[next] wifi: nl80211: emit CMD_START_AP on multicast group when an AP is started

  From: Alvin Šipraga <alsi@bang-olufsen.dk>

Userspace processes such as network daemons may wish to be informed when
any AP interface is brought up on the system, for example to initiate a
(re)configuration of IP settings or to start a DHCP server.

Currently nl80211 does not broadcast any such event on its multicast
groups, leaving userspace only two options:

1. the process must be the one that actually issued the
   NL80211_CMD_START_AP request, so that it can react on the response to
   that request;

2. the process must react to RTM_NEWLINK events indicating a change in
   carrier state, and may query for further information about the AP and
   react accordingly.

Option (1) is robust, but it does not cover all scenarios. It is easy to
imagine a situation where this is not the case (e.g. hostapd +
systemd-networkd).

Option (2) is not robust, because RTM_NEWLINK events may be silently
discarded by the linkwatch logic (cf. linkwatch_fire_event()).
Concretely, consider a scenario in which the carrier state flip-flops in
the following way:

 ^ carrier state (high/low = carrier/no carrier)
 |
 |        _______      _______ ...
 |       |       |    |
 | ______| "foo" |____| "bar"             (SSID in "quotes")
 |
 +-------A-------B----C---------> time

If the time interval between (A) and (C) is less than 1 second, then
linkwatch may emit only a single RTM_NEWLINK event indicating carrier
gain.

This is problematic because it is possible that the network
configuration that should be applied is a function of the AP's
properties such as SSID (cf. SSID= in systemd.network(5)). As
illustrated in the above diagram, it may be that the AP with SSID "bar"
ends up being configured as though it had SSID "foo".

Address the above issue by having nl80211 emit an NL80211_CMD_START_AP
message on the MLME nl80211 multicast group. This allows for arbitrary
processes to be reliably informed.

Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
---
 net/wireless/nl80211.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)
  

On Fri, 2022-12-09 at 16:28 +0100, Alvin Šipraga wrote:
> 
> This is problematic because it is possible that the network
> configuration that should be applied is a function of the AP's
> properties such as SSID (cf. SSID= in systemd.network(5)). As
> illustrated in the above diagram, it may be that the AP with SSID "bar"
> ends up being configured as though it had SSID "foo".
> 

You might not care if you want the SSID, but it still seems wrong:

> +static void nl80211_send_ap_started(struct wireless_dev *wdev)
> +{
> +	struct wiphy *wiphy = wdev->wiphy;
> +	struct cfg80211_registered_device *rdev = wiphy_to_rdev(wiphy);
> +	struct sk_buff *msg;
> +	void *hdr;
> +
> +	msg = nlmsg_new(NLMSG_DEFAULT_SIZE, GFP_KERNEL);
> +	if (!msg)
> +		return;
> +
> +	hdr = nl80211hdr_put(msg, 0, 0, 0, NL80211_CMD_START_AP);
> +	if (!hdr)
> +		goto out;
> +
> +	if (nla_put_u32(msg, NL80211_ATTR_WIPHY, rdev->wiphy_idx) ||
> +	    nla_put_u32(msg, NL80211_ATTR_IFINDEX, wdev->netdev->ifindex) ||
> +	    nla_put_u64_64bit(msg, NL80211_ATTR_WDEV, wdev_id(wdev),
> +			      NL80211_ATTR_PAD) ||
> +	    (wdev->u.ap.ssid_len &&
> +	     nla_put(msg, NL80211_ATTR_SSID, wdev->u.ap.ssid_len,
> +		     wdev->u.ap.ssid)))
> +		goto out;
> +
> +	genlmsg_end(msg, hdr);
> +
> +	genlmsg_multicast_netns(&nl80211_fam, wiphy_net(wiphy), msg, 0,
> +				NL80211_MCGRP_MLME, GFP_KERNEL);
> +	return;
> +out:
> +	nlmsg_free(msg);
> +}

This has no indication of the link, but with multi-link you could
actually be sending this event multiple times to userspace on the same
netdev.

>  static int nl80211_start_ap(struct sk_buff *skb, struct genl_info *info)
>  {
>  	struct cfg80211_registered_device *rdev = info->user_ptr[0];
> @@ -6050,6 +6083,8 @@ static int nl80211_start_ap(struct sk_buff *skb, struct genl_info *info)
>  
>  		if (info->attrs[NL80211_ATTR_SOCKET_OWNER])
>  			wdev->conn_owner_nlportid = info->snd_portid;
> +
> +		nl80211_send_ap_started(wdev);
>  	}

because this can be called multiple times, once for each link.

Seems like you should include the link ID or something?

johannes
  

Hi Johannes,

On Wed, Jan 18, 2023 at 05:26:19PM +0100, Johannes Berg wrote:
> On Fri, 2022-12-09 at 16:28 +0100, Alvin Šipraga wrote:
> > 
> > This is problematic because it is possible that the network
> > configuration that should be applied is a function of the AP's
> > properties such as SSID (cf. SSID= in systemd.network(5)). As
> > illustrated in the above diagram, it may be that the AP with SSID "bar"
> > ends up being configured as though it had SSID "foo".
> > 
> 
> You might not care if you want the SSID, but it still seems wrong:
> 
> > +static void nl80211_send_ap_started(struct wireless_dev *wdev)
> > +{
> > +	struct wiphy *wiphy = wdev->wiphy;
> > +	struct cfg80211_registered_device *rdev = wiphy_to_rdev(wiphy);
> > +	struct sk_buff *msg;
> > +	void *hdr;
> > +
> > +	msg = nlmsg_new(NLMSG_DEFAULT_SIZE, GFP_KERNEL);
> > +	if (!msg)
> > +		return;
> > +
> > +	hdr = nl80211hdr_put(msg, 0, 0, 0, NL80211_CMD_START_AP);
> > +	if (!hdr)
> > +		goto out;
> > +
> > +	if (nla_put_u32(msg, NL80211_ATTR_WIPHY, rdev->wiphy_idx) ||
> > +	    nla_put_u32(msg, NL80211_ATTR_IFINDEX, wdev->netdev->ifindex) ||
> > +	    nla_put_u64_64bit(msg, NL80211_ATTR_WDEV, wdev_id(wdev),
> > +			      NL80211_ATTR_PAD) ||
> > +	    (wdev->u.ap.ssid_len &&
> > +	     nla_put(msg, NL80211_ATTR_SSID, wdev->u.ap.ssid_len,
> > +		     wdev->u.ap.ssid)))
> > +		goto out;
> > +
> > +	genlmsg_end(msg, hdr);
> > +
> > +	genlmsg_multicast_netns(&nl80211_fam, wiphy_net(wiphy), msg, 0,
> > +				NL80211_MCGRP_MLME, GFP_KERNEL);
> > +	return;
> > +out:
> > +	nlmsg_free(msg);
> > +}
> 
> This has no indication of the link, but with multi-link you could
> actually be sending this event multiple times to userspace on the same
> netdev.
> 
> >  static int nl80211_start_ap(struct sk_buff *skb, struct genl_info *info)
> >  {
> >  	struct cfg80211_registered_device *rdev = info->user_ptr[0];
> > @@ -6050,6 +6083,8 @@ static int nl80211_start_ap(struct sk_buff *skb, struct genl_info *info)
> >  
> >  		if (info->attrs[NL80211_ATTR_SOCKET_OWNER])
> >  			wdev->conn_owner_nlportid = info->snd_portid;
> > +
> > +		nl80211_send_ap_started(wdev);
> >  	}
> 
> because this can be called multiple times, once for each link.
> 
> Seems like you should include the link ID or something?

Thanks for your review, you are quite right. I didn't give much thought
to MLO as I am not too familiar with it. Is something like the below
what you are looking for?

Speaking of which: I drew inspiration from nl80211_send_ap_stopped()
which see also doesn't include the link ID. Would you like me to include
a second patch in v2 which adds the link ID to that function along the
same lines?

Kind regards,
Alvin

-------------->8----------------------------------

diff --git a/net/wireless/nl80211.c b/net/wireless/nl80211.c
index ba4965f035b2..5720ffcbecc4 100644
--- a/net/wireless/nl80211.c
+++ b/net/wireless/nl80211.c
@@ -5770,7 +5770,7 @@ static bool nl80211_valid_auth_type(struct cfg80211_registered_device *rdev,
        }
 }
 
-static void nl80211_send_ap_started(struct wireless_dev *wdev)
+static void nl80211_send_ap_started(struct wireless_dev *wdev, unsigned int link_id)
 {
        struct wiphy *wiphy = wdev->wiphy;
        struct cfg80211_registered_device *rdev = wiphy_to_rdev(wiphy);
@@ -5791,7 +5791,9 @@ static void nl80211_send_ap_started(struct wireless_dev *wdev)
                              NL80211_ATTR_PAD) ||
            (wdev->u.ap.ssid_len &&
             nla_put(msg, NL80211_ATTR_SSID, wdev->u.ap.ssid_len,
-                    wdev->u.ap.ssid)))
+                    wdev->u.ap.ssid)) ||
+           (wdev->valid_links &&
+            nla_put_u8(msg, NL80211_ATTR_MLO_LINK_ID, link_id)))
                goto out;
 
        genlmsg_end(msg, hdr);
@@ -6084,7 +6086,7 @@ static int nl80211_start_ap(struct sk_buff *skb, struct genl_info *info)
                if (info->attrs[NL80211_ATTR_SOCKET_OWNER])
                        wdev->conn_owner_nlportid = info->snd_portid;
 
-               nl80211_send_ap_started(wdev);
+               nl80211_send_ap_started(wdev, link_id);
        }
 out_unlock:
        wdev_unlock(wdev);
  

Hi,

> > Seems like you should include the link ID or something?
> 
> Thanks for your review, you are quite right. I didn't give much thought
> to MLO as I am not too familiar with it. Is something like the below
> what you are looking for?

Yes, that looks good.

> Speaking of which: I drew inspiration from nl80211_send_ap_stopped()
> which see also doesn't include the link ID. Would you like me to include
> a second patch in v2 which adds the link ID to that function along the
> same lines?

Maybe have that as a separate patch, but yeah, good idea - thanks for
looking!

johannes