[PATCH v2 12/12] mesh: use deterministic channel on channel switch

[Markus Theil]

This patch uses a deterministic channel on DFS channel switch
in mesh networks. Otherwise, when switching to a usable but not
available channel, no CSA can be sent and a random channel is choosen
without notification of other nodes. It is then quite likely, that
the mesh network gets disconnected.

Fix this by using a deterministic number, based on the sha256 hash
of the mesh ID, in order to use at least a different number in each
mesh network.

Signed-off-by: Markus Theil <markus.theil at tu-ilmenau.de>
---
 src/ap/dfs.c                 | 20 +++++++++++++++++++-
 src/drivers/driver_nl80211.c |  4 ++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/ap/dfs.c b/src/ap/dfs.c
index 25dad3482..8f3630bf8 100644
--- a/src/ap/dfs.c
+++ b/src/ap/dfs.c
@@ -17,6 +17,7 @@
 #include "ap_drv_ops.h"
 #include "drivers/driver.h"
 #include "dfs.h"
+#include "crypto/crypto.h"
 
 
 static int dfs_get_used_n_chans(struct hostapd_iface *iface, int *seg1)
@@ -480,9 +481,14 @@ dfs_get_valid_channel(struct hostapd_iface *iface,
 	int num_available_chandefs;
 	int chan_idx, chan_idx2;
 	int sec_chan_idx_80p80 = -1;
+	bool is_mesh = false;
 	int i;
 	u32 _rand;
 
+#ifdef CONFIG_MESH
+	is_mesh = iface->mconf;
+#endif
+
 	wpa_printf(MSG_DEBUG, "DFS: Selecting random channel");
 	*secondary_channel = 0;
 	*oper_centr_freq_seg0_idx = 0;
@@ -502,8 +508,20 @@ dfs_get_valid_channel(struct hostapd_iface *iface,
 	if (num_available_chandefs == 0)
 		return NULL;
 
-	if (os_get_random((u8 *) &_rand, sizeof(_rand)) < 0)
+	/* try to use deterministic channel in mesh, so that both sides
+	 * have a chance to switch to the same channel */
+	if (is_mesh) {
+#ifdef CONFIG_MESH
+		u64 hash[4];
+		const u8 *meshid[1] = { &iface->mconf->meshid[0] };
+		const size_t meshid_len = iface->mconf->meshid_len;
+
+		sha256_vector(1, meshid, &meshid_len, (u8 *)&hash[0]);
+		_rand = hash[0] + hash[1] + hash[2] + hash[3];
+#endif
+	} else if (os_get_random((u8 *) &_rand, sizeof(_rand)) < 0)
 		return NULL;
+
 	chan_idx = _rand % num_available_chandefs;
 	dfs_find_channel(iface, &chan, chan_idx, skip_radar);
 	if (!chan) {
diff --git a/src/drivers/driver_nl80211.c b/src/drivers/driver_nl80211.c
index d922eda74..18521f832 100644
--- a/src/drivers/driver_nl80211.c
+++ b/src/drivers/driver_nl80211.c
@@ -9568,6 +9568,10 @@ static int nl80211_switch_channel(void *priv, struct csa_settings *settings)
 	if (ret)
 		goto error;
 
+	if (drv->nlmode == NL80211_IFTYPE_MESH_POINT) {
+		nla_put_flag(msg, NL80211_ATTR_HANDLE_DFS);
+	}
+
 	/* beacon_csa params */
 	beacon_csa = nla_nest_start(msg, NL80211_ATTR_CSA_IES);
 	if (!beacon_csa)
-- 
2.27.0