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

[Markus Theil]

This patch can also be left out, but then other measures should be taken,
to make most/all DFS channels available on each mesh point, in order for
channel switch and CSA to work so that network does not become disconnected/
partitioned.

On 6/30/20 2:19 PM, Markus Theil wrote:
> 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)