse@arm.com>

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>
[ code move to cpu_errata.c for backport ]
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/arm64/include/asm/cpufeature.h | 8 +++++++
arch/arm64/kernel/cpu_errata.c | 38 +++++++++++++++++++++++++++++++++---
2 files changed, 43 insertions(+), 3 deletions(-)

--- 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(v

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
--- 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 devic
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(struc

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;
+}