[PATCH 5.16 32/53] arm64: proton-pack: Report Spectre-BHB vulnerabilities as part of Spectre-v2

Greg Kroah-Hartman
2022-03-10T14:14:10Z

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>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/arm64/include/asm/spectre.h | 2 ++
arch/arm64/kernel/proton-pack.c | 36 ++++++++++++++++++++++++++++++++++--
2 files changed, 36 insertions(+), 2 deletions(-)

--- a/arch/arm64/include/asm/spectre.h
+++ b/arch/arm64/include/asm/spectre.h
@@ -93,5 +93,7 @@ void spectre_v4_enable_task_mitigation(s

enum mitigation_state arm64_get_meltdown_state(void);

+enum mitigation_state arm64_get_spectre_bhb_state(void);
+
#endif /* __ASSEMBLY__ */
#endif /* __ASM_SPECTRE_H */
--- a/arch/arm64/kernel/proton-pack.c
+++ b/arch/arm64/kernel/proton-pack.c
@@ -96,14 +96,39 @@ static bool spectre_v2_mitigations_off(v
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:
- return sprintf(buf, "Not affected\n");
+ 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:
@@ -771,6 +796,13 @@ int arch_prctl_spec_ctrl_get(struct task
}
}

+static enum mitigation_state spectre_bhb_state;
+
+enum mitigation_state arm64_get_spectre_bhb_state(void)
+{
+ return spectre_bhb_state;
+}
+
/* Patched to NOP when enabled */
void noinstr spectre_bhb_patch_loop_mitigation_enable(struct alt_instr *alt,
__le32 *origptr,