netfilter: ctnetlink: fix refcount leak on table dump

There is a reference count leak in ctnetlink_dump_table():
      if (res < 0) {
                nf_conntrack_get(&ct->ct_general); // HERE
                cb->args[1] = (unsigned long)ct;
                ...

While its very unlikely, its possible that ct == last.
If this happens, then the refcount of ct was already incremented.
This 2nd increment is never undone.

This prevents the conntrack object from being released, which in turn
keeps prevents cnet->count from dropping back to 0.

This will then block the netns dismantle (or conntrack rmmod) as
nf_conntrack_cleanup_net_list() will wait forever.

This can be reproduced by running conntrack_resize.sh selftest in a loop.
It takes ~20 minutes for me on a preemptible kernel on average before
I see a runaway kworker spinning in nf_conntrack_cleanup_net_list.

One fix would to change this to:
        if (res < 0) {
		if (ct != last)
	                nf_conntrack_get(&ct->ct_general);

But this reference counting isn't needed in the first place.
We can just store a cookie value instead.

A followup patch will do the same for ctnetlink_exp_dump_table,
it looks to me as if this has the same problem and like
ctnetlink_dump_table, we only need a 'skip hint', not the actual
object so we can apply the same cookie strategy there as well.

Fixes: d205dc40798d ("[NETFILTER]: ctnetlink: fix deadlock in table dumping")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index 486d52b45fe57e264326a914794b9dc2f3da2248..f403acd824372c41c9833139078b643121ab66da 100644 (file)
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -884,8 +884,6 @@ errout:
 
 static int ctnetlink_done(struct netlink_callback *cb)
 {
-       if (cb->args[1])
-               nf_ct_put((struct nf_conn *)cb->args[1]);
        kfree(cb->data);
        return 0;
 }
@@ -1208,19 +1206,26 @@ ignore_entry:
        return 0;
 }
 
+static unsigned long ctnetlink_get_id(const struct nf_conn *ct)
+{
+       unsigned long id = nf_ct_get_id(ct);
+
+       return id ? id : 1;
+}
+
 static int
 ctnetlink_dump_table(struct sk_buff *skb, struct netlink_callback *cb)
 {
        unsigned int flags = cb->data ? NLM_F_DUMP_FILTERED : 0;
        struct net *net = sock_net(skb->sk);
-       struct nf_conn *ct, *last;
+       unsigned long last_id = cb->args[1];
        struct nf_conntrack_tuple_hash *h;
        struct hlist_nulls_node *n;
        struct nf_conn *nf_ct_evict[8];
+       struct nf_conn *ct;
        int res, i;
        spinlock_t *lockp;
 
-       last = (struct nf_conn *)cb->args[1];
        i = 0;
 
        local_bh_disable();
@@ -1257,7 +1262,7 @@ restart:
                                continue;
 
                        if (cb->args[1]) {
-                               if (ct != last)
+                               if (ctnetlink_get_id(ct) != last_id)
                                        continue;
                                cb->args[1] = 0;
                        }
@@ -1270,8 +1275,7 @@ restart:
                                            NFNL_MSG_TYPE(cb->nlh->nlmsg_type),
                                            ct, true, flags);
                        if (res < 0) {
-                               nf_conntrack_get(&ct->ct_general);
-                               cb->args[1] = (unsigned long)ct;
+                               cb->args[1] = ctnetlink_get_id(ct);
                                spin_unlock(lockp);
                                goto out;
                        }
@@ -1284,12 +1288,10 @@ restart:
        }
 out:
        local_bh_enable();
-       if (last) {
+       if (last_id) {
                /* nf ct hash resize happened, now clear the leftover. */
-               if ((struct nf_conn *)cb->args[1] == last)
+               if (cb->args[1] == last_id)
                        cb->args[1] = 0;
-
-               nf_ct_put(last);
        }
 
        while (i) {