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netfilter: avoid race with exp->master ct

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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>
This commit is contained in:
Jesper Dangaard Brouer 2014-03-03 14:45:39 +01:00 committed by Pablo Neira Ayuso
parent b7779d06f9
commit e1b207dac1
2 changed files with 15 additions and 1 deletions
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2
net/netfilter/nf_conntrack_core.c
View file

@ -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);

14
net/netfilter/nf_conntrack_expect.c
View file

@ -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;
}