[PATCH 4.17 025/324] netfilter: nf_ct_helper: Fix possible panic after nf_conntrack_helper_unregister

From: Greg Kroah-Hartman
Date: Thu Aug 23 2018 - 04:39:59 EST

4.17-stable review patch. If anyone has any objections, please let me know.


From: Gao Feng <gfree.wind@xxxxxxxxxxx>

[ Upstream commit ad9852af97587b8abe8102f9ddcb05c9769656f6 ]

The helper module would be unloaded after nf_conntrack_helper_unregister,
so it may cause a possible panic caused by race.

nf_ct_iterate_destroy(unhelp, me) reset the helper of conntrack as NULL,
but maybe someone has gotten the helper pointer during this period. Then
it would panic, when it accesses the helper and the module was unloaded.

Take an example as following:
helper = rcu_dereference(help->helper);
set helper as NULL
unload helper module
helper->to_nlattr(skb, ct);

As above, the cpu0 tries to access the helper and its module is unloaded,
then the panic happens.

Signed-off-by: Gao Feng <gfree.wind@xxxxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
net/netfilter/nf_conntrack_helper.c | 5 +++++
1 file changed, 5 insertions(+)

--- a/net/netfilter/nf_conntrack_helper.c
+++ b/net/netfilter/nf_conntrack_helper.c
@@ -465,6 +465,11 @@ void nf_conntrack_helper_unregister(stru

nf_ct_expect_iterate_destroy(expect_iter_me, NULL);
nf_ct_iterate_destroy(unhelp, me);
+ /* Maybe someone has gotten the helper already when unhelp above.
+ * So need to wait it.
+ */
+ synchronize_rcu();