Add missing ret code checks in PEM module

Add missing return code checks in the functions pem_des_decrypt(),
pem_3des_decrypt() and pem_aes_decrypt() so that the calling function
mbedtls_pem_read_buffer() is notified of errors reported by the crypto
primitives AES, DES and 3DES.

ChangeLog

@@ -1,5 +1,15 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Bugfix
+   * Fix unchecked return codes from AES, DES and 3DES functions in
+     pem_aes_decrypt(), pem_des_decrypt() and pem_des3_decrypt() respectively.
+     If a call to one of the functions of the cryptographic primitive modules
+     failed, the error may not be noticed by the function
+     mbedtls_pem_read_buffer() causing it to return invalid values. Found by
+     Guido Vranken. #756
+
 = mbed TLS 2.5.0 branch released 2017-05-17
 
 Security

library/pem.c

@@ -134,45 +134,55 @@ static void pem_pbkdf1( unsigned char *key, size_t keylen,
 /*
  * Decrypt with DES-CBC, using PBKDF1 for key derivation
  */
-static void pem_des_decrypt( unsigned char des_iv[8],
+static int pem_des_decrypt( unsigned char des_iv[8],
                             unsigned char *buf, size_t buflen,
                             const unsigned char *pwd, size_t pwdlen )
 {
     mbedtls_des_context des_ctx;
     unsigned char des_key[8];
+    int ret;
 
     mbedtls_des_init( &des_ctx );
 
     pem_pbkdf1( des_key, 8, des_iv, pwd, pwdlen );
 
-    mbedtls_des_setkey_dec( &des_ctx, des_key );
+    if( ( ret = mbedtls_des_setkey_dec( &des_ctx, des_key ) ) != 0 )
-    mbedtls_des_crypt_cbc( &des_ctx, MBEDTLS_DES_DECRYPT, buflen,
+        goto exit;
+    ret = mbedtls_des_crypt_cbc( &des_ctx, MBEDTLS_DES_DECRYPT, buflen,
                      des_iv, buf, buf );
 
+exit:
     mbedtls_des_free( &des_ctx );
     mbedtls_zeroize( des_key, 8 );
+
+    return( ret );
 }
 
 /*
  * Decrypt with 3DES-CBC, using PBKDF1 for key derivation
  */
-static void pem_des3_decrypt( unsigned char des3_iv[8],
+static int pem_des3_decrypt( unsigned char des3_iv[8],
                              unsigned char *buf, size_t buflen,
                              const unsigned char *pwd, size_t pwdlen )
 {
     mbedtls_des3_context des3_ctx;
     unsigned char des3_key[24];
+    int ret;
 
     mbedtls_des3_init( &des3_ctx );
 
     pem_pbkdf1( des3_key, 24, des3_iv, pwd, pwdlen );
 
-    mbedtls_des3_set3key_dec( &des3_ctx, des3_key );
+    if( ( ret = mbedtls_des3_set3key_dec( &des3_ctx, des3_key ) ) != 0 )
-    mbedtls_des3_crypt_cbc( &des3_ctx, MBEDTLS_DES_DECRYPT, buflen,
+        goto exit;
+    ret = mbedtls_des3_crypt_cbc( &des3_ctx, MBEDTLS_DES_DECRYPT, buflen,
                      des3_iv, buf, buf );
 
+exit:
     mbedtls_des3_free( &des3_ctx );
     mbedtls_zeroize( des3_key, 24 );
+
+    return( ret );
 }
 #endif /* MBEDTLS_DES_C */
 
@@ -180,23 +190,28 @@ static void pem_des3_decrypt( unsigned char des3_iv[8],
 /*
  * Decrypt with AES-XXX-CBC, using PBKDF1 for key derivation
  */
-static void pem_aes_decrypt( unsigned char aes_iv[16], unsigned int keylen,
+static int pem_aes_decrypt( unsigned char aes_iv[16], unsigned int keylen,
                             unsigned char *buf, size_t buflen,
                             const unsigned char *pwd, size_t pwdlen )
 {
     mbedtls_aes_context aes_ctx;
     unsigned char aes_key[32];
+    int ret;
 
     mbedtls_aes_init( &aes_ctx );
 
     pem_pbkdf1( aes_key, keylen, aes_iv, pwd, pwdlen );
 
-    mbedtls_aes_setkey_dec( &aes_ctx, aes_key, keylen * 8 );
+    if( ( ret = mbedtls_aes_setkey_dec( &aes_ctx, aes_key, keylen * 8 ) ) != 0 )
-    mbedtls_aes_crypt_cbc( &aes_ctx, MBEDTLS_AES_DECRYPT, buflen,
+        goto exit;
+    ret = mbedtls_aes_crypt_cbc( &aes_ctx, MBEDTLS_AES_DECRYPT, buflen,
                      aes_iv, buf, buf );
 
+exit:
     mbedtls_aes_free( &aes_ctx );
     mbedtls_zeroize( aes_key, keylen );
+
+    return( ret );
 }
 #endif /* MBEDTLS_AES_C */
 
@@ -345,22 +360,30 @@ int mbedtls_pem_read_buffer( mbedtls_pem_context *ctx, const char *header, const
             return( MBEDTLS_ERR_PEM_PASSWORD_REQUIRED );
         }
 
+        ret = 0;
+
 #if defined(MBEDTLS_DES_C)
         if( enc_alg == MBEDTLS_CIPHER_DES_EDE3_CBC )
-            pem_des3_decrypt( pem_iv, buf, len, pwd, pwdlen );
+            ret = pem_des3_decrypt( pem_iv, buf, len, pwd, pwdlen );
         else if( enc_alg == MBEDTLS_CIPHER_DES_CBC )
-            pem_des_decrypt( pem_iv, buf, len, pwd, pwdlen );
+            ret = pem_des_decrypt( pem_iv, buf, len, pwd, pwdlen );
 #endif /* MBEDTLS_DES_C */
 
 #if defined(MBEDTLS_AES_C)
         if( enc_alg == MBEDTLS_CIPHER_AES_128_CBC )
-            pem_aes_decrypt( pem_iv, 16, buf, len, pwd, pwdlen );
+            ret = pem_aes_decrypt( pem_iv, 16, buf, len, pwd, pwdlen );
         else if( enc_alg == MBEDTLS_CIPHER_AES_192_CBC )
-            pem_aes_decrypt( pem_iv, 24, buf, len, pwd, pwdlen );
+            ret = pem_aes_decrypt( pem_iv, 24, buf, len, pwd, pwdlen );
         else if( enc_alg == MBEDTLS_CIPHER_AES_256_CBC )
-            pem_aes_decrypt( pem_iv, 32, buf, len, pwd, pwdlen );
+            ret = pem_aes_decrypt( pem_iv, 32, buf, len, pwd, pwdlen );
 #endif /* MBEDTLS_AES_C */
 
+        if( ret != 0 )
+        {
+            mbedtls_free( buf );
+            return( ret );
+        }
+
         /*
          * The result will be ASN.1 starting with a SEQUENCE tag, with 1 to 3
          * length bytes (allow 4 to be sure) in all known use cases.