From b75911b66ad508a3c3f006ce37d9f9ebee34da43 Mon Sep 17 00:00:00 2001 From: Liping Zhang Date: Thu, 18 Aug 2016 20:39:05 +0800 Subject: [PATCH] netfilter: cttimeout: fix use after free error when delete netns In general, when we want to delete a netns, cttimeout_net_exit will be called before ipt_unregister_table, i.e. before ctnl_timeout_put. But after call kfree_rcu in cttimeout_net_exit, we will still decrease the timeout object's refcnt in ctnl_timeout_put, this is incorrect, and will cause a use after free error. It is easy to reproduce this problem: # while : ; do ip netns add xxx ip netns exec xxx nfct add timeout testx inet icmp timeout 200 ip netns exec xxx iptables -t raw -p icmp -I OUTPUT -j CT --timeout testx ip netns del xxx done ======================================================================= BUG kmalloc-96 (Tainted: G B E ): Poison overwritten ----------------------------------------------------------------------- INFO: 0xffff88002b5161e8-0xffff88002b5161e8. First byte 0x6a instead of 0x6b INFO: Allocated in cttimeout_new_timeout+0xd4/0x240 [nfnetlink_cttimeout] age=104 cpu=0 pid=3330 ___slab_alloc+0x4da/0x540 __slab_alloc+0x20/0x40 __kmalloc+0x1c8/0x240 cttimeout_new_timeout+0xd4/0x240 [nfnetlink_cttimeout] nfnetlink_rcv_msg+0x21a/0x230 [nfnetlink] [ ... ] So only when the refcnt decreased to 0, we call kfree_rcu to free the timeout object. And like nfnetlink_acct do, use atomic_cmpxchg to avoid race between ctnl_timeout_try_del and ctnl_timeout_put. Signed-off-by: Liping Zhang Signed-off-by: Pablo Neira Ayuso --- net/netfilter/nfnetlink_cttimeout.c | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) diff --git a/net/netfilter/nfnetlink_cttimeout.c b/net/netfilter/nfnetlink_cttimeout.c index 4cdcd969b64c..68216cdc7083 100644 --- a/net/netfilter/nfnetlink_cttimeout.c +++ b/net/netfilter/nfnetlink_cttimeout.c @@ -330,16 +330,16 @@ static int ctnl_timeout_try_del(struct net *net, struct ctnl_timeout *timeout) { int ret = 0; - /* we want to avoid races with nf_ct_timeout_find_get. */ - if (atomic_dec_and_test(&timeout->refcnt)) { + /* We want to avoid races with ctnl_timeout_put. So only when the + * current refcnt is 1, we decrease it to 0. + */ + if (atomic_cmpxchg(&timeout->refcnt, 1, 0) == 1) { /* We are protected by nfnl mutex. */ list_del_rcu(&timeout->head); nf_ct_l4proto_put(timeout->l4proto); ctnl_untimeout(net, timeout); kfree_rcu(timeout, rcu_head); } else { - /* still in use, restore reference counter. */ - atomic_inc(&timeout->refcnt); ret = -EBUSY; } return ret; @@ -543,7 +543,9 @@ err: static void ctnl_timeout_put(struct ctnl_timeout *timeout) { - atomic_dec(&timeout->refcnt); + if (atomic_dec_and_test(&timeout->refcnt)) + kfree_rcu(timeout, rcu_head); + module_put(THIS_MODULE); } #endif /* CONFIG_NF_CONNTRACK_TIMEOUT */ @@ -591,7 +593,9 @@ static void __net_exit cttimeout_net_exit(struct net *net) list_for_each_entry_safe(cur, tmp, &net->nfct_timeout_list, head) { list_del_rcu(&cur->head); nf_ct_l4proto_put(cur->l4proto); - kfree_rcu(cur, rcu_head); + + if (atomic_dec_and_test(&cur->refcnt)) + kfree_rcu(cur, rcu_head); } } -- 2.20.1