Fix message for too long tls-crypt-v2 metadata

The current code only checks if the base64-encoded metadata is at most
980 characters. However, that can encode up to 735 bytes of data, while
only up to 733 bytes are allowed. When passing 734 or 735 bytes, openvpn
prints a misleading error message saying that the base64 cannot be
decoded.

This patch checks the decoded length to show an accurate error message.

v2: Remove now-unused macro and fix an off-by-one error.

Signed-off-by: Max Fillinger <maximilian.fillinger@foxcrypto.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20221214153414.12671-1-maximilian.fillinger@foxcrypto.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25694.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

Author: mfil

src/openvpn/base64.h

@@ -38,6 +38,10 @@
 #define OPENVPN_BASE64_LENGTH(binary_length) \
     ((((8 * binary_length) / 6) + 3) & ~3)
 
+/** Compute the maximal number of bytes encoded in a base64 string. */
+#define OPENVPN_BASE64_DECODED_LENGTH(base64_length) \
+    ((base64_length / 4) * 3)
+
 int openvpn_base64_encode(const void *data, int size, char **str);
 
 int openvpn_base64_decode(const char *str, void *data, int size);

src/openvpn/tls_crypt.c

@@ -627,15 +627,11 @@ tls_crypt_v2_write_client_key_file(const char *filename,
     }
     ASSERT(buf_write(&dst, client_key.keys, sizeof(client_key.keys)));
 
-    struct buffer metadata = alloc_buf_gc(TLS_CRYPT_V2_MAX_METADATA_LEN, &gc);
+    struct buffer metadata;
     if (b64_metadata)
     {
-        if (TLS_CRYPT_V2_MAX_B64_METADATA_LEN < strlen(b64_metadata))
-        {
-            msg(M_FATAL,
-                "ERROR: metadata too long (%d bytes, max %u bytes)",
-                (int)strlen(b64_metadata), TLS_CRYPT_V2_MAX_B64_METADATA_LEN);
-        }
+        size_t b64_length = strlen(b64_metadata);
+        metadata = alloc_buf_gc(OPENVPN_BASE64_DECODED_LENGTH(b64_length) + 1, &gc);
         ASSERT(buf_write(&metadata, &TLS_CRYPT_METADATA_TYPE_USER, 1));
         int decoded_len = openvpn_base64_decode(b64_metadata, BEND(&metadata),
                                                 BCAP(&metadata));
@@ -644,10 +640,18 @@ tls_crypt_v2_write_client_key_file(const char *filename,
             msg(M_FATAL, "ERROR: failed to base64 decode provided metadata");
             goto cleanup;
         }
+        if (decoded_len > TLS_CRYPT_V2_MAX_METADATA_LEN - 1)
+        {
+            msg(M_FATAL,
+                "ERROR: metadata too long (%d bytes, max %u bytes)",
+                decoded_len, TLS_CRYPT_V2_MAX_METADATA_LEN - 1);
+            goto cleanup;
+        }
         ASSERT(buf_inc_len(&metadata, decoded_len));
     }
     else
     {
+        metadata = alloc_buf_gc(1 + sizeof(int64_t), &gc);
         int64_t timestamp = htonll((uint64_t)now);
         ASSERT(buf_write(&metadata, &TLS_CRYPT_METADATA_TYPE_TIMESTAMP, 1));
         ASSERT(buf_write(&metadata, &timestamp, sizeof(timestamp)));

src/openvpn/tls_crypt.h

@@ -101,8 +101,6 @@
 #define TLS_CRYPT_V2_MAX_METADATA_LEN (unsigned)(TLS_CRYPT_V2_MAX_WKC_LEN \
                                                  - (TLS_CRYPT_V2_CLIENT_KEY_LEN + TLS_CRYPT_V2_TAG_SIZE \
                                                     + sizeof(uint16_t)))
-#define TLS_CRYPT_V2_MAX_B64_METADATA_LEN \
-    OPENVPN_BASE64_LENGTH(TLS_CRYPT_V2_MAX_METADATA_LEN - 1)
 
 /**
  * Initialize a key_ctx_bi structure for use with --tls-crypt.