Optional AP side workaround for key reinstallation attacks

This adds a new hostapd configuration parameter
wpa_disable_eapol_key_retries=1 that can be used to disable
retransmission of EAPOL-Key frames that are used to install
keys (EAPOL-Key message 3/4 and group message 1/2). This is
similar to setting wpa_group_update_count=1 and
wpa_pairwise_update_count=1, but with no impact to message 1/4
retries and with extended timeout for messages 4/4 and group
message 2/2 to avoid causing issues with stations that may use
aggressive power saving have very long time in replying to the
EAPOL-Key messages.

This option can be used to work around key reinstallation attacks
on the station (supplicant) side in cases those station devices
cannot be updated for some reason. By removing the
retransmissions the attacker cannot cause key reinstallation with
a delayed frame transmission. This is related to the station side
vulnerabilities CVE-2017-13077, CVE-2017-13078, CVE-2017-13079,
CVE-2017-13080, and CVE-2017-13081.

This workaround might cause interoperability issues and reduced
robustness of key negotiation especially in environments with
heavy traffic load due to the number of attempts to perform the
key exchange is reduced significantly. As such, this workaround
is disabled by default (unless overridden in build
configuration). To enable this, set the parameter to 1.

It is also possible to enable this in the build by default by
adding the following to the build configuration:

CFLAGS += -DDEFAULT_WPA_DISABLE_EAPOL_KEY_RETRIES=1

Signed-off-by: Jouni Malinen <j@w1.fi>

hostapd/config_file.c

@@ -2570,6 +2570,8 @@ static int hostapd_config_fill(struct hostapd_config *conf,
 			return 1;
 		}
 		bss->wpa_pairwise_update_count = (u32) val;
+	} else if (os_strcmp(buf, "wpa_disable_eapol_key_retries") == 0) {
+		bss->wpa_disable_eapol_key_retries = atoi(pos);
 	} else if (os_strcmp(buf, "wpa_passphrase") == 0) {
 		int len = os_strlen(pos);
 		if (len < 8 || len > 63) {

hostapd/defconfig

@@ -369,3 +369,7 @@ CONFIG_IPV6=y
 # Opportunistic Wireless Encryption (OWE)
 # Experimental implementation of draft-harkins-owe-07.txt
 #CONFIG_OWE=y
+
+# Override default value for the wpa_disable_eapol_key_retries configuration
+# parameter. See that parameter in hostapd.conf for more details.
+#CFLAGS += -DDEFAULT_WPA_DISABLE_EAPOL_KEY_RETRIES=1

hostapd/hostapd.conf

@@ -1315,6 +1315,30 @@ own_ip_addr=127.0.0.1
 # Range 1..4294967295; default: 4
 #wpa_pairwise_update_count=4
 
+# Workaround for key reinstallation attacks
+#
+# This parameter can be used to disable retransmission of EAPOL-Key frames that
+# are used to install keys (EAPOL-Key message 3/4 and group message 1/2). This
+# is similar to setting wpa_group_update_count=1 and
+# wpa_pairwise_update_count=1, but with no impact to message 1/4 and with
+# extended timeout on the response to avoid causing issues with stations that
+# may use aggressive power saving have very long time in replying to the
+# EAPOL-Key messages.
+#
+# This option can be used to work around key reinstallation attacks on the
+# station (supplicant) side in cases those station devices cannot be updated
+# for some reason. By removing the retransmissions the attacker cannot cause
+# key reinstallation with a delayed frame transmission. This is related to the
+# station side vulnerabilities CVE-2017-13077, CVE-2017-13078, CVE-2017-13079,
+# CVE-2017-13080, and CVE-2017-13081.
+#
+# This workaround might cause interoperability issues and reduced robustness of
+# key negotiation especially in environments with heavy traffic load due to the
+# number of attempts to perform the key exchange is reduced significantly. As
+# such, this workaround is disabled by default (unless overridden in build
+# configuration). To enable this, set the parameter to 1.
+#wpa_disable_eapol_key_retries=1
+
 # Enable IEEE 802.11i/RSN/WPA2 pre-authentication. This is used to speed up
 # roaming be pre-authenticating IEEE 802.1X/EAP part of the full RSN
 # authentication and key handshake before actually associating with a new AP.

src/ap/ap_config.c

@@ -37,6 +37,10 @@ static void hostapd_config_free_vlan(struct hostapd_bss_config *bss)
 }
 
 
+#ifndef DEFAULT_WPA_DISABLE_EAPOL_KEY_RETRIES
+#define DEFAULT_WPA_DISABLE_EAPOL_KEY_RETRIES 0
+#endif /* DEFAULT_WPA_DISABLE_EAPOL_KEY_RETRIES */
+
 void hostapd_config_defaults_bss(struct hostapd_bss_config *bss)
 {
 	dl_list_init(&bss->anqp_elem);
@@ -58,6 +62,8 @@ void hostapd_config_defaults_bss(struct hostapd_bss_config *bss)
 	bss->wpa_gmk_rekey = 86400;
 	bss->wpa_group_update_count = 4;
 	bss->wpa_pairwise_update_count = 4;
+	bss->wpa_disable_eapol_key_retries =
+		DEFAULT_WPA_DISABLE_EAPOL_KEY_RETRIES;
 	bss->wpa_key_mgmt = WPA_KEY_MGMT_PSK;
 	bss->wpa_pairwise = WPA_CIPHER_TKIP;
 	bss->wpa_group = WPA_CIPHER_TKIP;

src/ap/ap_config.h

@@ -333,6 +333,7 @@ struct hostapd_bss_config {
 	int wpa_ptk_rekey;
 	u32 wpa_group_update_count;
 	u32 wpa_pairwise_update_count;
+	int wpa_disable_eapol_key_retries;
 	int rsn_pairwise;
 	int rsn_preauth;
 	char *rsn_preauth_interfaces;

src/ap/wpa_auth.c

@@ -65,6 +65,7 @@ static u8 * ieee80211w_kde_add(struct wpa_state_machine *sm, u8 *pos);
 static const u32 eapol_key_timeout_first = 100; /* ms */
 static const u32 eapol_key_timeout_subseq = 1000; /* ms */
 static const u32 eapol_key_timeout_first_group = 500; /* ms */
+static const u32 eapol_key_timeout_no_retrans = 4000; /* ms */
 
 /* TODO: make these configurable */
 static const int dot11RSNAConfigPMKLifetime = 43200;
@@ -1604,6 +1605,9 @@ static void wpa_send_eapol(struct wpa_authenticator *wpa_auth,
 			eapol_key_timeout_first_group;
 	else
 		timeout_ms = eapol_key_timeout_subseq;
+	if (wpa_auth->conf.wpa_disable_eapol_key_retries &&
+	    (!pairwise || (key_info & WPA_KEY_INFO_MIC)))
+		timeout_ms = eapol_key_timeout_no_retrans;
 	if (pairwise && ctr == 1 && !(key_info & WPA_KEY_INFO_MIC))
 		sm->pending_1_of_4_timeout = 1;
 	wpa_printf(MSG_DEBUG, "WPA: Use EAPOL-Key timeout of %u ms (retry "
@@ -2855,6 +2859,11 @@ SM_STATE(WPA_PTK, PTKINITNEGOTIATING)
 	sm->TimeoutEvt = FALSE;
 
 	sm->TimeoutCtr++;
+	if (sm->wpa_auth->conf.wpa_disable_eapol_key_retries &&
+	    sm->TimeoutCtr > 1) {
+		/* Do not allow retransmission of EAPOL-Key msg 3/4 */
+		return;
+	}
 	if (sm->TimeoutCtr > sm->wpa_auth->conf.wpa_pairwise_update_count) {
 		/* No point in sending the EAPOL-Key - we will disconnect
 		 * immediately following this. */
@@ -3197,7 +3206,9 @@ SM_STEP(WPA_PTK)
 			 sm->EAPOLKeyPairwise && sm->MICVerified)
 			SM_ENTER(WPA_PTK, PTKINITDONE);
 		else if (sm->TimeoutCtr >
-			 sm->wpa_auth->conf.wpa_pairwise_update_count) {
+			 sm->wpa_auth->conf.wpa_pairwise_update_count ||
+			 (sm->wpa_auth->conf.wpa_disable_eapol_key_retries &&
+			  sm->TimeoutCtr > 1)) {
 			wpa_auth->dot11RSNA4WayHandshakeFailures++;
 			wpa_auth_vlogger(
 				sm->wpa_auth, sm->addr, LOGGER_DEBUG,
@@ -3237,6 +3248,11 @@ SM_STATE(WPA_PTK_GROUP, REKEYNEGOTIATING)
 	SM_ENTRY_MA(WPA_PTK_GROUP, REKEYNEGOTIATING, wpa_ptk_group);
 
 	sm->GTimeoutCtr++;
+	if (sm->wpa_auth->conf.wpa_disable_eapol_key_retries &&
+	    sm->GTimeoutCtr > 1) {
+		/* Do not allow retransmission of EAPOL-Key group msg 1/2 */
+		return;
+	}
 	if (sm->GTimeoutCtr > sm->wpa_auth->conf.wpa_group_update_count) {
 		/* No point in sending the EAPOL-Key - we will disconnect
 		 * immediately following this. */
@@ -3340,7 +3356,9 @@ SM_STEP(WPA_PTK_GROUP)
 		    !sm->EAPOLKeyPairwise && sm->MICVerified)
 			SM_ENTER(WPA_PTK_GROUP, REKEYESTABLISHED);
 		else if (sm->GTimeoutCtr >
-			 sm->wpa_auth->conf.wpa_group_update_count)
+			 sm->wpa_auth->conf.wpa_group_update_count ||
+			 (sm->wpa_auth->conf.wpa_disable_eapol_key_retries &&
+			  sm->GTimeoutCtr > 1))
 			SM_ENTER(WPA_PTK_GROUP, KEYERROR);
 		else if (sm->TimeoutEvt)
 			SM_ENTER(WPA_PTK_GROUP, REKEYNEGOTIATING);

src/ap/wpa_auth.h

@@ -165,6 +165,7 @@ struct wpa_auth_config {
 	int wpa_ptk_rekey;
 	u32 wpa_group_update_count;
 	u32 wpa_pairwise_update_count;
+	int wpa_disable_eapol_key_retries;
 	int rsn_pairwise;
 	int rsn_preauth;
 	int eapol_version;

src/ap/wpa_auth_glue.c

@@ -46,6 +46,8 @@ static void hostapd_wpa_auth_conf(struct hostapd_bss_config *conf,
 	wconf->wpa_gmk_rekey = conf->wpa_gmk_rekey;
 	wconf->wpa_ptk_rekey = conf->wpa_ptk_rekey;
 	wconf->wpa_group_update_count = conf->wpa_group_update_count;
+	wconf->wpa_disable_eapol_key_retries =
+		conf->wpa_disable_eapol_key_retries;
 	wconf->wpa_pairwise_update_count = conf->wpa_pairwise_update_count;
 	wconf->rsn_pairwise = conf->rsn_pairwise;
 	wconf->rsn_preauth = conf->rsn_preauth;