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author | Steffan Karger <steffan.karger@fox-it.com> | 2015-07-29 12:30:26 +0200 |
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committer | Gert Doering <gert@greenie.muc.de> | 2015-08-01 15:08:03 +0200 |
commit | cc377dec820f9e6e7e72981013eb3857aa6ea5ce (patch) | |
tree | 6e1ece2a42e606eccfcf20ed0d88a4d9f26852b8 | |
parent | 710c439817522ac8f4dfa7411baa787c5e2e2f89 (diff) | |
download | openvpn-cc377dec820f9e6e7e72981013eb3857aa6ea5ce.tar.gz openvpn-cc377dec820f9e6e7e72981013eb3857aa6ea5ce.tar.xz openvpn-cc377dec820f9e6e7e72981013eb3857aa6ea5ce.zip |
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 <steffan.karger@fox-it.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <1438165826-32762-1-git-send-email-steffan.karger@fox-it.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9974
Signed-off-by: Gert Doering <gert@greenie.muc.de>
-rw-r--r-- | src/openvpn/crypto.c | 19 | ||||
-rw-r--r-- | 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 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)); dmsg (D_PACKET_CONTENT, "DECRYPT TO: %s", 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 |