net/l2tp: fix warning in l2tp_exit_net found by syzbot

In l2tp's net exit handler, we check that an IDR is empty before
destroying it:

	WARN_ON_ONCE(!idr_is_empty(&pn->l2tp_tunnel_idr));
	idr_destroy(&pn->l2tp_tunnel_idr);

By forcing memory allocation failures in idr_alloc_32, syzbot is able
to provoke a condition where idr_is_empty returns false despite there
being no items in the IDR. This turns out to be because the radix tree
of the IDR contains only internal radix-tree nodes and it is this that
causes idr_is_empty to return false. The internal nodes are cleaned by
idr_destroy.

Use idr_for_each to check that the IDR is empty instead of
idr_is_empty to avoid the problem.

Reported-by: syzbot+332fe1e67018625f63c9@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=332fe1e67018625f63c9
Fixes: 73d33bd ("l2tp: avoid using drain_workqueue in l2tp_pre_exit_net")
Signed-off-by: James Chapman <jchapman@katalix.com>
Link: https://patch.msgid.link/20241118140411.1582555-1-jchapman@katalix.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/l2tp/l2tp_core.c

@@ -1870,15 +1870,31 @@ static __net_exit void l2tp_pre_exit_net(struct net *net)
 	}
 }
 
+static int l2tp_idr_item_unexpected(int id, void *p, void *data)
+{
+	const char *idr_name = data;
+
+	pr_err("l2tp: %s IDR not empty at net %d exit\n", idr_name, id);
+	WARN_ON_ONCE(1);
+	return 1;
+}
+
 static __net_exit void l2tp_exit_net(struct net *net)
 {
 	struct l2tp_net *pn = l2tp_pernet(net);
 
-	WARN_ON_ONCE(!idr_is_empty(&pn->l2tp_v2_session_idr));
+	/* Our per-net IDRs should be empty. Check that is so, to
+	 * help catch cleanup races or refcnt leaks.
+	 */
+	idr_for_each(&pn->l2tp_v2_session_idr, l2tp_idr_item_unexpected,
+		     "v2_session");
+	idr_for_each(&pn->l2tp_v3_session_idr, l2tp_idr_item_unexpected,
+		     "v3_session");
+	idr_for_each(&pn->l2tp_tunnel_idr, l2tp_idr_item_unexpected,
+		     "tunnel");
+
 	idr_destroy(&pn->l2tp_v2_session_idr);
-	WARN_ON_ONCE(!idr_is_empty(&pn->l2tp_v3_session_idr));
 	idr_destroy(&pn->l2tp_v3_session_idr);
-	WARN_ON_ONCE(!idr_is_empty(&pn->l2tp_tunnel_idr));
 	idr_destroy(&pn->l2tp_tunnel_idr);
 }