Prevent signed integer overflow in CSR parsing

Modify the function mbedtls_x509_csr_parse_der() so that it checks the
parsed CSR version integer before it increments the value. This prevents
a potential signed integer overflow, as these have undefined behaviour
in the C standard.

ChangeLog

@@ -17,6 +17,13 @@ Bugfix
      encoded X509 CRLs. The overflow would enable maliciously constructed CRLs
      to bypass the version verification check. Found by Peng Li/Yueh-Hsun Lin,
      KNOX Security, Samsung Research America
+   * Fix a potential integer overflow in the version verification for DER
+     encoded X509 certificates. The overflow would enable maliciously
+     constructed certificates to bypass the certificate verification check.
+   * Fix potential integer overflow in the version verification for DER
+     encoded X509 CSRs. The overflow would enable maliciously constructed CSRs
+     to bypass the version verification check. Found by Peng Li/Yueh-Hsun Lin,
+     KNOX Security, Samsung Research America
 
 = mbed TLS 2.1.8 branch released 2017-06-21
 
@@ -131,9 +138,6 @@ Bugfix
      digits. Found and fixed by Guido Vranken.
    * Fix unlisted DES configuration dependency in some pkparse test cases. Found
      by inestlerode. #555
-   * Fix a potential integer overflow in the version verification for DER
-     encoded X509 certificates. The overflow would enable maliciously
-     constructed certificates to bypass the certificate verification check.
 
 = mbed TLS 2.1.6 branch released 2016-10-17
 

library/x509_csr.c

@@ -168,14 +168,14 @@ int mbedtls_x509_csr_parse_der( mbedtls_x509_csr *csr,
         return( ret );
     }
 
-    csr->version++;
+    if( csr->version != 0 )
-
-    if( csr->version != 1 )
     {
         mbedtls_x509_csr_free( csr );
         return( MBEDTLS_ERR_X509_UNKNOWN_VERSION );
     }
 
+    csr->version++;
+
     /*
      *  subject               Name
      */