powerpc/security: Fix Speculation_Store_Bypass reporting on Power10

Nageswara reported that /proc/self/status was showing "vulnerable" for
the Speculation_Store_Bypass feature on Power10, eg:

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

But at the same time the sysfs files, and lscpu, were showing "Not
affected".

This turns out to simply be a bug in the reporting of the
Speculation_Store_Bypass, aka. PR_SPEC_STORE_BYPASS, case.

When SEC_FTR_STF_BARRIER was added, so that firmware could communicate
the vulnerability was not present, the code in ssb_prctl_get() was not
updated to check the new flag.

So add the check for SEC_FTR_STF_BARRIER being disabled. Rather than
adding the new check to the existing if block and expanding the comment
to cover both cases, rewrite the three cases to be separate so they can
be commented separately for clarity.

Fixes: 84ed26f ("powerpc/security: Add a security feature for STF barrier")
Cc: stable@vger.kernel.org # v5.14+
Reported-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
Tested-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
Reviewed-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230517074945.53188-1-mpe@ellerman.id.au

arch/powerpc/kernel/security.c

@@ -364,26 +364,27 @@ ssize_t cpu_show_spec_store_bypass(struct device *dev, struct device_attribute *
 
 static int ssb_prctl_get(struct task_struct *task)
 {
+	/*
+	 * The STF_BARRIER feature is on by default, so if it's off that means
+	 * firmware has explicitly said the CPU is not vulnerable via either
+	 * the hypercall or device tree.
+	 */
+	if (!security_ftr_enabled(SEC_FTR_STF_BARRIER))
+		return PR_SPEC_NOT_AFFECTED;
+
+	/*
+	 * If the system's CPU has no known barrier (see setup_stf_barrier())
+	 * then assume that the CPU is not vulnerable.
+	 */
 	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;
+
+	/*
+	 * Otherwise the CPU is vulnerable. The barrier is not a global or
+	 * per-process mitigation, so the only value that can be reported here
+	 * is PR_SPEC_ENABLE, which appears as "vulnerable" in /proc.
+	 */
+	return PR_SPEC_ENABLE;
 }
 
 int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)