powerpc/64: Update Speculation_Store_Bypass in /proc/<pid>/status

Currently we don't report anything useful in /proc/<pid>/status:

  $ grep Speculation_Store_Bypass /proc/self/status
  Speculation_Store_Bypass:       unknown

Our mitigation is currently always a barrier instruction, which
doesn't map that well onto the existing possibilities for the PR_SPEC
values.

However even if we added a "barrier" type PR_SPEC value, userspace
would still need to consult some other source to work out which type
of barrier to use. So reporting "vulnerable" seems sufficient, as
userspace can see that and then consult its source to determine what
barrier to use.

Signed-off-by: Gustavo Walbon <gwalbon@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200402124929.3574166-1-mpe@ellerman.id.au

arch/powerpc/kernel/security.c

@@ -7,6 +7,8 @@
 #include <linux/cpu.h>
 #include <linux/kernel.h>
 #include <linux/device.h>
+#include <linux/nospec.h>
+#include <linux/prctl.h>
 #include <linux/seq_buf.h>
 
 #include <asm/asm-prototypes.h>
@@ -353,6 +355,40 @@ ssize_t cpu_show_spec_store_bypass(struct device *dev, struct device_attribute *
 	return sprintf(buf, "Vulnerable\n");
 }
 
+static int ssb_prctl_get(struct task_struct *task)
+{
+	if (stf_enabled_flush_types == STF_BARRIER_NONE)
+		/*
+		 * We don't have an explicit signal from firmware that we're
+		 * vulnerable or not, we only have certain CPU revisions that
+		 * are known to be vulnerable.
+		 *
+		 * We assume that if we're on another CPU, where the barrier is
+		 * NONE, then we are not vulnerable.
+		 */
+		return PR_SPEC_NOT_AFFECTED;
+	else
+		/*
+		 * If we do have a barrier type then we are vulnerable. The
+		 * barrier is not a global or per-process mitigation, so the
+		 * only value we can report here is PR_SPEC_ENABLE, which
+		 * appears as "vulnerable" in /proc.
+		 */
+		return PR_SPEC_ENABLE;
+
+	return -EINVAL;
+}
+
+int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)
+{
+	switch (which) {
+	case PR_SPEC_STORE_BYPASS:
+		return ssb_prctl_get(task);
+	default:
+		return -ENODEV;
+	}
+}
+
 #ifdef CONFIG_DEBUG_FS
 static int stf_barrier_set(void *data, u64 val)
 {