Avoid generating unecessary mbed debug messages

The main motivation to make this change is to avoid a crash in mbed TLS
2.25 with --verb < 8.

mbed TLS 2.25 has a nasty bug that the print function for Montgomery style
EC curves (Curve25519 and Curve448) does segfault. See also the issue
reported here: https://github.com/ARMmbed/mbedtls/issues/4208

We request always debug level 3 from mbed TLS but filter out any debug
output of level 3 unless verb 8 or higher is set. This commeit sets
the debug level to 2 to avoid this problem by makeing mbed TLS not
generatin the problematic debug output.

For the affected version to still use --verb 8 with mbed TLS 2.25 is to
restrict the EC groups to ones that do not crash the print function
like with '--tls-groups secp521r1:secp384r1:secp256r1'.

This patch has no patch on user-visible behaviour on unaffected mbed TLS
versions.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Patch V2: Replace magic constant with proper define. Highlight more this
          avoding generating unessary debug output than crash workaround.
Acked-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan@karger.me>
Message-Id: <20210316124421.1635-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21667.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>

diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 3a2c7a10f8bc5849ce78d41e04aa89aa4524d567..86e78b057d5783823e8d24594d90376a76be1678 100644 (file)
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -5883,6 +5883,12 @@ add_option(struct options *options,
     {
         VERIFY_PERMISSION(OPT_P_MESSAGES);
         options->verbosity = positive_atoi(p[1]);
+        if (options->verbosity >= (D_TLS_DEBUG_MED & M_DEBUG_LEVEL))
+        {
+            /* We pass this flag to the SSL library to avoid
+             * mbed TLS always generating debug level logging */
+            options->ssl_flags |= SSLF_TLS_DEBUG_ENABLED;
+        }
 #if !defined(ENABLE_DEBUG) && !defined(ENABLE_SMALL)
         /* Warn when a debug verbosity is supplied when built without debug support */
         if (options->verbosity >= 7)

diff --git a/src/openvpn/ssl_common.h b/src/openvpn/ssl_common.h
index b493cbb77ec5d9cd98ec7a8090f01676413a9eac..2e3c98dbcc7e4afe4edc38c7a9a190d447f040a3 100644 (file)
--- a/src/openvpn/ssl_common.h
+++ b/src/openvpn/ssl_common.h
@@ -352,6 +352,7 @@ struct tls_options
 #define SSLF_TLS_VERSION_MIN_MASK     0xF  /* (uses bit positions 6 to 9) */
 #define SSLF_TLS_VERSION_MAX_SHIFT    10
 #define SSLF_TLS_VERSION_MAX_MASK     0xF  /* (uses bit positions 10 to 13) */
+#define SSLF_TLS_DEBUG_ENABLED        (1<<14)
     unsigned int ssl_flags;
 
 #ifdef ENABLE_MANAGEMENT

diff --git a/src/openvpn/ssl_mbedtls.c b/src/openvpn/ssl_mbedtls.c
index b30b6b9dba8a12dde9ff7e281922c5b299347b65..4626e98389ab2ab2729411bc1974526dae75ef4e 100644 (file)
--- a/src/openvpn/ssl_mbedtls.c
+++ b/src/openvpn/ssl_mbedtls.c
@@ -1058,7 +1058,18 @@ key_state_ssl_init(struct key_state_ssl *ks_ssl,
     mbedtls_ssl_config_defaults(ks_ssl->ssl_config, ssl_ctx->endpoint,
                                 MBEDTLS_SSL_TRANSPORT_STREAM, MBEDTLS_SSL_PRESET_DEFAULT);
 #ifdef MBEDTLS_DEBUG_C
-    mbedtls_debug_set_threshold(3);
+    /* We only want to have mbed TLS generate debug level logging when we would
+     * also display it.
+     * In fact mbed TLS 2.25.0 crashes generating debug log if Curve25591 is
+     * selected for DH (https://github.com/ARMmbed/mbedtls/issues/4208) */
+    if (session->opt->ssl_flags & SSLF_TLS_DEBUG_ENABLED)
+    {
+        mbedtls_debug_set_threshold(3);
+    }
+    else
+    {
+        mbedtls_debug_set_threshold(2);
+    }
 #endif
     mbedtls_ssl_conf_dbg(ks_ssl->ssl_config, my_debug, NULL);
     mbedtls_ssl_conf_rng(ks_ssl->ssl_config, mbedtls_ctr_drbg_random,