[Openvpn-devel,v3] Always disable TLS renegotiations

Message ID 20210401105918.19396-1-arne@rfc2549.org
State Superseded
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  • [Openvpn-devel,v3] Always disable TLS renegotiations
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Commit Message

Arne Schwabe April 1, 2021, 10:59 a.m.
Renegotiations have been troublesome in the past and also the recent OpenSSL
security problem (CVE-2021-3449) is only exploitable if TLS renegotiation
is enabled.

mbed TLS disables it by default and says in the documentation:

Warning: It is recommended to always disable renegotation unless you know you
need it and you know what you're doing. In the past, there have been
several issues associated with renegotiation or a poor understanding of
its properties.

TLS renegotiation can be used to restart a session with diffferent
parameters (e.g. now with client certs). This somethign that OpenVPN does
not use.

For OpenSSL 1.0.2 the workaround to disable renegotiation is rather
cumbersome. So we keep this to 1.1.1 only since 1.0.2 is on its way to
deprecation anyway.

Furthermore because of all these problems, also TLS 1.3 completely
drops support for renegotiations.

Patch V2: Improve commments and commit message
Patch V3: Only disable renegotiation where the SSL_OP_NO_RENEGOTIATION
          define is available. LibreSSL, wolfSSL and OpenSSL 1.0.2 are
          lacking this macro.
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
---
 src/openvpn/ssl_mbedtls.c | 4 ++++
 src/openvpn/ssl_openssl.c | 4 ++++
 2 files changed, 8 insertions(+)

Patch

diff --git a/src/openvpn/ssl_mbedtls.c b/src/openvpn/ssl_mbedtls.c
index 4626e9838..8917fb188 100644
--- a/src/openvpn/ssl_mbedtls.c
+++ b/src/openvpn/ssl_mbedtls.c
@@ -1086,6 +1086,10 @@  key_state_ssl_init(struct key_state_ssl *ks_ssl,
     {
         mbedtls_ssl_conf_curves(ks_ssl->ssl_config, ssl_ctx->groups);
     }
+    /* Disable TLS renegotiations. OpenVPN's renegotiation creates new SSL
+     * session and does not depend on this feature. And TLS renegotiations have
+     * been problematic in the past */
+    mbedtls_ssl_conf_renegotiation(ks_ssl->ssl_config, MBEDTLS_SSL_RENEGOTIATION_DISABLED);
 
     /* Disable record splitting (for now).  OpenVPN assumes records are sent
      * unfragmented, and changing that will require thorough review and
diff --git a/src/openvpn/ssl_openssl.c b/src/openvpn/ssl_openssl.c
index b85f95be1..1f14b5d2c 100644
--- a/src/openvpn/ssl_openssl.c
+++ b/src/openvpn/ssl_openssl.c
@@ -320,6 +320,10 @@  tls_ctx_set_options(struct tls_root_ctx *ctx, unsigned int ssl_flags)
     sslopt |= SSL_OP_CIPHER_SERVER_PREFERENCE;
 #endif
     sslopt |= SSL_OP_NO_COMPRESSION;
+    /* Disable TLS renegotiations. OpenVPN's renegotiation creates new SSL
+     * session and does not depend on this feature. And TLS renegotiations have
+     * been problematic in the past */
+    sslopt |= SSL_OP_NO_RENEGOTIATION;
 
     SSL_CTX_set_options(ctx->ctx, sslopt);