Hi,
No full review yet, but this version does seem to address my previous
comments. Some minor nits I noticed on my first run through v2:
On 29-07-2020 13:38, Arne Schwabe wrote:
> This reworks the NCP logic to be more strict about what is
> considered an acceptable result of an NCP negotiation. It also
> us to finally drop BF-CBC support by default.
>
> All new behaviour is currently limited to server/client
> mode with pull enabled. P2p mode without pull does not change.
>
> New Server behaviour:
> - when a client announces its supported ciphers through either
> OCC or IV_CIPHER/IV_NCP we reject the client with a
> AUTH_FAILED message if we have no common cipher.
>
> - When a client does not announce any cipher in either
> OCC or NCP we by reject it unless fallback-cipher is
> specified in either ccd or config.
>
> New client behaviour:
> - When no cipher is pushed (or a cipher we refused to support)
> and we also cannot support the server's server announced in
> OCC we fail the connection and log why
>
> - If fallback-cipher is specified we will in the case that
> cipher is missing from occ use the fallback cipher instead
> of failing the connection
>
> Both client and server behaviour:
> - We only announce --cipher xyz in occ if we are willing
> to support that cipher.
>
> It means that we only announce the fallback-cipher if
> it is also contained in --data-ciphers
>
> Compatiblity behaviour:
>
> In 2.5 both client and server will automatically set
> fallback-cipher xyz if --cipher xyz is in the config and
> also add append the cipher to the end of data-ciphers.
>
> We log a warn user about this and point to --data-ciphers and
> --fallback-cipher. This also happens if the configuration
> contains an explicit --cipher BF-CBC.
>
> If --cipher is not set, we only warn that previous versions
> allowed BF-CBC and point how to reenable BF-CBC. This might
> break very few config where someone connects a very old
> client to a 2.5 server but at some point we need to drop
> the BF-CBC and those affected use will already have a the
> scary SWEET32 warning in their logs.
>
> In short: If --cipher is explicitly set 2.6 will work the same as
> 2.4 did. When --cipher is not set, BF-CBC support is dropped and
> we warn about it.
>
> Examples how breaking the default BF-CBC will be logged:
>
> Client side:
> - Client connecting to server that does not push cipher but
> has --cipher in OCC
>
> OPTIONS ERROR: failed to negotiate cipher with server. Add the server's
> cipher ('BF-CBC') to --data-ciphers (currently 'AES-256-GCM:AES-128-CBC') if
> you want to connect to this server.
>
> - Client connecting to a server that does not support OCC:
>
> OPTIONS ERROR: failed to negotioate cipher with server. Configure
> --fallback-cipher if you want connect to this server.
>
> Server Side:
>
> - Server has a client only supporting BF-CBC connecting:
>
> styx/IP PUSH: No common cipher between server and client. Server
> data-ciphers:
> 'CHACHA20-POLY1305:AES-128-GCM:AES-256-GCM:AES-256-CBC:AES-128-CBC', client
> supports cipher 'BF-CBC'.
>
> - Client without OCC:
>
> styx/IP PUSH:No NCP or OCC cipher data received from peer.
> styx/IP Use --fallback-cipher with the cipher the client is using if you
> want to allow the client to connect
>
> In all reject cases on the client:
>
> AUTH: Received control message: AUTH_FAILED,Data channel cipher
> negotiation failed (no shared cipher)
>
> Signed-off-by: Arne Schwabe <a...@rfc2549.org>
>
> Patch V2: rename fallback-cipher to data-ciphers-fallback
> add all corrections from Steffan
> Ignore occ cipher for clients sending IV_CIPHERS
> move client side ncp in its own function
> do not print INSECURE cipher warning if BF-CBC is not allowed
>
> Signed-off-by: Arne Schwabe <a...@rfc2549.org>
> ---
> doc/man-sections/protocol-options.rst | 22 ++++-
> src/openvpn/crypto.c | 4 +-
> src/openvpn/init.c | 18 ++--
> src/openvpn/multi.c | 135 ++++++++++++++++----------
> src/openvpn/options.c | 117 +++++++++++++++++-----
> src/openvpn/options.h | 2 +
> src/openvpn/ssl.c | 17 ++--
> src/openvpn/ssl_ncp.c | 82 +++++++++++++---
> src/openvpn/ssl_ncp.h | 18 ++--
> tests/unit_tests/openvpn/test_ncp.c | 26 +++--
> 10 files changed, 311 insertions(+), 130 deletions(-)
>
> diff --git a/doc/man-sections/protocol-options.rst
> b/doc/man-sections/protocol-options.rst
> index 051f1d32..69d3bc67 100644
> --- a/doc/man-sections/protocol-options.rst
> +++ b/doc/man-sections/protocol-options.rst
> @@ -57,6 +57,9 @@ configured in a compatible way between both the local and
> remote side.
> http://www.cs.ucsd.edu/users/mihir/papers/hmac.html
>
> --cipher alg
> + This option is deprecated for server-client mode and ``--data-ciphers``
> + or rarely `--data-ciphers-fallback`` should be used instead.
> +
> Encrypt data channel packets with cipher algorithm ``alg``.
>
> The default is :code:`BF-CBC`, an abbreviation for Blowfish in Cipher
> @@ -183,8 +186,9 @@ configured in a compatible way between both the local and
> remote side.
> ``--server`` ), or if ``--pull`` is specified (client-side, implied by
> setting --client).
>
> - If both peers support and do not disable NCP, the negotiated cipher will
> - override the cipher specified by ``--cipher``.
> + If no common cipher is found during cipher negotiation, the connection
> + is terminated. To support old clients/server that do not provide any cipher
> + negotiation support see ``data-ciphers-fallback``.
>
> Additionally, to allow for more smooth transition, if NCP is enabled,
> OpenVPN will inherit the cipher of the peer if that cipher is different
> @@ -201,8 +205,18 @@ configured in a compatible way between both the local
> and remote side.
> This list is restricted to be 127 chars long after conversion to OpenVPN
> ciphers.
>
> - This option was called ``ncp-ciphers`` in OpenVPN 2.4 but has been renamed
> - to ``data-ciphers`` in OpenVPN 2.5 to more accurately reflect its meaning.
> + This option was called ``--ncp-ciphers`` in OpenVPN 2.4 but has been
> renamed
> + to ``--data-ciphers`` in OpenVPN 2.5 to more accurately reflect its
> meaning.
> +
> +--data-ciphers-fallback alg
> +
> + Configure a cipher that is used to fall back to if we could not determine
> + which cipher the peer is willing to use.
> +
> + This option should only be needed to
> + connect to peers that are running OpenVPN 2.3 and older version, and
> + have been configured with `--enable-small`
> + (typically used on routers or other embedded devices).
>
> --ncp-disable
> Disable "Negotiable Crypto Parameters". This completely disables cipher
> diff --git a/src/openvpn/crypto.c b/src/openvpn/crypto.c
> index e92a0dc1..3a0bfbec 100644
> --- a/src/openvpn/crypto.c
> +++ b/src/openvpn/crypto.c
> @@ -727,7 +727,9 @@ warn_insecure_key_type(const char *ciphername, const
> cipher_kt_t *cipher)
> {
> msg(M_WARN, "WARNING: INSECURE cipher (%s) with block size less than
> 128"
> " bit (%d bit). This allows attacks like SWEET32. Mitigate by "
> - "using a --cipher with a larger block size (e.g. AES-256-CBC).",
> + "using a --cipher with a larger block size (e.g. AES-256-CBC). "
> + "Support for these insecure ciphers will be removed in "
> + "OpenVPN 2.6.",
> ciphername, cipher_kt_block_size(cipher)*8);
> }
> }
> diff --git a/src/openvpn/init.c b/src/openvpn/init.c
> index 1ea4735d..402d2652 100644
> --- a/src/openvpn/init.c
> +++ b/src/openvpn/init.c
> @@ -2365,16 +2365,9 @@ do_deferred_options(struct context *c, const unsigned
> int found)
> /* process (potentially pushed) crypto options */
> if (c->options.pull)
> {
> - struct tls_session *session = &c->c2.tls_multi->session[TM_ACTIVE];
> - if (found & OPT_P_NCP)
> - {
> - msg(D_PUSH, "OPTIONS IMPORT: data channel crypto options
> modified");
> - }
> - else if (c->options.ncp_enabled)
> + if (!check_pull_client_ncp(c, found))
> {
> - /* If the server did not push a --cipher, we will switch to the
> - * remote cipher if it is in our ncp-ciphers list */
> - tls_poor_mans_ncp(&c->options,
> c->c2.tls_multi->remote_ciphername);
> + return false;
> }
> struct frame *frame_fragment = NULL;
> #ifdef ENABLE_FRAGMENT
> @@ -2384,6 +2377,7 @@ do_deferred_options(struct context *c, const unsigned
> int found)
> }
> #endif
>
> + struct tls_session *session = &c->c2.tls_multi->session[TM_ACTIVE];
> if (!tls_session_update_crypto_params(session, &c->options,
> &c->c2.frame,
> frame_fragment))
> {
> @@ -2757,9 +2751,13 @@ do_init_crypto_tls_c1(struct context *c)
> #endif /* if P2MP */
> }
>
> + /* Do not warn if only have BF-CBC in options->ciphername
> + * because it is still the default cipher */
> + bool warn = !streq(options->ciphername, "BF-CBC")
> + || options->enable_ncp_fallback;
> /* Get cipher & hash algorithms */
> init_key_type(&c->c1.ks.key_type, options->ciphername,
> options->authname,
> - options->keysize, true, true);
> + options->keysize, true, warn);
>
> /* Initialize PRNG with config-specified digest */
> prng_init(options->prng_hash, options->prng_nonce_secret_len);
> diff --git a/src/openvpn/multi.c b/src/openvpn/multi.c
> index 0f9c586b..79b5c0c3 100644
> --- a/src/openvpn/multi.c
> +++ b/src/openvpn/multi.c
> @@ -1777,7 +1777,7 @@ multi_client_connect_setenv(struct multi_context *m,
> * - choosen cipher
> * - peer id
> */
> -static void
> +static bool
> multi_client_set_protocol_options(struct context *c)
> {
>
> @@ -1807,56 +1807,85 @@ multi_client_set_protocol_options(struct context *c)
> }
>
> /* Select cipher if client supports Negotiable Crypto Parameters */
> - if (o->ncp_enabled)
> + if (!o->ncp_enabled)
> {
> - /* if we have already created our key, we cannot *change* our own
> - * cipher -> so log the fact and push the "what we have now" cipher
> - * (so the client is always told what we expect it to use)
> - */
> - const struct tls_session *session = &tls_multi->session[TM_ACTIVE];
> - if (session->key[KS_PRIMARY].crypto_options.key_ctx_bi.initialized)
> + return true;
> + }
> +
> + /* if we have already created our key, we cannot *change* our own
> + * cipher -> so log the fact and push the "what we have now" cipher
> + * (so the client is always told what we expect it to use)
> + */
> + const struct tls_session *session = &tls_multi->session[TM_ACTIVE];
> + if (session->key[KS_PRIMARY].crypto_options.key_ctx_bi.initialized)
> + {
> + msg( M_INFO, "PUSH: client wants to negotiate cipher (NCP), but "
No space after ( .
> + "server has already generated data channel keys, "
> + "re-sending previously negotiated cipher '%s'",
> + o->ciphername );
> + return true;
> + }
> +
> + /*
> + * Push the first cipher from --data-ciphers to the client that
> + * the client announces to be supporting.
> + */
> + char *push_cipher = ncp_get_best_cipher(o->ncp_ciphers, peer_info,
> + tls_multi->remote_ciphername,
> + &o->gc);
> +
> + if (push_cipher)
> + {
> + o->ciphername = push_cipher;
> + return true;
> + }
> +
> + /* NCP cipher negotiation failed. Try to figure out why exactly it
> + * failed and give good error messages and potentially do a fallback
> + * for non NCP clients */
> + struct gc_arena gc = gc_new();
> + bool ret = false;
> +
> + const char *peer_ciphers = tls_peer_ncp_list(peer_info, &gc);
> + /* If we are in a situation where we know the client ciphers, there is no
> + * reason to fall back to a cipher that will not be accepted by the other
> + * side, in this situation we fail the auth*/
> + if (strlen(peer_ciphers) > 0)
> + {
> + msg(M_INFO, "PUSH: No common cipher between server and client. "
> + "Server data-ciphers: '%s', client supported ciphers '%s'",
> + o->ncp_ciphers, peer_ciphers);
> + }
> + else if (tls_multi->remote_ciphername)
> + {
> + msg(M_INFO, "PUSH: No common cipher between server and client. "
> + "Server data-ciphers: '%s', client supports cipher '%s'",
> + o->ncp_ciphers, tls_multi->remote_ciphername);
> + }
> + else
> + {
> + msg(M_INFO, "PUSH: No NCP or OCC cipher data received from peer.");
> +
> + if (o->enable_ncp_fallback && !tls_multi->remote_ciphername)
> {
> - msg( M_INFO, "PUSH: client wants to negotiate cipher (NCP), but "
> - "server has already generated data channel keys, "
> - "re-sending previously negotiated cipher '%s'",
> - o->ciphername );
> + msg(M_INFO, "Using data channel cipher '%s' since "
> + "--data-ciphers-fallback is set.", o->ciphername);
> + ret = true;
> }
> else
> {
> - /*
> - * Push the first cipher from --data-ciphers to the client that
> - * the client announces to be supporting.
> - */
> - char *push_cipher = ncp_get_best_cipher(o->ncp_ciphers,
> o->ciphername,
> - peer_info,
> -
> tls_multi->remote_ciphername,
> - &o->gc);
> -
> - if (push_cipher)
> - {
> - o->ciphername = push_cipher;
> - }
> - else
> - {
> - struct gc_arena gc = gc_new();
> - const char *peer_ciphers = tls_peer_ncp_list(peer_info, &gc);
> - if (strlen(peer_ciphers) > 0)
> - {
> - msg(M_INFO, "PUSH: No common cipher between server and "
> - "client. Expect this connection not to work. Server "
> - "data-ciphers: '%s', client supported ciphers '%s'",
> - o->ncp_ciphers, peer_ciphers);
> - }
> - else
> - {
> - msg(M_INFO, "No NCP data received from peer, falling
> back "
> - "to --cipher '%s'. Peer reports in OCC --cipher
> '%s'",
> - o->ciphername, np(tls_multi->remote_ciphername));
> - }
> - gc_free(&gc);
> - }
> + msg(M_INFO, "Use --data-ciphers-fallback with the cipher the "
> + "client is using if you want to allow the client to
> connect");
> }
> }
> + if (!ret)
> + {
> + auth_set_client_reason(tls_multi, "Data channel cipher negotiation "
> + "failed (no shared cipher)");
> + }
> +
> + gc_free(&gc);
> + return ret;
> }
>
> /**
> @@ -2322,7 +2351,7 @@ multi_client_connect_late_setup(struct multi_context *m,
> if (!mi->context.c2.push_ifconfig_defined)
> {
> msg(D_MULTI_ERRORS, "MULTI: no dynamic or static remote"
> - "--ifconfig address is available for %s",
> + "--ifconfig address is available for %s",
> multi_instance_string(mi, false, &gc));
> }
>
> @@ -2338,7 +2367,7 @@ multi_client_connect_late_setup(struct multi_context *m,
>
> /* JYFIXME -- this should cause the connection to fail */
> msg(D_MULTI_ERRORS, "MULTI ERROR: primary virtual IP for %s (%s)"
> - "violates tunnel network/netmask constraint (%s/%s)",
> + "violates tunnel network/netmask constraint
> (%s/%s)",
> multi_instance_string(mi, false, &gc),
> print_in_addr_t(mi->context.c2.push_ifconfig_local, 0, &gc),
> ifconfig_constraint_network, ifconfig_constraint_netmask);
> @@ -2387,7 +2416,7 @@ multi_client_connect_late_setup(struct multi_context *m,
> else if (mi->context.options.iroutes)
> {
> msg(D_MULTI_ERRORS, "MULTI: --iroute options rejected for %s --
> iroute"
> - "only works with tun-style tunnels",
> + "only works with tun-style tunnels",
> multi_instance_string(mi, false, &gc));
> }
>
> @@ -2399,11 +2428,15 @@ multi_client_connect_late_setup(struct multi_context
> *m,
> mi->context.c2.context_auth = CAS_SUCCEEDED;
>
> /* authentication complete, calculate dynamic client specific options */
> - multi_client_set_protocol_options(&mi->context);
> -
> - /* Generate data channel keys */
> - if (!multi_client_generate_tls_keys(&mi->context))
> + if (!multi_client_set_protocol_options(&mi->context))
> + {
> + mi->context.c2.context_auth = CAS_FAILED;
> + }
> + /* Generate data channel keys only if setting protocol options
> + * has not failed */
> + else if (!multi_client_generate_tls_keys(&mi->context))
> {
> +
> mi->context.c2.context_auth = CAS_FAILED;
> }
>
> diff --git a/src/openvpn/options.c b/src/openvpn/options.c
> index bc256b18..c53ef7f9 100644
> --- a/src/openvpn/options.c
> +++ b/src/openvpn/options.c
> @@ -855,7 +855,6 @@ init_options(struct options *o, const bool init_gc)
> #if P2MP
> o->scheduled_exit_interval = 5;
> #endif
> - o->ciphername = "BF-CBC";
> o->ncp_enabled = true;
> o->ncp_ciphers = "AES-256-GCM:AES-128-GCM";
> o->authname = "SHA1";
> @@ -2053,7 +2052,7 @@ options_postprocess_verify_ce(const struct options
> *options, const struct connec
> if (options->inetd)
> {
> msg(M_WARN, "DEPRECATED OPTION: --inetd mode is deprecated "
> - "and will be removed in OpenVPN 2.6");
> + "and will be removed in OpenVPN 2.6");
> }
>
> if (options->lladdr && dev != DEV_TYPE_TAP)
> @@ -3046,6 +3045,67 @@ options_postprocess_verify(const struct options *o)
> }
> }
>
> +static void
> +options_postprocess_cipher(struct options *o)
> +{
> + if (!o->pull && !(o->mode == MODE_SERVER))
> + {
> + /* we are in the classic P2P mode */
> + o->ncp_enabled = false;
> + msg( M_WARN, "Cipher negotiation is disabled since neither "
> + "P2MP client nor server mode is enabled");
> +
> + /* If the cipher is not set, use the old default ofo BF-CBC. We will
> + * warn that this is deprecated on cipher initialisation, no need
> + * to warn here as well */
> + if (!o->ciphername)
> + {
> + o->ciphername = "BF-CBC";
> + }
> + return;
> + }
> +
> + /* pull or P2MP mode */
> + if (!o->ciphername)
> + {
> + if (!o->ncp_enabled)
> + {
> + msg(M_USAGE, "--ncp-disable needs an explicit --cipher or "
> + "--data-ciphers-fallback config option");
> + }
> +
> + msg(M_WARN, "--cipher is not set. Previous OpenVPN version defaulted
> to "
> + "BF-CBC as fallback when cipher negotiation failed in this case.
> "
> + "If you need this fallback please add '--data-ciphers-fallback "
> + "BF-CBC' to your configuration and/or add BF-CBC to "
> + "--data-ciphers.");
> +
> + /* We still need to set the ciphername to BF-CBC since various other
> + * parts of OpenVPN assert that the ciphername is set */
> + o->ciphername = "BF-CBC";
> + }
> + else if (!o->enable_ncp_fallback
> + && !tls_item_in_cipher_list(o->ciphername, o->ncp_ciphers))
> + {
> + msg(M_WARN, "DEPRECATED OPTION: --cipher set to '%s' but missing in"
> + " --data-ciphers (%s). Future OpenVPN version will "
> + "ignore --cipher for cipher negotiations. "
> + "Add '%s' to --data-ciphers or change --cipher '%s' to "
> + "--data-ciphers-fallback '%s' to silence this warning.",
> + o->ciphername, o->ncp_ciphers, o->ciphername,
> + o->ciphername, o->ciphername);
> + o->enable_ncp_fallback = true;
> +
> + /* Append the --cipher to ncp_ciphers to allow it in NCP */
> + size_t newlen = strlen(o->ncp_ciphers) + 1 + strlen(o->ciphername)
> +1;
Missing space after the last + .
> + char *ncp_ciphers = gc_malloc(newlen, false, &o->gc);
> +
> + ASSERT(openvpn_snprintf(ncp_ciphers, newlen, "%s:%s", o->ncp_ciphers,
> + o->ciphername));
> + o->ncp_ciphers = ncp_ciphers;
> + }
> +}
> +
> static void
> options_postprocess_mutate(struct options *o)
> {
> @@ -3058,6 +3118,7 @@ options_postprocess_mutate(struct options *o)
> helper_keepalive(o);
> helper_tcp_nodelay(o);
>
> + options_postprocess_cipher(o);
> options_postprocess_mutate_invariant(o);
>
> if (o->ncp_enabled)
> @@ -3118,16 +3179,6 @@ options_postprocess_mutate(struct options *o)
> "include this in your server configuration");
> o->dh_file = NULL;
> }
> -
> - /* cipher negotiation (NCP) currently assumes --pull or --mode server */
> - if (o->ncp_enabled
> - && !(o->pull || o->mode == MODE_SERVER) )
> - {
> - msg( M_WARN, "disabling NCP mode (--ncp-disable) because not "
> - "in P2MP client or server mode" );
> - o->ncp_enabled = false;
> - }
> -
> #if ENABLE_MANAGEMENT
> if (o->http_proxy_override)
> {
> @@ -3663,14 +3714,21 @@ options_string(const struct options *o,
> */
>
> buf_printf(&out, ",dev-type %s", dev_type_string(o->dev, o->dev_type));
> - buf_printf(&out, ",link-mtu %u", (unsigned int)
> calc_options_string_link_mtu(o, frame));
> + /* the link-mtu that we send has only a meaning if have a fixed
> + * cipher (p2p) or have a fallback cipher configured for older non
> + * ncp clients. But not sending it, will make even 2.4 complain
> + * about it missing. So still send it. */
> + buf_printf(&out, ",link-mtu %u",
> + (unsigned int) calc_options_string_link_mtu(o, frame));
> +
> buf_printf(&out, ",tun-mtu %d", PAYLOAD_SIZE(frame));
> buf_printf(&out, ",proto %s", proto_remote(o->ce.proto, remote));
>
> + bool p2p_nopull = o->mode == MODE_POINT_TO_POINT && !PULL_DEFINED(o);
> /* send tun_ipv6 only in peer2peer mode - in client/server mode, it
> * is usually pushed by the server, triggering a non-helpful warning
> */
> - if (o->ifconfig_ipv6_local && o->mode == MODE_POINT_TO_POINT &&
> !PULL_DEFINED(o))
> + if (o->ifconfig_ipv6_local && p2p_nopull)
> {
> buf_printf(&out, ",tun-ipv6");
> }
> @@ -3700,7 +3758,7 @@ options_string(const struct options *o,
> }
> }
>
> - if (tt && o->mode == MODE_POINT_TO_POINT && !PULL_DEFINED(o))
> + if (tt && p2p_nopull)
> {
> const char *ios = ifconfig_options_string(tt, remote,
> o->ifconfig_nowarn, gc);
> if (ios && strlen(ios))
> @@ -3756,8 +3814,14 @@ options_string(const struct options *o,
>
> init_key_type(&kt, o->ciphername, o->authname, o->keysize, true,
> false);
> -
> - buf_printf(&out, ",cipher %s", cipher_kt_name(kt.cipher));
> + /* Only announce the cipher to our peer if we are willing to
> + * support it */
> + const char *ciphername = cipher_kt_name(kt.cipher);
> + if (p2p_nopull || !o->ncp_enabled
> + || tls_item_in_cipher_list(ciphername, o->ncp_ciphers))
> + {
> + buf_printf(&out, ",cipher %s", ciphername);
> + }
> buf_printf(&out, ",auth %s", md_kt_name(kt.digest));
> buf_printf(&out, ",keysize %d", kt.cipher_length * 8);
> if (o->shared_secret_file)
> @@ -3875,7 +3939,8 @@ options_warning_safe_scan2(const int msglevel,
> || strprefix(p1, "keydir ")
> || strprefix(p1, "proto ")
> || strprefix(p1, "tls-auth ")
> - || strprefix(p1, "tun-ipv6"))
> + || strprefix(p1, "tun-ipv6")
> + || strprefix(p1, "cipher "))
> {
> return;
> }
> @@ -7863,14 +7928,20 @@ add_option(struct options *options,
> VERIFY_PERMISSION(OPT_P_NCP|OPT_P_INSTANCE);
> options->ciphername = p[1];
> }
> + else if (streq(p[0], "data-ciphers-fallback") && p[1] && !p[2])
> + {
> + VERIFY_PERMISSION(OPT_P_GENERAL|OPT_P_INSTANCE);
> + options->ciphername = p[1];
> + options->enable_ncp_fallback = true;
> + }
> else if ((streq(p[0], "data-ciphers") || streq(p[0], "ncp-ciphers"))
> - && p[1] && !p[2])
> + && p[1] && !p[2])
> {
> VERIFY_PERMISSION(OPT_P_GENERAL|OPT_P_INSTANCE);
> if (streq(p[0], "ncp-ciphers"))
> {
> msg(M_INFO, "Note: Treating option '--ncp-ciphers' as "
> - " '--data-ciphers' (renamed in OpenVPN 2.5).");
> + " '--data-ciphers' (renamed in OpenVPN 2.5).");
> }
> options->ncp_ciphers = p[1];
> }
> @@ -7878,9 +7949,9 @@ add_option(struct options *options,
> {
> VERIFY_PERMISSION(OPT_P_GENERAL|OPT_P_INSTANCE);
> options->ncp_enabled = false;
> - msg(M_WARN, "DEPRECATED OPTION: ncp-disable. Disabling dynamic "
> - "cipher negotiation is a deprecated debug feature that "
> - "will be removed in OpenVPN 2.6");
> + msg(M_WARN, "DEPRECATED OPTION: ncp-disable. Disabling "
> + "cipher negotiation is a deprecated debug feature that "
> + "will be removed in OpenVPN 2.6");
> }
> else if (streq(p[0], "prng") && p[1] && !p[3])
> {
> diff --git a/src/openvpn/options.h b/src/openvpn/options.h
> index c5df2d18..877e9396 100644
> --- a/src/openvpn/options.h
> +++ b/src/openvpn/options.h
> @@ -503,6 +503,8 @@ struct options
> bool shared_secret_file_inline;
> int key_direction;
> const char *ciphername;
> + bool enable_ncp_fallback; /**< If defined fall back to
> + * ciphername if NCP fails */
> bool ncp_enabled;
> const char *ncp_ciphers;
> const char *authname;
> diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
> index 91ab3bf6..06dc9f8f 100644
> --- a/src/openvpn/ssl.c
> +++ b/src/openvpn/ssl.c
> @@ -1932,13 +1932,14 @@ tls_session_update_crypto_params(struct tls_session
> *session,
> return true;
> }
>
> - if (!session->opt->server
> - && 0 != strcmp(options->ciphername, session->opt->config_ciphername)
> + bool cipher_allowed_as_fallback = options->enable_ncp_fallback
> + && streq(options->ciphername,
> session->opt->config_ciphername);
> +
> + if (!session->opt->server && !cipher_allowed_as_fallback
> && !tls_item_in_cipher_list(options->ciphername,
> options->ncp_ciphers))
> {
> - msg(D_TLS_ERRORS, "Error: pushed cipher not allowed - %s not in %s
> or %s",
> - options->ciphername, session->opt->config_ciphername,
> - options->ncp_ciphers);
> + msg(D_TLS_ERRORS, "Error: pushed cipher not allowed - %s not in %s",
> + options->ciphername, options->ncp_ciphers);
> /* undo cipher push, abort connection setup */
> options->ciphername = session->opt->config_ciphername;
> return false;
> @@ -1956,9 +1957,9 @@ tls_session_update_crypto_params(struct tls_session
> *session,
> }
> else
> {
> - /* Very hacky workaround and quick fix for frame calculation
> - * different when adjusting frame size when the original and new cipher
> - * are identical to avoid a regression with client without NCP */
> + /* Very hacky workaround and quick fix for frame calculation
> + * different when adjusting frame size when the original and new
> cipher
> + * are identical to avoid a regression with client without NCP */
> return tls_session_generate_data_channel_keys(session);
> }
>
> diff --git a/src/openvpn/ssl_ncp.c b/src/openvpn/ssl_ncp.c
> index 8e432fb7..2d3983c2 100644
> --- a/src/openvpn/ssl_ncp.c
> +++ b/src/openvpn/ssl_ncp.c
> @@ -48,6 +48,7 @@
> #include "common.h"
>
> #include "ssl_ncp.h"
> +#include "openvpn.h"
>
> /**
> * Return the Negotiable Crypto Parameters version advertised in the peer
> info
> @@ -211,9 +212,8 @@ tls_peer_ncp_list(const char *peer_info, struct gc_arena
> *gc)
> }
>
> char *
> -ncp_get_best_cipher(const char *server_list, const char *server_cipher,
> - const char *peer_info, const char *remote_cipher,
> - struct gc_arena *gc)
> +ncp_get_best_cipher(const char *server_list, const char *peer_info,
> + const char *remote_cipher, struct gc_arena *gc)
> {
> /*
> * The gc of the parameter is tied to the VPN session, create a
> @@ -226,7 +226,9 @@ ncp_get_best_cipher(const char *server_list, const char
> *server_cipher,
> const char *peer_ncp_list = tls_peer_ncp_list(peer_info, &gc_tmp);
>
> /* non-NCP client without OCC? "assume nothing" */
> - if (remote_cipher == NULL)
> + /* For client doing the newer version of NCP (that send IV_CIPHER)
> + * we cannot assume that they will accept remote_cipher */
> + if (remote_cipher == NULL || strstr(peer_info, "IV_CIPHERS="))
Just noting the missing NULL check that Gert found with testing. Can you
add a regression test while at it?
> {
> remote_cipher = "";
> }
> @@ -242,15 +244,6 @@ ncp_get_best_cipher(const char *server_list, const char
> *server_cipher,
> break;
> }
> }
> - /* We have not found a common cipher, as a last resort check if the
> - * server cipher can be used
> - */
> - if (token == NULL
> - && (tls_item_in_cipher_list(server_cipher, peer_ncp_list)
> - || streq(server_cipher, remote_cipher)))
> - {
> - token = server_cipher;
> - }
>
> char *ret = NULL;
> if (token != NULL)
> @@ -262,16 +255,75 @@ ncp_get_best_cipher(const char *server_list, const char
> *server_cipher,
> return ret;
> }
>
> -void
> +/**
> + * "Poor man's NCP": Use peer cipher if it is an allowed (NCP) cipher.
> + * Allows non-NCP peers to upgrade their cipher individually.
> + *
> + * Returns true if we switched to the peer's cipher
> + *
> + * Make sure to call tls_session_update_crypto_params() after calling this
> + * function.
> + */
> +static bool
> tls_poor_mans_ncp(struct options *o, const char *remote_ciphername)
> {
> - if (o->ncp_enabled && remote_ciphername
> + if (remote_ciphername
> && 0 != strcmp(o->ciphername, remote_ciphername))
> {
> if (tls_item_in_cipher_list(remote_ciphername, o->ncp_ciphers))
> {
> o->ciphername = string_alloc(remote_ciphername, &o->gc);
> msg(D_TLS_DEBUG_LOW, "Using peer cipher '%s'", o->ciphername);
> + return true;
> }
> }
> + return false;
> }
> +
> +bool
> +check_pull_client_ncp(struct context *c, const int found)
> +{
> + if (found & OPT_P_NCP)
> + {
> + msg(D_PUSH, "OPTIONS IMPORT: data channel crypto options modified");
> + return true;
> + }
> +
> + if (!c->options.ncp_enabled)
> + {
> + return true;
> + }
> + /* If the server did not push a --cipher, we will switch to the
> + * remote cipher if it is in our ncp-ciphers list */
> + bool useremotecipher = tls_poor_mans_ncp(&c->options,
> +
> c->c2.tls_multi->remote_ciphername);
> +
> +
> + /* We could not figure out the peer's cipher but we have fallback
> + * enable */
enableD.
> + if (!useremotecipher && c->options.enable_ncp_fallback)
> + {
> + return true;
> + }
> +
> + /* We failed negotiation, give appropiate error message */
> + if (c->c2.tls_multi->remote_ciphername)
> + {
> + msg(D_TLS_ERRORS, "OPTIONS ERROR: failed to negotiate "
> + "cipher with server. Add the server's "
> + "cipher ('%s') to --data-ciphers (currently '%s') if "
> + "you want to connect to this server.",
> + c->c2.tls_multi->remote_ciphername,
> + c->options.ncp_ciphers);
> + return false;
> +
> + }
> + else
> + {
> + msg(D_TLS_ERRORS, "OPTIONS ERROR: failed to negotiate "
> + "cipher with server. Configure "
> + "--data-ciphers-fallback if you want to connect "
> + "to this server.");
> + return false;
> + }
> +}
> \ No newline at end of file
> diff --git a/src/openvpn/ssl_ncp.h b/src/openvpn/ssl_ncp.h
> index d00c222d..39158a56 100644
> --- a/src/openvpn/ssl_ncp.h
> +++ b/src/openvpn/ssl_ncp.h
> @@ -40,14 +40,17 @@
> bool
> tls_peer_supports_ncp(const char *peer_info);
>
> +/* forward declaration to break include dependency loop */
> +struct context;
> +
> /**
> - * "Poor man's NCP": Use peer cipher if it is an allowed (NCP) cipher.
> - * Allows non-NCP peers to upgrade their cipher individually.
> + * Checks whether the cipher negotiation is in an acceptable state
> + * and we continue to connect or should abort.
> *
> - * Make sure to call tls_session_update_crypto_params() after calling this
> - * function.
> + * @return Wether the client NCP process suceeded or failed
> */
> -void tls_poor_mans_ncp(struct options *o, const char *remote_ciphername);
> +bool
> +check_pull_client_ncp(struct context *c, int found);
>
> /**
> * Iterates through the ciphers in server_list and return the first
> @@ -67,9 +70,8 @@ void tls_poor_mans_ncp(struct options *o, const char
> *remote_ciphername);
> * cipher
> */
> char *
> -ncp_get_best_cipher(const char *server_list, const char *server_cipher,
> - const char *peer_info, const char *remote_cipher,
> - struct gc_arena *gc);
> +ncp_get_best_cipher(const char *server_list, const char *peer_info,
> + const char *remote_cipher, struct gc_arena *gc);
>
>
> /**
> diff --git a/tests/unit_tests/openvpn/test_ncp.c
> b/tests/unit_tests/openvpn/test_ncp.c
> index 19432410..ea950030 100644
> --- a/tests/unit_tests/openvpn/test_ncp.c
> +++ b/tests/unit_tests/openvpn/test_ncp.c
> @@ -139,21 +139,29 @@ test_poor_man(void **state)
> char *best_cipher;
>
> const char *serverlist = "CHACHA20_POLY1305:AES-128-GCM";
> + const char *serverlistbfcbc = "CHACHA20_POLY1305:AES-128-GCM:BF-CBC";
>
> - best_cipher = ncp_get_best_cipher(serverlist, "BF-CBC",
> + best_cipher = ncp_get_best_cipher(serverlist,
> + "IV_YOLO=NO\nIV_BAR=7",
> + "BF-CBC", &gc);
> +
> + assert_ptr_equal(best_cipher, NULL);
> +
> +
> + best_cipher = ncp_get_best_cipher(serverlistbfcbc,
> "IV_YOLO=NO\nIV_BAR=7",
> "BF-CBC", &gc);
>
> assert_string_equal(best_cipher, "BF-CBC");
>
> - best_cipher = ncp_get_best_cipher(serverlist, "BF-CBC",
> +
> + best_cipher = ncp_get_best_cipher(serverlist,
> "IV_NCP=1\nIV_BAR=7",
> "AES-128-GCM", &gc);
>
> assert_string_equal(best_cipher, "AES-128-GCM");
>
> - best_cipher = ncp_get_best_cipher(serverlist, "BF-CBC",
> - NULL,
> + best_cipher = ncp_get_best_cipher(serverlist, NULL,
> "AES-128-GCM", &gc);
>
> assert_string_equal(best_cipher, "AES-128-GCM");
> @@ -170,29 +178,27 @@ test_ncp_best(void **state)
>
> const char *serverlist = "CHACHA20_POLY1305:AES-128-GCM:AES-256-GCM";
>
> - best_cipher = ncp_get_best_cipher(serverlist, "BF-CBC",
> + best_cipher = ncp_get_best_cipher(serverlist,
> "IV_YOLO=NO\nIV_NCP=2\nIV_BAR=7",
> "BF-CBC", &gc);
>
> assert_string_equal(best_cipher, "AES-128-GCM");
>
> /* Best cipher is in --cipher of client */
> - best_cipher = ncp_get_best_cipher(serverlist, "BF-CBC",
> - "IV_NCP=2\nIV_BAR=7",
> + best_cipher = ncp_get_best_cipher(serverlist, "IV_NCP=2\nIV_BAR=7",
> "CHACHA20_POLY1305", &gc);
>
> assert_string_equal(best_cipher, "CHACHA20_POLY1305");
>
> /* Best cipher is in --cipher of client */
> - best_cipher = ncp_get_best_cipher(serverlist, "BF-CBC",
> - "IV_CIPHERS=AES-128-GCM",
> + best_cipher = ncp_get_best_cipher(serverlist, "IV_CIPHERS=AES-128-GCM",
> "AES-256-CBC", &gc);
>
>
> assert_string_equal(best_cipher, "AES-128-GCM");
>
> /* IV_NCP=2 should be ignored if IV_CIPHERS is sent */
> - best_cipher = ncp_get_best_cipher(serverlist, "BF-CBC",
> + best_cipher = ncp_get_best_cipher(serverlist,
>
> "IV_FOO=7\nIV_CIPHERS=AES-256-GCM\nIV_NCP=2",
> "AES-256-CBC", &gc);
>
>
I still try to find time to do more review and testing, but don't wait
for me if someone else has taken a good look and/or given this patch a
good beating.
-Steffan
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