diff options
authorSteffan Karger <>2015-07-29 12:30:26 +0200
committerGert Doering <>2015-08-01 15:08:03 +0200
commitcc377dec820f9e6e7e72981013eb3857aa6ea5ce (patch)
parent710c439817522ac8f4dfa7411baa787c5e2e2f89 (diff)
Fix overflow check in openvpn_decrypt()
Sebastian Krahmer from the SuSE security team reported that the buffer overflow check in openvpn_decrypt() was too strict according to the cipher update function contract: "The amount of data written depends on the block alignment of the encrypted data: as a result the amount of data written may be anything from zero bytes to (inl + cipher_block_size - 1) so outl should contain sufficient room." This stems from the way CBC mode works, which caches input and 'flushes' it block-wise to the output buffer. We do allocate enough space for this extra block in the output buffer for CBC mode, but not for CFB/OFB modes. This patch: * updates the overflow check to also verify that the extra block required according to the function contract is available. * uses buf_inc_len() to double-check for overflows during en/decryption. * also reserves the extra block for non-CBC cipher modes. In practice, I could not find a way in which this would fail. The plaintext is never longer than the ciphertext, and the implementations of CBC/OFB/CBC for AES and BF in both OpenSSL and PolarSSL/mbed TLS do not use the buffer beyond the plaintext length when decrypting. However, some funky OpenSSL engine I did not check *might* use the buffer space required by the function contract. So we should still make sure we have enough room anyway. v2 - always ASSERT() on buf_inc_len(). It is a double-check so should really not fail, but if it fails there has been a buffer overflow. At that point the best thing we can do is assert out. (The primary check *is* handled gracefully, and just drops the packet.) Signed-off-by: Steffan Karger <> Acked-by: Arne Schwabe <> Message-Id: <> URL: Signed-off-by: Gert Doering <>
2 files changed, 10 insertions, 11 deletions
diff --git a/src/openvpn/crypto.c b/src/openvpn/crypto.c
index 588d9f0..1ceb411 100644
--- a/src/openvpn/crypto.c
+++ b/src/openvpn/crypto.c
@@ -166,11 +166,11 @@ openvpn_encrypt (struct buffer *buf, struct buffer work,
/* Encrypt packet ID, payload */
ASSERT (cipher_ctx_update (ctx->cipher, BPTR (&work), &outlen, BPTR (buf), BLEN (buf)));
- work.len += outlen;
+ ASSERT (buf_inc_len(&work, outlen));
/* Flush the encryption buffer */
- ASSERT(cipher_ctx_final(ctx->cipher, BPTR (&work) + outlen, &outlen));
- work.len += outlen;
+ ASSERT (cipher_ctx_final(ctx->cipher, BPTR (&work) + outlen, &outlen));
+ ASSERT (buf_inc_len(&work, outlen));
/* For all CBC mode ciphers, check the last block is complete */
ASSERT (cipher_kt_mode (cipher_kt) != OPENVPN_MODE_CBC ||
@@ -305,18 +305,18 @@ openvpn_decrypt (struct buffer *buf, struct buffer work,
CRYPT_ERROR ("cipher init failed");
/* Buffer overflow check (should never happen) */
- if (!buf_safe (&work, buf->len))
- CRYPT_ERROR ("buffer overflow");
+ if (!buf_safe (&work, buf->len + cipher_ctx_block_size(ctx->cipher)))
+ CRYPT_ERROR ("potential buffer overflow");
/* Decrypt packet ID, payload */
if (!cipher_ctx_update (ctx->cipher, BPTR (&work), &outlen, BPTR (buf), BLEN (buf)))
CRYPT_ERROR ("cipher update failed");
- work.len += outlen;
+ ASSERT (buf_inc_len(&work, outlen));
/* Flush the decryption buffer */
if (!cipher_ctx_final (ctx->cipher, BPTR (&work) + outlen, &outlen))
CRYPT_ERROR ("cipher final failed");
- work.len += outlen;
+ ASSERT (buf_inc_len(&work, outlen));
format_hex (BPTR (&work), BLEN (&work), 80, &gc));
@@ -413,9 +413,8 @@ crypto_adjust_frame_parameters(struct frame *frame,
if (use_iv)
crypto_overhead += cipher_kt_iv_size (kt->cipher);
- if (cipher_kt_mode_cbc (kt->cipher))
- /* worst case padding expansion */
- crypto_overhead += cipher_kt_block_size (kt->cipher);
+ /* extra block required by cipher_ctx_update() */
+ crypto_overhead += cipher_kt_block_size (kt->cipher);
crypto_overhead += kt->hmac_length;
diff --git a/src/openvpn/crypto_backend.h b/src/openvpn/crypto_backend.h
index 4e45df0..4c1ce9f 100644
--- a/src/openvpn/crypto_backend.h
+++ b/src/openvpn/crypto_backend.h
@@ -333,7 +333,7 @@ int cipher_ctx_reset (cipher_ctx_t *ctx, uint8_t *iv_buf);
* Note that if a complete block cannot be written, data is cached in the
* context, and emitted at a later call to \c cipher_ctx_update, or by a call
* to \c cipher_ctx_final(). This implies that dst should have enough room for
- * src_len + \c cipher_ctx_block_size() - 1.
+ * src_len + \c cipher_ctx_block_size().
* @param ctx Cipher's context. May not be NULL.
* @param dst Destination buffer