netfilter: avoid race with exp->master ct

Preparation for disconnecting the nf_conntrack_lock from the
expectations code.  Once the nf_conntrack_lock is lifted, a race
condition is exposed.

The expectations master conntrack exp->master, can race with
delete operations, as the refcnt increment happens too late in
init_conntrack().  Race is against other CPUs invoking
->destroy() (destroy_conntrack()), or nf_ct_delete() (via timeout
or early_drop()).

Avoid this race in nf_ct_find_expectation() by using atomic_inc_not_zero(),
and checking if nf_ct_is_dying() (path via nf_ct_delete()).

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/net/netfilter/nf_conntrack_expect.c b/net/netfilter/nf_conntrack_expect.c
index da2f84f41777..f02805e0c7c5 100644
--- a/net/netfilter/nf_conntrack_expect.c
+++ b/net/netfilter/nf_conntrack_expect.c
@@ -155,6 +155,18 @@ nf_ct_find_expectation(struct net *net, u16 zone,
 	if (!nf_ct_is_confirmed(exp->master))
 		return NULL;
 
+	/* Avoid race with other CPUs, that for exp->master ct, is
+	 * about to invoke ->destroy(), or nf_ct_delete() via timeout
+	 * or early_drop().
+	 *
+	 * The atomic_inc_not_zero() check tells:  If that fails, we
+	 * know that the ct is being destroyed.  If it succeeds, we
+	 * can be sure the ct cannot disappear underneath.
+	 */
+	if (unlikely(nf_ct_is_dying(exp->master) ||
+		     !atomic_inc_not_zero(&exp->master->ct_general.use)))
+		return NULL;
+
 	if (exp->flags & NF_CT_EXPECT_PERMANENT) {
 		atomic_inc(&exp->use);
 		return exp;
@@ -162,6 +174,8 @@ nf_ct_find_expectation(struct net *net, u16 zone,
 		nf_ct_unlink_expect(exp);
 		return exp;
 	}
+	/* Undo exp->master refcnt increase, if del_timer() failed */
+	nf_ct_put(exp->master);
 
 	return NULL;
 }