Add option to disable SAE key_mgmt without PMF

Add the `sae_check_mfp` global option to limit SAE when PMF will
not be selected for the connection.
With this option SAE is avoided when the hardware is not capable
of PMF due to missing ciphers.
With this option SAE is avoided on capable hardware when the AP
does not enable PMF.

Allows falling back to PSK on drivers with the
WPA_DRIVER_FLAGS_SAE capability but do not support the BIP cipher
necessary for PMF. This enables configurations that can fall back
to WPA-PSK and avoid problems associating with APs configured
with `sae_require_mfp=1`.

Useful when `pmf=1` and `sae_check_mfp=1` are enabled and networks
are configured with ieee80211w=3 (default) and key_mgmt="WPA-PSK SAE".
In this configuration if the device is unable to use PMF due to
lacking BIP group ciphers it will avoid SAE and fallback to
WPA-PSK for that connection.

Signed-off-by: Jeffery Miller <jefferymiller@google.com>

tests/hwsim/test_sae.py

@@ -424,6 +424,72 @@ def test_sae_mixed_mfp(dev, apdev):
     dev[2].connect("test-sae", psk="12345678", ieee80211w="0", scan_freq="2412")
     dev[2].dump_monitor()
 
+def _test_sae_mixed_check_mfp(dev, apdev):
+    """Mixed SAE and non-SAE network with the sae_check_mfp option"""
+    check_sae_capab(dev[0])
+    check_sae_capab(dev[1])
+    check_sae_capab(dev[2])
+
+    params = hostapd.wpa2_params(ssid="test-sae", passphrase="12345678")
+    params['wpa_key_mgmt'] = 'SAE WPA-PSK'
+    params["ieee80211w"] = "1"
+    hostapd.add_ap(apdev[0], params)
+
+    params = hostapd.wpa2_params(ssid="test-sae-no-mfp", passphrase="12345678")
+    params['wpa_key_mgmt'] = 'SAE WPA-PSK'
+    params["ieee80211w"] = "0"
+    hostapd.add_ap(apdev[1], params)
+
+    dev[0].set("sae_check_mfp", "0")
+    dev[0].set("sae_groups", "")
+    dev[0].connect("test-sae", psk="12345678", key_mgmt="SAE WPA-PSK",
+                   ieee80211w="0", scan_freq="2412")
+    dev[0].dump_monitor()
+    status = dev[0].get_status()
+    if status['key_mgmt'] != "SAE":
+        raise Exception("SAE without sae_check_mfp was not allowed")
+
+    # Confirm SAE is used when sae_check_mfp is not set for non-PMF AP.
+    dev[2].set("sae_check_mfp", "0")
+    dev[2].set("sae_groups", "")
+    dev[2].connect("test-sae-no-mfp", psk="12345678", key_mgmt="SAE WPA-PSK",
+                   ieee80211w="1", scan_freq="2412")
+    status = dev[2].get_status()
+    if status['key_mgmt'] != "SAE":
+        raise Exception("SAE without sae_check_mfp was not allowed")
+    dev[2].dump_monitor()
+
+    # Confirm SAE is not used with the PMF disabled network configuration.
+    dev[1].set("sae_check_mfp", "1")
+    dev[1].set("sae_groups", "")
+    dev[1].connect("test-sae", psk="12345678", key_mgmt="SAE WPA-PSK",
+                   ieee80211w="0", scan_freq="2412")
+    status = dev[1].get_status()
+    dev[1].request("DISCONNECT")
+    dev[1].wait_disconnected()
+    dev[1].dump_monitor()
+    if status['key_mgmt'] != "WPA2-PSK":
+        raise Exception("SAE without MFP was allowed")
+    dev[1].request("REMOVE_NETWORK all")
+
+    # Confirm SAE is not used connecting to PMF disabled AP.
+    dev[1].set("sae_check_mfp", "1")
+    dev[1].set("sae_groups", "")
+    dev[1].connect("test-sae-no-mfp", psk="12345678", key_mgmt="SAE WPA-PSK",
+                   ieee80211w="1", scan_freq="2412")
+    status = dev[1].get_status()
+    if status['key_mgmt'] != "WPA2-PSK":
+        raise Exception("SAE without MFP was allowed")
+
+def test_sae_mixed_check_mfp(dev, apdev):
+    """Mixed SAE and non-SAE network with the sae_check_mfp option"""
+    try:
+        _test_sae_mixed_check_mfp(dev, apdev)
+    finally:
+        dev[0].set("sae_check_mfp", "0")
+        dev[1].set("sae_check_mfp", "0")
+        dev[2].set("sae_check_mfp", "0")
+
 def test_sae_and_psk_transition_disable(dev, apdev):
     """SAE and PSK transition disable indication"""
     check_sae_capab(dev[0])

wpa_supplicant/config.c

@@ -5280,6 +5280,7 @@ static const struct global_parse_data global_fields[] = {
 	{ INT_RANGE(auto_interworking, 0, 1), 0 },
 	{ INT(okc), 0 },
 	{ INT(pmf), 0 },
+	{ INT_RANGE(sae_check_mfp, 0, 1), 0 },
 	{ FUNC(sae_groups), 0 },
 	{ INT_RANGE(sae_pwe, 0, 3), 0 },
 	{ INT_RANGE(sae_pmkid_in_assoc, 0, 1), 0 },

wpa_supplicant/config.h

@@ -1238,6 +1238,25 @@ struct wpa_config {
 	 */
 	enum mfp_options pmf;
 
+	/**
+	 * sae_check_mfp - Whether to limit SAE based on PMF capabilities
+	 *
+	 * With this check SAE key_mgmt will not be selected if PMF is
+	 * not enabled.
+	 * Scenarios where enabling this check will limit SAE:
+	 *  1) ieee8011w=0 is set for the network.
+	 *  2) The AP does not have PMF enabled.
+	 *  3) ieee8011w for the network is the default(3), pmf=1 is enabled
+	 *     globally and the device does not support the BIP cipher.
+	 *
+	 * Useful to allow the BIP cipher check that occurs for ieee80211w=3
+	 * and pmf=1 to also avoid using SAE key_mgmt.
+	 * Useful when hardware does not support BIP to still to allow
+	 * connecting to sae_require_mfp=1 WPA2+WPA3-Personal transition mode
+	 *access points by automatically selecting PSK instead of SAE.
+	 */
+	int sae_check_mfp;
+
 	/**
 	 * sae_groups - Preference list of enabled groups for SAE
 	 *

wpa_supplicant/config_file.c

@@ -1356,6 +1356,9 @@ static void wpa_config_write_global(FILE *f, struct wpa_config *config)
 	if (config->beacon_int)
 		fprintf(f, "beacon_int=%d\n", config->beacon_int);
 
+	if (config->sae_check_mfp)
+		fprintf(f, "sae_check_mfp=%d\n", config->sae_check_mfp);
+
 	if (config->sae_groups) {
 		int i;
 		fprintf(f, "sae_groups=");

wpa_supplicant/sme.c

@@ -413,8 +413,13 @@ static void sme_send_authentication(struct wpa_supplicant *wpa_s,
 #endif /* CONFIG_DPP */
 		} else if (wpa_parse_wpa_ie(rsn, 2 + rsn[1], &ied) == 0 &&
 			   wpa_key_mgmt_sae(ied.key_mgmt)) {
+			if (wpas_is_sae_avoided(wpa_s, ssid, &ied)) {
+				wpa_dbg(wpa_s, MSG_DEBUG,
+					"SAE enabled, but disallowing SAE auth_alg without PMF");
+			} else {
 				wpa_dbg(wpa_s, MSG_DEBUG, "Using SAE auth_alg");
 				params.auth_alg = WPA_AUTH_ALG_SAE;
+			}
 		} else {
 			wpa_dbg(wpa_s, MSG_DEBUG,
 				"SAE enabled, but target BSS does not advertise SAE AKM for RSN");

wpa_supplicant/wpa_cli.c

@@ -503,7 +503,7 @@ static char ** wpa_cli_complete_set(const char *str, int pos)
 		"autoscan", "wps_nfc_dev_pw_id", "wps_nfc_dh_pubkey",
 		"wps_nfc_dh_privkey", "wps_nfc_dev_pw", "ext_password_backend",
 		"p2p_go_max_inactivity", "auto_interworking", "okc", "pmf",
-		"sae_groups", "dtim_period", "beacon_int",
+		"sae_check_mfp", "sae_groups", "dtim_period", "beacon_int",
 		"ap_vendor_elements", "ignore_old_scan_res", "freq_list",
 		"scan_cur_freq", "scan_res_valid_for_connect",
 		"sched_scan_interval",
@@ -602,6 +602,7 @@ static char ** wpa_cli_complete_get(const char *str, int pos)
 		"go_venue_group", "go_venue_type",
 		"wps_nfc_dev_pw_id", "ext_password_backend",
 		"p2p_go_max_inactivity", "auto_interworking", "okc", "pmf",
+		"sae_check_mfp",
 		"dtim_period", "beacon_int", "ignore_old_scan_res",
 		"scan_cur_freq", "scan_res_valid_for_connect",
 		"sched_scan_interval",

wpa_supplicant/wpa_supplicant.c

@@ -1651,7 +1651,8 @@ int wpa_supplicant_set_suites(struct wpa_supplicant *wpa_s,
 
 	sel = ie.key_mgmt & ssid->key_mgmt;
 #ifdef CONFIG_SAE
-	if (!(wpa_s->drv_flags & WPA_DRIVER_FLAGS_SAE))
+	if (!(wpa_s->drv_flags & WPA_DRIVER_FLAGS_SAE) ||
+	    wpas_is_sae_avoided(wpa_s, ssid, &ie))
 		sel &= ~(WPA_KEY_MGMT_SAE | WPA_KEY_MGMT_SAE_EXT_KEY |
 			 WPA_KEY_MGMT_FT_SAE | WPA_KEY_MGMT_FT_SAE_EXT_KEY);
 #endif /* CONFIG_SAE */
@@ -8211,6 +8212,19 @@ int wpas_get_ssid_pmf(struct wpa_supplicant *wpa_s, struct wpa_ssid *ssid)
 }
 
 
+#ifdef CONFIG_SAE
+bool wpas_is_sae_avoided(struct wpa_supplicant *wpa_s,
+			 struct wpa_ssid *ssid,
+			 const struct wpa_ie_data *ie)
+{
+	return wpa_s->conf->sae_check_mfp &&
+		(!(ie->capabilities &
+		   (WPA_CAPABILITY_MFPC | WPA_CAPABILITY_MFPR)) ||
+		 wpas_get_ssid_pmf(wpa_s, ssid) == NO_MGMT_FRAME_PROTECTION);
+}
+#endif /* CONFIG_SAE */
+
+
 int pmf_in_use(struct wpa_supplicant *wpa_s, const u8 *addr)
 {
 	if (wpa_s->current_ssid == NULL ||

wpa_supplicant/wpa_supplicant.conf

@@ -419,6 +419,34 @@ fast_reauth=1
 # RSN.
 #pmf=0
 
+# sae_check_mfp: Require PMF support to select SAE key_mgmt
+# 0 = Do not check PMF for SAE (default)
+# 1 = Limit SAE when PMF is not enabled
+#
+# When enabled SAE will not be selected if PMF will not be used
+# for the connection.
+# Scenarios where this check will limit SAE:
+#  1) ieee80211w=0 is set for the network
+#  2) The AP does not have PMF enabled.
+#  3) ieee80211w is unset, pmf=1 is enabled globally, and
+#     the device does not support the BIP cipher.
+# Consider the configuration of global parameterss sae_check_mfp=1, pmf=1 and a
+# network configured with ieee80211w unset and key_mgmt=SAE WPA-PSK.
+# In the example WPA-PSK will be used if the device does not support
+# the BIP cipher or the AP has PMF disabled.
+# Limiting SAE with this check can avoid failing to associate to an AP
+# that is configured with sae_requires_mfp=1 if the device does
+# not support PMF due to lack of the BIP cipher.
+#
+# Enabling this check helps with compliance of the WPA3
+# specification for WPA3-Personal transition mode.
+# The WPA3 specification section 2.3 "WPA3-Personal transition mode" item 8
+# states "A STA shall negotiate PMF when associating to an AP using SAE".
+# With this check WPA3 capable devices when connecting
+# to transition mode APs that do not advertise PMF support
+# will not use SAE and instead fallback to PSK.
+#sae_check_mfp=0
+
 # Enabled SAE finite cyclic groups in preference order
 # By default (if this parameter is not set), the mandatory group 19 (ECC group
 # defined over a 256-bit prime order field, NIST P-256) is preferred and groups

wpa_supplicant/wpa_supplicant_i.h

@@ -1794,6 +1794,10 @@ int wpas_get_ssid_pmf(struct wpa_supplicant *wpa_s, struct wpa_ssid *ssid);
 int pmf_in_use(struct wpa_supplicant *wpa_s, const u8 *addr);
 void wpa_s_setup_sae_pt(struct wpa_config *conf, struct wpa_ssid *ssid);
 
+bool wpas_is_sae_avoided(struct wpa_supplicant *wpa_s,
+			struct wpa_ssid *ssid,
+			const struct wpa_ie_data *ie);
+
 int wpas_init_ext_pw(struct wpa_supplicant *wpa_s);
 
 void dump_freq_data(struct wpa_supplicant *wpa_s, const char *title,