[v2,1/3] dpp: scan to pick up extra frequencies when enrolling

Commit Message

James Prestwood Nov. 10, 2023, 2:16 p.m. UTC

The DPP-PKEX spec provides a very limited list of frequencies used
to discover configurators, only 3 on 2.4 and 5GHz bands. Since
configurators (at least in IWD's implementation) are only allowed
on the current operating frequency its very unlikely an enrollee
will find a configurator on these frequencies out of the entire
spectrum.

The spec does mention that the 3 default frequencies should be used
"In lieu of specific channel information obtained in a manner outside
the scope of this specification, ...". This allows the implementation
some flexibility in using a broader range of frequencies.

To increase the chances of finding a configurator shared code
enrollees will first issue a scan to determine what access points are
around, then iterate these frequencies. This is especially helpful
when the configurators are IWD-based since we know that they'll be
on the same channels as the APs in the area.
---
 src/dpp.c | 98 +++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 88 insertions(+), 10 deletions(-)

v2:
 * Removed user-option, scan by default
 * Fixed potential NULL return from dpp_default_freqs

Patch

diff --git a/src/dpp.c b/src/dpp.c
index c54bd484..daa49410 100644
--- a/src/dpp.c
+++ b/src/dpp.c
@@ -182,6 +182,7 @@  struct dpp_sm {
 	size_t z_len;
 	uint8_t u[L_ECC_SCALAR_MAX_BYTES];
 	size_t u_len;
+	uint32_t pkex_scan_id;
 
 	bool mcast_support : 1;
 	bool roc_started : 1;
@@ -507,6 +508,11 @@  static void dpp_reset(struct dpp_sm *dpp)
 		dpp->retry_timeout = NULL;
 	}
 
+	if (dpp->pkex_scan_id) {
+		scan_cancel(dpp->wdev_id, dpp->pkex_scan_id);
+		dpp->pkex_scan_id = 0;
+	}
+
 	dpp->state = DPP_STATE_NOTHING;
 	dpp->new_freq = 0;
 	dpp->frame_retry = 0;
@@ -3956,6 +3962,14 @@  static struct l_dbus_message *dpp_dbus_stop(struct l_dbus *dbus,
 	return l_dbus_message_new_method_return(message);
 }
 
+static void dpp_pkex_scan_trigger(int err, void *user_data)
+{
+	struct dpp_sm *dpp = user_data;
+
+	if (err < 0)
+		dpp_reset(dpp);
+}
+
 /*
  * Section 5.6.1
  * In lieu of specific channel information obtained in a manner outside
@@ -3994,6 +4008,62 @@  static uint32_t *dpp_default_freqs(struct dpp_sm *dpp, size_t *out_len)
 	return freqs_out;
 }
 
+static bool dpp_pkex_scan_notify(int err, struct l_queue *bss_list,
+					const struct scan_freq_set *freqs,
+					void *user_data)
+{
+	struct dpp_sm *dpp = user_data;
+	const struct l_queue_entry *e;
+	_auto_(scan_freq_set_free) struct scan_freq_set *freq_set = NULL;
+
+	if (err < 0)
+		goto failed;
+
+	freq_set = scan_freq_set_new();
+
+	if (!bss_list || l_queue_isempty(bss_list)) {
+		dpp->freqs = dpp_default_freqs(dpp, &dpp->freqs_len);
+		if (!dpp->freqs)
+			goto failed;
+
+		l_debug("No BSS's seen, using default frequency list");
+		goto start;
+	}
+
+	for (e = l_queue_get_entries(bss_list); e; e = e->next) {
+		const struct scan_bss *bss = e->data;
+
+		scan_freq_set_add(freq_set, bss->frequency);
+	}
+
+	l_debug("Found %u frequencies to search for configurator",
+			l_queue_length(bss_list));
+
+	dpp->freqs = scan_freq_set_to_fixed_array(freq_set, &dpp->freqs_len);
+
+start:
+	dpp->current_freq = dpp->freqs[0];
+
+	dpp_reset_protocol_timer(dpp, DPP_PKEX_PROTO_TIMEOUT);
+
+	l_debug("PKEX start enrollee (id=%s)", dpp->pkex_id ?: "unset");
+
+	dpp_start_offchannel(dpp, dpp->current_freq);
+
+	return false;
+
+failed:
+	dpp_reset(dpp);
+	return false;
+}
+
+static void dpp_pkex_scan_destroy(void *user_data)
+{
+	struct dpp_sm *dpp = user_data;
+
+	dpp->pkex_scan_id = 0;
+}
+
 static bool dpp_start_pkex_enrollee(struct dpp_sm *dpp, const char *key,
 				const char *identifier)
 {
@@ -4045,17 +4115,25 @@  static bool dpp_start_pkex_enrollee(struct dpp_sm *dpp, const char *key,
 
 	dpp_property_changed_notify(dpp);
 
-	dpp->freqs = dpp_default_freqs(dpp, &dpp->freqs_len);
-	if (!dpp->freqs)
-		goto failed;
-
-	dpp->current_freq = dpp->freqs[dpp->freqs_idx];
-
-	dpp_reset_protocol_timer(dpp, DPP_PKEX_PROTO_TIMEOUT);
-
-	l_debug("PKEX start enrollee (id=%s)", dpp->pkex_id ?: "unset");
+	/*
+	 * The 'dpp_default_freqs' function returns the default frequencies
+	 * outlined in section 5.6.1. For 2.4/5GHz this is only 3 frequencies
+	 * which is unlikely to result in discovery of a configurator. The spec
+	 * does allow frequencies to be "obtained in a manner outside the scope
+	 * of this specification" which is what is being done here.
+	 *
+	 * This is mainly geared towards IWD-based configurators; banking on the
+	 * fact that they are currently connected to nearby APs. Scanning lets
+	 * us see nearby BSS's which should be the same frequencies as our
+	 * target configurator.
+	 */
+	l_debug("Performing scan for frequencies to start PKEX");
 
-	dpp_start_offchannel(dpp, dpp->current_freq);
+	dpp->pkex_scan_id = scan_active(dpp->wdev_id, NULL, 0,
+				dpp_pkex_scan_trigger, dpp_pkex_scan_notify,
+				dpp, dpp_pkex_scan_destroy);
+	if (!dpp->pkex_scan_id)
+		goto failed;
 
 	return true;