Fix verify out flags from x509_crt_verify_top()

This change fixes a regression introduced by an earlier commit that
modified x509_crt_verify_top() to ensure that valid certificates
that are after past or future valid in the chain are processed. However
the change introduced a change in behaviour that caused the
verification flags MBEDTLS_X509_BADCERT_EXPIRED and
MBEDTLS_BADCERT_FUTURE to always be set whenever there is a failure in
the verification regardless of the cause.

The fix maintains both behaviours:
  * Ensure that valid certificates after future and past are verified
  * Ensure that the correct verification flags are set.

To do so, a temporary pointer to the first future or past valid
certificate is maintained while traversing the chain. If a truly valid
certificate is found then that one is used, otherwise if no valid
certificate is found and the end of the chain is reached, the program
reverts back to using the future or past valid certificate.

ChangeLog

@@ -1,5 +1,14 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Bugfix
+   * Fix output certificate verification flags set by x509_crt_verify_top() when
+     traversing a chain of trusted CA. The issue would cause both flags,
+     MBEDTLS_X509_BADCERT_NOT_TRUSTED and MBEDTLS_X509_BADCERT_EXPIRED, to be
+     set when the verification conditions are not met regardless of the cause.
+     Found by Harm Verhagen and inestlerode. #665 #561
+
 = mbed TLS 2.1.6 branch released 2016-10-17
 
 Security

library/x509_crt.c

@@ -1893,6 +1893,7 @@ static int x509_crt_verify_top(
     int check_path_cnt;
     unsigned char hash[MBEDTLS_MD_MAX_SIZE];
     const mbedtls_md_info_t *md_info;
+    mbedtls_x509_crt *future_past_ca = NULL;
 
     if( mbedtls_x509_time_is_past( &child->valid_to ) )
         *flags |= MBEDTLS_X509_BADCERT_EXPIRED;
@@ -1947,16 +1948,6 @@ static int x509_crt_verify_top(
             continue;
         }
 
-        if( mbedtls_x509_time_is_past( &trust_ca->valid_to ) )
-        {
-            continue;
-        }
-
-        if( mbedtls_x509_time_is_future( &trust_ca->valid_from ) )
-        {
-            continue;
-        }
-
         if( mbedtls_pk_verify_ext( child->sig_pk, child->sig_opts, &trust_ca->pk,
                            child->sig_md, hash, mbedtls_md_get_size( md_info ),
                            child->sig.p, child->sig.len ) != 0 )
@@ -1964,6 +1955,20 @@ static int x509_crt_verify_top(
             continue;
         }
 
+        if( mbedtls_x509_time_is_past( &trust_ca->valid_to ) ||
+            mbedtls_x509_time_is_future( &trust_ca->valid_from ) )
+        {
+            if ( future_past_ca == NULL )
+                future_past_ca = trust_ca;
+
+            continue;
+        }
+
+        break;
+    }
+
+    if( trust_ca != NULL || ( trust_ca = future_past_ca ) != NULL )
+    {
         /*
          * Top of chain is signed by a trusted CA
          */
@@ -1971,8 +1976,6 @@ static int x509_crt_verify_top(
 
         if( x509_profile_check_key( profile, child->sig_pk, &trust_ca->pk ) != 0 )
             *flags |= MBEDTLS_X509_BADCERT_BAD_KEY;
-
-        break;
     }
 
     /*
@@ -1992,6 +1995,12 @@ static int x509_crt_verify_top(
         ((void) ca_crl);
 #endif
 
+        if( mbedtls_x509_time_is_past( &trust_ca->valid_to ) )
+            ca_flags |= MBEDTLS_X509_BADCERT_EXPIRED;
+
+        if( mbedtls_x509_time_is_future( &trust_ca->valid_from ) )
+            ca_flags |= MBEDTLS_X509_BADCERT_FUTURE;
+
         if( NULL != f_vrfy )
         {
             if( ( ret = f_vrfy( p_vrfy, trust_ca, path_cnt + 1,