arm64: proton-pack: Include unprivileged eBPF status in Spectre v2 mitigation reporting

The mitigations for Spectre-BHB are only applied when an exception is
taken from user-space. The mitigation status is reported via the spectre_v2
sysfs vulnerabilities file.

When unprivileged eBPF is enabled the mitigation in the exception vectors
can be avoided by an eBPF program.

When unprivileged eBPF is enabled, print a warning and report vulnerable
via the sysfs vulnerabilities file.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>

arch/arm64/kernel/proton-pack.c

@@ -18,6 +18,7 @@
  */
 
 #include <linux/arm-smccc.h>
+#include <linux/bpf.h>
 #include <linux/cpu.h>
 #include <linux/device.h>
 #include <linux/nospec.h>
@@ -111,6 +112,15 @@ static const char *get_bhb_affected_string(enum mitigation_state bhb_state)
 	}
 }
 
+static bool _unprivileged_ebpf_enabled(void)
+{
+#ifdef CONFIG_BPF_SYSCALL
+	return !sysctl_unprivileged_bpf_disabled;
+#else
+	return false;
+#endif
+}
+
 ssize_t cpu_show_spectre_v2(struct device *dev, struct device_attribute *attr,
 			    char *buf)
 {
@@ -130,6 +140,9 @@ ssize_t cpu_show_spectre_v2(struct device *dev, struct device_attribute *attr,
 		v2_str = "CSV2";
 		fallthrough;
 	case SPECTRE_MITIGATED:
+		if (bhb_state == SPECTRE_MITIGATED && _unprivileged_ebpf_enabled())
+			return sprintf(buf, "Vulnerable: Unprivileged eBPF enabled\n");
+
 		return sprintf(buf, "Mitigation: %s%s\n", v2_str, bhb_str);
 	case SPECTRE_VULNERABLE:
 		fallthrough;
@@ -1125,3 +1138,16 @@ void __init spectre_bhb_patch_clearbhb(struct alt_instr *alt,
 	*updptr++ = cpu_to_le32(aarch64_insn_gen_nop());
 	*updptr++ = cpu_to_le32(aarch64_insn_gen_nop());
 }
+
+#ifdef CONFIG_BPF_SYSCALL
+#define EBPF_WARN "Unprivileged eBPF is enabled, data leaks possible via Spectre v2 BHB attacks!\n"
+void unpriv_ebpf_notify(int new_state)
+{
+	if (spectre_v2_state == SPECTRE_VULNERABLE ||
+	    spectre_bhb_state != SPECTRE_MITIGATED)
+		return;
+
+	if (!new_state)
+		pr_err("WARNING: %s", EBPF_WARN);
+}
+#endif