[PATCH net] net/l2tp: fix warning in l2tp_exit_net found by syzbot

From: James Chapman
Date: Mon Nov 18 2024 - 09:14:32 EST


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@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=332fe1e67018625f63c9
Fixes: 73d33bd063c4 ("l2tp: avoid using drain_workqueue in l2tp_pre_exit_net")
Signed-off-by: James Chapman <jchapman@xxxxxxxxxxx>
---
net/l2tp/l2tp_core.c | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 3eec23ac5ab1..369a2f2e459c 100644
--- a/net/l2tp/l2tp_core.c
+++ b/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);
}

--
2.34.1