[Openvpn-devel,v2] Always disable TLS renegotiations

Message ID 20210326160545.26836-1-arne@rfc2549.org
State Changes Requested
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  • [Openvpn-devel,v2] Always disable TLS renegotiations
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Commit Message

Arne Schwabe March 26, 2021, 4:05 p.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.

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

Patch V2: Improve commments and commit message

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(+)

Comments

Antonio Quartulli March 26, 2021, 4:37 p.m. | #1
Hi,

On 26/03/2021 17:05, Arne Schwabe wrote:
> 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

too many f

> parameters (e.g. now with client certs). This somethign that OpenVPN does

somethign -> something

> not use.
> 
> Furthermore because of all these problems, also TLS 1.3 completely
> drops support for renegotiations.
> 
> Patch V2: Improve commments and commit message

too many m :D

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

Acked-by: Antonio Quartulli <antonio@openvpn.net>


Basic connection tests passed, with OpenVPN renegotiations performed
with no issue.
Gert Doering March 26, 2021, 4:56 p.m. | #2
Hi,

On Fri, Mar 26, 2021 at 05:05:45PM +0100, Arne Schwabe wrote:
> +    /* 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;

Unfortunately this seems to be not available in OpenSSL 1.0.2, which
is what the OS ships on FreeBSD 11.4 (which is still supported).

../../../openvpn/src/openvpn/ssl_openssl.c:326:15: error: use of undeclared identifier
      'SSL_OP_NO_RENEGOTIATION'
    sslopt |= SSL_OP_NO_RENEGOTIATION;

gert

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 d161f48b8..c311dd08e 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);