wifi: cfg80211: fix missing interfaces when dumping

[ Upstream commit a6e4f85 ]

The nl80211_dump_interface() supports resumption
in case nl80211_send_iface() doesn't have the
resources to complete its work.

The logic would store the progress as iteration
offsets for rdev and wdev loops.

However the logic did not properly handle
resumption for non-last rdev. Assuming a system
with 2 rdevs, with 2 wdevs each, this could
happen:

 dump(cb=[0, 0]):
  if_start=cb[1] (=0)
  send rdev0.wdev0 -> ok
  send rdev0.wdev1 -> yield
  cb[1] = 1

 dump(cb=[0, 1]):
  if_start=cb[1] (=1)
  send rdev0.wdev1 -> ok
  // since if_start=1 the rdev0.wdev0 got skipped
  // through if_idx < if_start
  send rdev1.wdev1 -> ok

The if_start needs to be reset back to 0 upon wdev
loop end.

The problem is actually hard to hit on a desktop,
and even on most routers. The prerequisites for
this manifesting was:
 - more than 1 wiphy
 - a few handful of interfaces
 - dump without rdev or wdev filter

I was seeing this with 4 wiphys 9 interfaces each.
It'd miss 6 interfaces from the last wiphy
reported to userspace.

Signed-off-by: Michal Kazior <michal@plume.com>
Link: https://msgid.link/20240116142340.89678-1-kazikcz@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

net/wireless/nl80211.c

@@ -4012,6 +4012,7 @@ static int nl80211_dump_interface(struct sk_buff *skb, struct netlink_callback *
 			if_idx++;
 		}
 
+		if_start = 0;
 		wp_idx++;
 	}
  out: