From: Steffan Karger <[email protected]>
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 <[email protected]>
---
src/openvpn/crypto.c | 19 +++++++++----------
src/openvpn/crypto_backend.h | 2 +-
2 files changed, 10 insertions(+), 11 deletions(-)
diff --git a/src/openvpn/crypto.c b/src/openvpn/crypto.c
index d15c85a..aab53f2 100644
--- a/src/openvpn/crypto.c
+++ b/src/openvpn/crypto.c
@@ -156,11 +156,11 @@ 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 ||
@@ -298,18 +298,18 @@ 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));
dmsg (D_PACKET_CONTENT, "DECRYPT TO: %s",
format_hex (BPTR (&work), BLEN (&work), 80, &gc));
@@ -395,9 +395,8 @@ 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
--
2.1.0
