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@xxxxxxxxxx>
Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
---
V2: Address Florian Westphal concerns
* add check after !nf_ct_is_confirmed()
* handle exit case if del_timer(&exp->timeout) fails
net/netfilter/nf_conntrack_core.c | 2 +-
net/netfilter/nf_conntrack_expect.c | 14 ++++++++++++++
2 files changed, 15 insertions(+), 1 deletions(-)
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index 289b279..92d5977 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -902,6 +902,7 @@ init_conntrack(struct net *net, struct nf_conn *tmpl,
ct, exp);
/* Welcome, Mr. Bond. We've been expecting you... */
__set_bit(IPS_EXPECTED_BIT, &ct->status);
+ /* exp->master safe, refcnt bumped in nf_ct_find_expectation */
ct->master = exp->master;
if (exp->helper) {
help = nf_ct_helper_ext_add(ct, exp->helper,
@@ -916,7 +917,6 @@ init_conntrack(struct net *net, struct nf_conn *tmpl,
#ifdef CONFIG_NF_CONNTRACK_SECMARK
ct->secmark = exp->master->secmark;
#endif
- nf_conntrack_get(&ct->master->ct_general);
NF_CT_STAT_INC(net, expect_new);
} else {
__nf_ct_try_assign_helper(ct, tmpl, GFP_ATOMIC);
diff --git a/net/netfilter/nf_conntrack_expect.c b/net/netfilter/nf_conntrack_expect.c
index 4fd1ca9..1867bad 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;
}
--
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