crypto: clear out buffer after timing pbkdf algorithm

The 'out' buffer will hold a key derived from master
password, so it is best practice to clear this buffer
when no longer required.

At this time, the code isn't worrying about locking
buffers into RAM to prevent swapping sensitive data
to disk.
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

crypto/pbkdf.c

@@ -67,13 +67,14 @@ uint64_t qcrypto_pbkdf2_count_iters(QCryptoHashAlgorithm hash,
                                     const uint8_t *salt, size_t nsalt,
                                     Error **errp)
 {
+    uint64_t ret = -1;
     uint8_t out[32];
     uint64_t iterations = (1 << 15);
     unsigned long long delta_ms, start_ms, end_ms;
 
     while (1) {
         if (qcrypto_pbkdf2_get_thread_cpu(&start_ms, errp) < 0) {
-            return -1;
+            goto cleanup;
         }
         if (qcrypto_pbkdf2(hash,
                            key, nkey,
@@ -81,10 +82,10 @@ uint64_t qcrypto_pbkdf2_count_iters(QCryptoHashAlgorithm hash,
                            iterations,
                            out, sizeof(out),
                            errp) < 0) {
-            return -1;
+            goto cleanup;
         }
         if (qcrypto_pbkdf2_get_thread_cpu(&end_ms, errp) < 0) {
-            return -1;
+            goto cleanup;
         }
 
         delta_ms = end_ms - start_ms;
@@ -100,5 +101,9 @@ uint64_t qcrypto_pbkdf2_count_iters(QCryptoHashAlgorithm hash,
 
     iterations = iterations * 1000 / delta_ms;
 
-    return iterations;
+    ret = iterations;
+
+ cleanup:
+    memset(out, 0, sizeof(out));
+    return ret;
 }