Do not underestimate number of encrypted/decrypted AEAD blocks

Even though the current code typically counts all the encrypted/decrypted
traffic, this is only the case because of the specific implementation
of OpenSSL at the moment.

Instead of counting the length returned by one call only, count all
the encrypted/decrypted bytes.

Other implementations that use AES-GCM (like IPSec, MacSEC, TLS 1.2)
(currently) do not honour these usage limits at all. This is the reason that
I also currently do not consider the lack/improper validation in our code
to be a security vulnerability. In the current state implementations/protocol
that lack this feature altogether are not considered vulnerable.

Reported by: <[email protected]>

Change-Id: I429d768fb33ef2c58484287d4091440ad8599053
Signed-off-by: Arne Schwabe <[email protected]>
Acked-by: Gert Doering <[email protected]>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1358
Message-Id: <[email protected]>
Signed-off-by: Gert Doering <[email protected]>

Author: schwabe

src/openvpn/crypto.c

@@ -152,15 +152,15 @@ openvpn_encrypt_aead(struct buffer *buf, struct buffer work, struct crypto_optio
     ASSERT(cipher_ctx_update(ctx->cipher, BEND(&work), &outlen, BPTR(buf), BLEN(buf)));
     ASSERT(buf_inc_len(&work, outlen));
 
-    /* update number of plaintext blocks encrypted. Use the (x + (n-1))/n trick
-     * to round up the result to the number of blocks used */
-    const int blocksize = AEAD_LIMIT_BLOCKSIZE;
-    opt->key_ctx_bi.encrypt.plaintext_blocks += (outlen + (blocksize - 1)) / blocksize;
-
     /* Flush the encryption buffer */
     ASSERT(cipher_ctx_final(ctx->cipher, BEND(&work), &outlen));
     ASSERT(buf_inc_len(&work, outlen));
 
+    /* update number of plaintext blocks encrypted. Use the (x + (n-1))/n trick
+     * to round up the result to the number of blocks used */
+    const int blocksize = AEAD_LIMIT_BLOCKSIZE;
+    opt->key_ctx_bi.encrypt.plaintext_blocks += (BLEN(&work) + (blocksize - 1)) / blocksize;
+
     /* if the tag is at end the end, allocate it now */
     if (use_epoch_data_format)
     {
@@ -580,11 +580,10 @@ openvpn_decrypt_aead(struct buffer *buf, struct buffer work, struct crypto_optio
         goto error_exit;
     }
 
-
     /* update number of plaintext blocks decrypted. Use the (x + (n-1))/n trick
      * to round up the result to the number of blocks used. */
     const int blocksize = AEAD_LIMIT_BLOCKSIZE;
-    opt->key_ctx_bi.decrypt.plaintext_blocks += (outlen + (blocksize - 1)) / blocksize;
+    opt->key_ctx_bi.decrypt.plaintext_blocks += (BLEN(&work) + (blocksize - 1)) / blocksize;
 
     *buf = work;