[Openvpn-devel,v3] Allow running a default configuration with TLS libraries without BF-CBC

Message ID 20210215143146.9770-1-arne@rfc2549.org
State Superseded
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Series
  • [Openvpn-devel,v3] Allow running a default configuration with TLS libraries without BF-CBC
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Commit Message

Arne Schwabe Feb. 15, 2021, 2:31 p.m.
Modern TLS libraries might drop Blowfish by default or distributions
might disable Blowfish in OpenSSL/mbed TLS. We still signal OCC
options with BF-CBC compatible strings. To avoid requiring BF-CBC
for this, special this one usage of BF-CBC enough to avoid a hard
requirement on Blowfish in the default configuration.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>

Patch v2: add more clarifying comment, do not warn about OCC only insecure
          ciphers, code improvements

Patch V3: Put ciphername resolution via ciper_kt_name in the right branch
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
---
 src/openvpn/crypto_backend.h |  2 ++
 src/openvpn/init.c           | 32 ++++++++++++++++++------
 src/openvpn/options.c        | 47 +++++++++++++++++++++++++++++++-----
 3 files changed, 67 insertions(+), 14 deletions(-)

Comments

Gert Doering Feb. 16, 2021, 7:32 p.m. | #1
Hi,

On Mon, Feb 15, 2021 at 03:31:46PM +0100, Arne Schwabe wrote:
> Modern TLS libraries might drop Blowfish by default or distributions
> might disable Blowfish in OpenSSL/mbed TLS. We still signal OCC
> options with BF-CBC compatible strings. To avoid requiring BF-CBC
> for this, special this one usage of BF-CBC enough to avoid a hard
> requirement on Blowfish in the default configuration.
> 
> Signed-off-by: Arne Schwabe <arne@rfc2549.org>
> 
> Patch v2: add more clarifying comment, do not warn about OCC only insecure
>           ciphers, code improvements
> 
> Patch V3: Put ciphername resolution via ciper_kt_name in the right branch

This still fails one of my test cases - but only one (v2 failed two).

The test case is "udp / p2mp tun, 2.4 server with --ncp-disable" (on
the server).

The client is called as 
   ... --dev tun --proto udp --data-ciphers AES-256-GCM:AES-128-GCM:BF-CBC

(no --cipher setting) and logS

2021-02-16 20:04:51 --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.
...
2021-02-16 20:04:53 PUSH: Received control message: 'PUSH_REPLY,route 10.194.0.0255.255.0.0,route-ipv6 fd00:abcd:194::/48,tun-ipv6,route 10.194.103.1,topology net30,ping 10,ping-restart 30,ifconfig-ipv6 fd00:abcd:194:103::1000/64 fd00:abcd:194:103::1,ifconfig 10.194.103.6 10.194.103.5,peer-id 0'
2021-02-16 20:04:53 Using peer cipher 'BF-CBC'

but something seems to get confused about things:

2021-02-16 20:04:53 Initialization Sequence Completed
2021-02-16 20:05:03 Bad LZO decompression header byte: 166


The server agrees on BF-CBC (same log, different time zone):

Feb 16 14:04:51 phillip tun-udp-p2mp-2.4-noncp[29923]: 2001:608:0:814::f000:11 peer info: IV_CIPHERS=AES-256-GCM:AES-128-GCM:BF-CBC
Feb 16 14:04:51 phillip tun-udp-p2mp-2.4-noncp[29923]: 2001:608:0:814::f000:11 Outgoing Data Channel: Cipher 'BF-CBC' initialized with 128 bit key
Feb 16 14:04:51 phillip tun-udp-p2mp-2.4-noncp[29923]: 2001:608:0:814::f000:11 Incoming Data Channel: Cipher 'BF-CBC' initialized with 128 bit key

but packets fail decryption:

Feb 16 14:04:53 phillip tun-udp-p2mp-2.4-noncp[29923]: cron2-gentoo.ov-amd64/2001:608:0:814::f000:11 Authenticate/Decrypt packet error: packet HMAC authentication failed
Feb 16 14:05:06 phillip syslogd: last message repeated 103 times


Noticeable fact in the client log: there is no "Data Channel: Cipher..."
line on the client, so maybe this is triggering a new corner case?


Notice 2: there is a test case talking to a 2.3 server *which succeeds*,
but that one has "--cipher BF-CBC".  So what fails seems to be "cipher
initialization for 'implicit BF-CBC'".

gert

Patch

diff --git a/src/openvpn/crypto_backend.h b/src/openvpn/crypto_backend.h
index 384ffc80..93f7e475 100644
--- a/src/openvpn/crypto_backend.h
+++ b/src/openvpn/crypto_backend.h
@@ -241,6 +241,8 @@  const cipher_kt_t *cipher_kt_get(const char *ciphername);
  * The returned name is normalised to the OpenVPN config name in case the
  * name differs from the name used by the crypto library.
  *
+ * Returns [null-cipher] in case the cipher_kt is NULL.
+ *
  * @param cipher_kt     Static cipher parameters
  *
  * @return a statically allocated string describing the cipher.
diff --git a/src/openvpn/init.c b/src/openvpn/init.c
index 46c933b1..5cf257eb 100644
--- a/src/openvpn/init.c
+++ b/src/openvpn/init.c
@@ -2769,14 +2769,30 @@  do_init_crypto_tls_c1(struct context *c)
 #endif /* if P2MP */
         }
 
-        /* Do not warn if we 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, warn);
-
+       /*
+        * BF-CBC is allowed to be used only when explicitly configured
+        * as NCP-fallback or when NCP has been disabled.
+        * In all other cases don't attempt to initialize BF-CBC as it
+        * may not even be supported by the underlying SSL library.
+        *
+        * Therefore, the key structure has to be initialized when:
+        * - any non-BF-CBC cipher was selected; or
+        * - BF-CBC is selected and NCP is disabled (explicit request to
+        *   use the BF-CBC cipher); or
+        * - BF-CBC is selected, NCP is enabled and fallback is enabled
+        *   (BF-CBC will be the fallback).
+        *
+        * Note that BF-CBC will still be part of the OCC string to retain
+        * backwards compatibility with older clients.
+        */
+        if (!streq(options->ciphername, "BF-CBC") || !options->ncp_enabled
+            || options->enable_ncp_fallback)
+        {
+            /* Do not warn if the if the cipher is used only in OCC */
+            bool warn = !options->ncp_enabled || options->enable_ncp_fallback;
+            init_key_type(&c->c1.ks.key_type, options->ciphername, options->authname,
+                          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/options.c b/src/openvpn/options.c
index 059386b3..c02ad051 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -3609,9 +3609,29 @@  calc_options_string_link_mtu(const struct options *o, const struct frame *frame)
     {
         struct frame fake_frame = *frame;
         struct key_type fake_kt;
-        init_key_type(&fake_kt, o->ciphername, o->authname, o->keysize, true,
-                      false);
+
         frame_remove_from_extra_frame(&fake_frame, crypto_max_overhead());
+
+
+        /* o->ciphername might be BF-CBC even though the underlying SSL library
+         * does not support it. For this reason we workaround this corner case
+         * by pretending to have no encryption enabled and by manually adding
+         * the required packet overhead to the MTU computation.
+         */
+        const char* ciphername = o->ciphername;
+
+        if (strcmp(o->ciphername, "BF-CBC") == 0)
+        {
+            /* none has no overhead, so use this to later add only --auth
+             * overhead */
+
+            /* overhead of BF-CBC: 64 bit block size, 64 bit IV size */
+            frame_add_to_extra_frame(&fake_frame, 64/8 + 64/8);
+        }
+
+        init_key_type(&fake_kt, ciphername, o->authname, o->keysize, true,
+                      false);
+
         crypto_adjust_frame_parameters(&fake_frame, &fake_kt, o->replay,
                                        cipher_kt_mode_ofb_cfb(fake_kt.cipher));
         frame_finalize(&fake_frame, o->ce.link_mtu_defined, o->ce.link_mtu,
@@ -3781,18 +3801,33 @@  options_string(const struct options *o,
                + (TLS_SERVER == true)
                <= 1);
 
-        init_key_type(&kt, o->ciphername, o->authname, o->keysize, true,
-                      false);
+        /* Skip resolving BF-CBC to allow SSL libraries without BF-CBC
+         * to work here in the default configuration */
+        const char *ciphername = o->ciphername;
+        int keysize;
+
+        if (strcmp(o->ciphername, "BF-CBC") == 0)
+        {
+            init_key_type(&kt, "none", o->authname, o->keysize, true,
+                          false);
+            keysize = 128;
+        }
+        else
+        {
+            init_key_type(&kt, o->ciphername, o->authname, o->keysize, true,
+                          false);
+            ciphername = cipher_kt_name(kt.cipher);
+            keysize = kt.cipher_length * 8;
+        }
         /* 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);
+        buf_printf(&out, ",keysize %d", keysize);
         if (o->shared_secret_file)
         {
             buf_printf(&out, ",secret");