netfilter: conntrack: fix race between nf_conntrack proc read and hash resize

When we do "cat /proc/net/nf_conntrack", and meanwhile resize the conntrack
hash table via /sys/module/nf_conntrack/parameters/hashsize, race will
happen, because reader can observe a newly allocated hash but the old size
(or vice versa). So oops will happen like follows：

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000017
  IP: [<ffffffffa0418e21>] seq_print_acct+0x11/0x50 [nf_conntrack]
  Call Trace:
  [<ffffffffa0412f4e>] ? ct_seq_show+0x14e/0x340 [nf_conntrack]
  [<ffffffff81261a1c>] seq_read+0x2cc/0x390
  [<ffffffff812a8d62>] proc_reg_read+0x42/0x70
  [<ffffffff8123bee7>] __vfs_read+0x37/0x130
  [<ffffffff81347980>] ? security_file_permission+0xa0/0xc0
  [<ffffffff8123cf75>] vfs_read+0x95/0x140
  [<ffffffff8123e475>] SyS_read+0x55/0xc0
  [<ffffffff817c2572>] entry_SYSCALL_64_fastpath+0x1a/0xa4

It is very easy to reproduce this kernel crash.
1. open one shell and input the following cmds:
  while : ; do
    echo $RANDOM > /sys/module/nf_conntrack/parameters/hashsize
  done
2. open more shells and input the following cmds:
  while : ; do
    cat /proc/net/nf_conntrack
  done
3. just wait a monent, oops will happen soon.

The solution in this patch is based on Florian's Commit 5e3c61f9
("netfilter: conntrack: fix lookup race during hash resize"). And
add a wrapper function nf_conntrack_get_ht to get hash and hsize
suggested by Florian Westphal.
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_conntrack_core.h

@@ -51,6 +51,8 @@ bool nf_ct_invert_tuple(struct nf_conntrack_tuple *inverse,
 			const struct nf_conntrack_l3proto *l3proto,
 			const struct nf_conntrack_l4proto *l4proto);
 
+void nf_conntrack_get_ht(struct hlist_nulls_head **hash, unsigned int *hsize);
+
 /* Find a connection corresponding to a tuple. */
 struct nf_conntrack_tuple_hash *
 nf_conntrack_find_get(struct net *net,

net/ipv4/netfilter/nf_conntrack_l3proto_ipv4_compat.c

@@ -26,6 +26,8 @@
 
 struct ct_iter_state {
 	struct seq_net_private p;
+	struct hlist_nulls_head *hash;
+	unsigned int htable_size;
 	unsigned int bucket;
 };
 
@@ -35,10 +37,10 @@ static struct hlist_nulls_node *ct_get_first(struct seq_file *seq)
 	struct hlist_nulls_node *n;
 
 	for (st->bucket = 0;
-	     st->bucket < nf_conntrack_htable_size;
+	     st->bucket < st->htable_size;
 	     st->bucket++) {
 		n = rcu_dereference(
-			hlist_nulls_first_rcu(&nf_conntrack_hash[st->bucket]));
+			hlist_nulls_first_rcu(&st->hash[st->bucket]));
 		if (!is_a_nulls(n))
 			return n;
 	}
@@ -53,11 +55,11 @@ static struct hlist_nulls_node *ct_get_next(struct seq_file *seq,
 	head = rcu_dereference(hlist_nulls_next_rcu(head));
 	while (is_a_nulls(head)) {
 		if (likely(get_nulls_value(head) == st->bucket)) {
-			if (++st->bucket >= nf_conntrack_htable_size)
+			if (++st->bucket >= st->htable_size)
 				return NULL;
 		}
 		head = rcu_dereference(
-			hlist_nulls_first_rcu(&nf_conntrack_hash[st->bucket]));
+			hlist_nulls_first_rcu(&st->hash[st->bucket]));
 	}
 	return head;
 }
@@ -75,7 +77,11 @@ static struct hlist_nulls_node *ct_get_idx(struct seq_file *seq, loff_t pos)
 static void *ct_seq_start(struct seq_file *seq, loff_t *pos)
 	__acquires(RCU)
 {
+	struct ct_iter_state *st = seq->private;
+
 	rcu_read_lock();
+
+	nf_conntrack_get_ht(&st->hash, &st->htable_size);
 	return ct_get_idx(seq, *pos);
 }
 

net/netfilter/nf_conntrack_core.c

@@ -460,6 +460,23 @@ nf_ct_key_equal(struct nf_conntrack_tuple_hash *h,
 	       net_eq(net, nf_ct_net(ct));
 }
 
+/* must be called with rcu read lock held */
+void nf_conntrack_get_ht(struct hlist_nulls_head **hash, unsigned int *hsize)
+{
+	struct hlist_nulls_head *hptr;
+	unsigned int sequence, hsz;
+
+	do {
+		sequence = read_seqcount_begin(&nf_conntrack_generation);
+		hsz = nf_conntrack_htable_size;
+		hptr = nf_conntrack_hash;
+	} while (read_seqcount_retry(&nf_conntrack_generation, sequence));
+
+	*hash = hptr;
+	*hsize = hsz;
+}
+EXPORT_SYMBOL_GPL(nf_conntrack_get_ht);
+
 /*
  * Warning :
  * - Caller must take a reference on returned object

net/netfilter/nf_conntrack_standalone.c

@@ -48,6 +48,8 @@ EXPORT_SYMBOL_GPL(print_tuple);
 
 struct ct_iter_state {
 	struct seq_net_private p;
+	struct hlist_nulls_head *hash;
+	unsigned int htable_size;
 	unsigned int bucket;
 	u_int64_t time_now;
 };
@@ -58,9 +60,10 @@ static struct hlist_nulls_node *ct_get_first(struct seq_file *seq)
 	struct hlist_nulls_node *n;
 
 	for (st->bucket = 0;
-	     st->bucket < nf_conntrack_htable_size;
+	     st->bucket < st->htable_size;
 	     st->bucket++) {
-		n = rcu_dereference(hlist_nulls_first_rcu(&nf_conntrack_hash[st->bucket]));
+		n = rcu_dereference(
+			hlist_nulls_first_rcu(&st->hash[st->bucket]));
 		if (!is_a_nulls(n))
 			return n;
 	}
@@ -75,12 +78,11 @@ static struct hlist_nulls_node *ct_get_next(struct seq_file *seq,
 	head = rcu_dereference(hlist_nulls_next_rcu(head));
 	while (is_a_nulls(head)) {
 		if (likely(get_nulls_value(head) == st->bucket)) {
-			if (++st->bucket >= nf_conntrack_htable_size)
+			if (++st->bucket >= st->htable_size)
 				return NULL;
 		}
 		head = rcu_dereference(
-				hlist_nulls_first_rcu(
-					&nf_conntrack_hash[st->bucket]));
+			hlist_nulls_first_rcu(&st->hash[st->bucket]));
 	}
 	return head;
 }
@@ -102,6 +104,8 @@ static void *ct_seq_start(struct seq_file *seq, loff_t *pos)
 
 	st->time_now = ktime_get_real_ns();
 	rcu_read_lock();
+
+	nf_conntrack_get_ht(&st->hash, &st->htable_size);
 	return ct_get_idx(seq, *pos);
 }