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Message-Id: <20241107173134.542802-1-jchapman@katalix.com>
Date: Thu, 7 Nov 2024 17:31:34 +0000
From: James Chapman <jchapman@...alix.com>
To: netdev@...r.kernel.org
Subject: [PATCH RFC] l2tp: fix warning in l2tp_exit_net found by syzbot
The following patch fixes a WARN_ON in l2tp found by syzbot but I am
not confident that it's the right approach, hence this RFC.
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);
However, 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 one or more internal
radix-tree nodes and these cause idr_is_empty to return false. These
internal nodes are cleaned by idr_destroy.
Using idr_for_each to check that the IDR is empty instead of
idr_is_empty, as per the included patch, avoids the problem. But the
idr_is_empty pattern is used in other code too,
e.g. drivers/net/ethernet/mellanox/mlxsw/spectrum_pgt.c
Should idr_is_empty be returning false if the radix tree contains only
internal nodes?
Ref: https://syzkaller.appspot.com/bug?extid=332fe1e67018625f63c9
---
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
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