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

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>

diff --git a/arch/arm64/include/asm/spectre.h b/arch/arm64/include/asm/spectre.h
index f62ca39..c617fe9 100644 (file)
--- a/arch/arm64/include/asm/spectre.h
+++ b/arch/arm64/include/asm/spectre.h
@@ -93,5 +93,7 @@ void spectre_v4_enable_task_mitigation(struct task_struct *tsk);
 
 enum mitigation_state arm64_get_meltdown_state(void);
 
+enum mitigation_state arm64_get_spectre_bhb_state(void);
+
 #endif /* __ASSEMBLY__ */
 #endif /* __ASM_SPECTRE_H */

diff --git a/arch/arm64/kernel/proton-pack.c b/arch/arm64/kernel/proton-pack.c
index 6a5eeb8..8a499b8 100644 (file)
--- a/arch/arm64/kernel/proton-pack.c
+++ b/arch/arm64/kernel/proton-pack.c
@@ -96,14 +96,39 @@ 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:
-               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_struct *task, unsigned long which)
        }
 }
 
+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,