mac80211: fix scan vs. interface removal race

When we remove an interface, we can currently end up having
a pointer to it left in local->scan_sdata after it has been
set down, and then with a hardware scan the scan completion
can try to access it which is a bug. Alternatively, a scan
that started as a hardware scan may terminate as though it
was a software scan, if the timing is just right.

On SMP systems, software scan also has a similar problem,
just canceling the delayed work and setting a flag isn't
enough since it may be running concurrently; in this case
we would also never restore state of other interfaces.

This patch hopefully fixes the problems by always invoking
ieee80211_scan_completed or requiring it to be invoked by
the driver, I suspect the drivers that have ->hw_scan() are
buggy. The bug will not manifest itself unless you remove
the interface while hw-scanning which will also turn off
the hw, and then add a new interface which will be unusable
until you scan once.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 25 insertions, 8 deletions

diff --git a/net/mac80211/main.c b/net/mac80211/main.c
index ebdec7106d63..4bfac4b41c51 100644
--- a/net/mac80211/main.c
+++ b/net/mac80211/main.c
@@ -564,14 +564,6 @@ static int ieee80211_stop(struct net_device *dev)
                 synchronize_rcu();
                 skb_queue_purge(&sdata->u.sta.skb_queue);
 
-                if (local->scan_sdata == sdata) {
-                        if (!local->ops->hw_scan) {
-                                local->sta_sw_scanning = 0;
-                                cancel_delayed_work(&local->scan_work);
-                        } else
-                                local->sta_hw_scanning = 0;
-                }
-
                 sdata->u.sta.flags &= ~IEEE80211_STA_PRIVACY_INVOKED;
                 kfree(sdata->u.sta.extra_ie);
                 sdata->u.sta.extra_ie = NULL;
@@ -585,6 +577,31 @@ static int ieee80211_stop(struct net_device *dev)
                 }
                 /* fall through */
         default:
+                if (local->scan_sdata == sdata) {
+                        if (!local->ops->hw_scan)
+                                cancel_delayed_work_sync(&local->scan_work);
+                        /*
+                         * The software scan can no longer run now, so we can
+                         * clear out the scan_sdata reference. However, the
+                         * hardware scan may still be running. The complete
+                         * function must be prepared to handle a NULL value.
+                         */
+                        local->scan_sdata = NULL;
+                        /*
+                         * The memory barrier guarantees that another CPU
+                         * that is hardware-scanning will now see the fact
+                         * that this interface is gone.
+                         */
+                        smp_mb();
+                        /*
+                         * If software scanning, complete the scan but since
+                         * the scan_sdata is NULL already don't send out a
+                         * scan event to userspace -- the scan is incomplete.
+                         */
+                        if (local->sta_sw_scanning)
+                                ieee80211_scan_completed(&local->hw);
+                }
+
                 conf.vif = &sdata->vif;
                 conf.type = sdata->vif.type;
                 conf.mac_addr = dev->dev_addr;