powerpc/64s: Wire up cpu_show_spectre_v1()

CVE-2018-3639 (powerpc)

Add a definition for cpu_show_spectre_v1() to override the generic
version. Currently this just prints "Not affected" or "Vulnerable"
based on the firmware flag.

Although the kernel does have array_index_nospec() in a few places, we
haven't yet audited all the powerpc code to see where it's necessary,
so for now we don't list that as a mitigation.

(cherry picked from commit 56986016)
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>

arch/powerpc/kernel/security.c

@@ -50,3 +50,11 @@ ssize_t cpu_show_meltdown(struct device *dev, struct device_attribute *attr, cha
 
 	return sprintf(buf, "Vulnerable\n");
 }
+
+ssize_t cpu_show_spectre_v1(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	if (!security_ftr_enabled(SEC_FTR_BNDS_CHK_SPEC_BAR))
+		return sprintf(buf, "Not affected\n");
+
+	return sprintf(buf, "Vulnerable\n");
+}