[Openvpn-devel,1/3] Refactor key_state_export_keying_material functions

This refactors the common code between mbed SSL and OpenSSL into
export_user_keying_material and also prepares the backend functions
to export more than one key.

Also fix checking the return value of SSL_export_keying_material
only 1 is a sucess, -1 is also an error.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
---
 src/openvpn/ssl.c         | 33 ++++++++++++++++++++++++-
 src/openvpn/ssl_backend.h | 18 ++++++++++++--
 src/openvpn/ssl_mbedtls.c | 22 ++++++-----------
 src/openvpn/ssl_openssl.c | 51 +++++++++++++++++++++------------------
 4 files changed, 83 insertions(+), 41 deletions(-)

Hi,

Couldn't resist giving this a quick look.

Feature-ACK on the patch set, but some comments on the approach:

On 12-08-2020 10:55, Arne Schwabe wrote:
> This refactors the common code between mbed SSL and OpenSSL into
> export_user_keying_material and also prepares the backend functions
> to export more than one key.
> 
> Also fix checking the return value of SSL_export_keying_material
> only 1 is a sucess, -1 is also an error.
> 
> Signed-off-by: Arne Schwabe <arne@rfc2549.org>
> ---
>  src/openvpn/ssl.c         | 33 ++++++++++++++++++++++++-
>  src/openvpn/ssl_backend.h | 18 ++++++++++++--
>  src/openvpn/ssl_mbedtls.c | 22 ++++++-----------
>  src/openvpn/ssl_openssl.c | 51 +++++++++++++++++++++------------------
>  4 files changed, 83 insertions(+), 41 deletions(-)
> 
> diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
> index f16114c2..390114e1 100644
> --- a/src/openvpn/ssl.c
> +++ b/src/openvpn/ssl.c
> @@ -2412,6 +2412,37 @@ error:
>      return false;
>  }
>  
> +static void
> +export_user_keying_material(struct key_state_ssl *ssl,
> +                            struct tls_session *session)
> +{
> +    if (session->opt->ekm_size > 0)
> +    {
> +        unsigned int size = session->opt->ekm_size;
> +        struct gc_arena gc = gc_new();
> +
> +        unsigned char *ekm;
> +        if ((ekm = key_state_export_keying_material(ssl, session,
> +                                                    EXPORT_KEY_USER, &gc)))
> +        {

Hmm, I don't think these abstractions are right. I would argue that the
crypto backends should know as few about OpenVPN specifics as possible.
So things like "EXPORT_KEY_USER" should probably not be passed to the
backend, and instead be handled in ssl.c.

Why not give the backend a prototype like

  key_state_export_keying_material(label, label_len, ekm, ekm_len)

I understand that this would mean an extra memcpy() for the mbedtls
code, but this is not performance-critical at all, right?

(In the follow-up patch, just cache the master secret and client/server
random, instead of the derived key material in the mbedtls backend, so
you can generate the export at will. For OpenSSL, this API should be
trivial.)

> +            unsigned int len = (size * 2) + 2;
> +
> +            const char *key = format_hex_ex(ekm, size, len, 0, NULL, &gc);
> +            setenv_str(session->opt->es, "exported_keying_material", key);
> +
> +            dmsg(D_TLS_DEBUG_MED, "%s: exported keying material: %s",
> +                 __func__, key);
> +            secure_memzero(ekm, size);
> +        }
> +        else
> +        {
> +            msg(M_WARN, "WARNING: Export keying material failed!");
> +            setenv_del(session->opt->es, "exported_keying_material");
> +        }
> +        gc_free(&gc);
> +    }
> +}
> +
>  /**
>   * Handle reading key data, peer-info, username/password, OCC
>   * from the TLS control channel (cleartext).
> @@ -2541,7 +2572,7 @@ key_method_2_read(struct buffer *buf, struct tls_multi *multi, struct tls_sessio
>      if ((ks->authenticated > KS_AUTH_FALSE)
>          && plugin_defined(session->opt->plugins, OPENVPN_PLUGIN_TLS_FINAL))
>      {
> -        key_state_export_keying_material(&ks->ks_ssl, session);
> +        export_user_keying_material(&ks->ks_ssl, session);
>  
>          if (plugin_call(session->opt->plugins, OPENVPN_PLUGIN_TLS_FINAL, NULL, NULL, session->opt->es) != OPENVPN_PLUGIN_FUNC_SUCCESS)
>          {
> diff --git a/src/openvpn/ssl_backend.h b/src/openvpn/ssl_backend.h
> index 7f52ab1e..8faaefd5 100644
> --- a/src/openvpn/ssl_backend.h
> +++ b/src/openvpn/ssl_backend.h
> @@ -389,18 +389,32 @@ void key_state_ssl_free(struct key_state_ssl *ks_ssl);
>  void backend_tls_ctx_reload_crl(struct tls_root_ctx *ssl_ctx,
>                                  const char *crl_file, bool crl_inline);
>  
> +
> +/* defines the different RFC5705 that are used in OpenVPN */

Make this /** to have it turn up nicely in the doxygen.

> +enum export_key_identifier {
> +    EXPORT_KEY_USER
> +};
> +
>  /**
>   * Keying Material Exporters [RFC 5705] allows additional keying material to be
>   * derived from existing TLS channel. This exported keying material can then be
>   * used for a variety of purposes.
>   *
> + * Note
> + *

Note ... what?

>   * @param ks_ssl       The SSL channel's state info
>   * @param session      The session associated with the given key_state
> + * @param key          The key to export.
> + * @param gc           gc_arena that might be used to allocate a string

Where "a string" is "the keying material", right? Maybe just say that :)

> + * @returns            The exported key material, the caller may zero the
> + *                     string but should not free it
>   */
>  
> -void
> +unsigned char*
>  key_state_export_keying_material(struct key_state_ssl *ks_ssl,
> -                                 struct tls_session *session) __attribute__((nonnull));
> +                                 struct tls_session *session,
> +                                 enum export_key_identifier export_key,
> +                                 struct gc_arena *gc) __attribute__((nonnull));
>  
>  /**************************************************************************/
>  /** @addtogroup control_tls
> diff --git a/src/openvpn/ssl_mbedtls.c b/src/openvpn/ssl_mbedtls.c
> index 9c874788..8ae6ec7b 100644
> --- a/src/openvpn/ssl_mbedtls.c
> +++ b/src/openvpn/ssl_mbedtls.c
> @@ -231,24 +231,18 @@ mbedtls_ssl_export_keys_cb(void *p_expkey, const unsigned char *ms,
>  }
>  #endif /* HAVE_EXPORT_KEYING_MATERIAL */
>  
> -void
> +unsigned char *
>  key_state_export_keying_material(struct key_state_ssl *ssl,
> -                                 struct tls_session *session)
> +                                 struct tls_session *session,
> +                                 enum export_key_identifier key_id,
> +                                 struct gc_arena *gc)
>  {
> -    if (ssl->exported_key_material)
> +    if (key_id == EXPORT_KEY_USER)
>      {
> -        unsigned int size = session->opt->ekm_size;
> -        struct gc_arena gc = gc_new();
> -        unsigned int len = (size * 2) + 2;
> -
> -        const char *key = format_hex_ex(ssl->exported_key_material,
> -                                        size, len, 0, NULL, &gc);
> -        setenv_str(session->opt->es, "exported_keying_material", key);
> -
> -        dmsg(D_TLS_DEBUG_MED, "%s: exported keying material: %s",
> -             __func__, key);
> -        gc_free(&gc);
> +        return ssl->exported_key_material;
>      }
> +
> +    return  NULL;
>  }
>  
>  
> diff --git a/src/openvpn/ssl_openssl.c b/src/openvpn/ssl_openssl.c
> index 5ba74402..1aad4f89 100644
> --- a/src/openvpn/ssl_openssl.c
> +++ b/src/openvpn/ssl_openssl.c
> @@ -158,35 +158,38 @@ tls_ctx_initialised(struct tls_root_ctx *ctx)
>      return NULL != ctx->ctx;
>  }
>  
> -void
> +unsigned char *
>  key_state_export_keying_material(struct key_state_ssl *ssl,
> -                                 struct tls_session *session)
> +                                 struct tls_session *session,
> +                                 enum export_key_identifier key_id,
> +                                 struct gc_arena *gc)
> +
>  {
> -    if (session->opt->ekm_size > 0)
> -    {
> -        unsigned int size = session->opt->ekm_size;
> -        struct gc_arena gc = gc_new();
> -        unsigned char *ekm = (unsigned char *) gc_malloc(size, true, &gc);
> +    const char *label;
> +    size_t label_size, ekm_size;
>  
> -        if (SSL_export_keying_material(ssl->ssl, ekm, size,
> -                                       session->opt->ekm_label,
> -                                       session->opt->ekm_label_size,
> -                                       NULL, 0, 0))
> -        {
> -            unsigned int len = (size * 2) + 2;
> +    if (key_id == EXPORT_KEY_USER)
> +    {
> +        label = session->opt->ekm_label;
> +        label_size = session->opt->ekm_label_size;
> +        ekm_size = session->opt->ekm_size;
> +    }
> +    else
> +    {
> +        ASSERT(0);
> +    }
>  
> -            const char *key = format_hex_ex(ekm, size, len, 0, NULL, &gc);
> -            setenv_str(session->opt->es, "exported_keying_material", key);
> +    unsigned char *ekm = (unsigned char *) gc_malloc(ekm_size, true, gc);
>  
> -            dmsg(D_TLS_DEBUG_MED, "%s: exported keying material: %s",
> -                 __func__, key);
> -        }
> -        else
> -        {
> -            msg(M_WARN, "WARNING: Export keying material failed!");
> -            setenv_del(session->opt->es, "exported_keying_material");
> -        }
> -        gc_free(&gc);
> +    if (SSL_export_keying_material(ssl->ssl, ekm, ekm_size, label,
> +                                   label_size, NULL, 0, 0) == 1)
> +    {
> +        return ekm;
> +    }
> +    else
> +    {
> +        secure_memzero(ekm, ekm_size);
> +        return NULL;
>      }
>  }
>  
> 

-Steffan.