nl80211: fix attrbuf access race by allocating a separate one

Since my commit 3713b4e364 ("nl80211: allow splitting wiphy
information in dumps"), nl80211_dump_wiphy() uses the global
nl80211_fam.attrbuf for parsing the incoming data. This wouldn't
be a problem if it only did so on the first dump iteration which
is locked against other commands in generic netlink, but due to
space constraints in cb->args (the needed state doesn't fit) I
decided to always parse the original message. That's racy though
since nl80211_fam.attrbuf could be used by some other parsing in
generic netlink concurrently.

For now, fix this by allocating a separate parse buffer (it's a
bit too big for the stack, currently 1448 bytes on 64-bit). For
-next, I'll change the code to parse into the global buffer in
the first round only and then allocate a smaller buffer to keep
the data in cb->args.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -1564,12 +1564,17 @@ static int nl80211_dump_wiphy(struct sk_buff *skb, struct netlink_callback *cb)
 	struct cfg80211_registered_device *dev;
 	s64 filter_wiphy = -1;
 	bool split = false;
-	struct nlattr **tb = nl80211_fam.attrbuf;
+	struct nlattr **tb;
 	int res;
 
+	/* will be zeroed in nlmsg_parse() */
+	tb = kmalloc(sizeof(*tb) * (NL80211_ATTR_MAX + 1), GFP_KERNEL);
+	if (!tb)
+		return -ENOMEM;
+
 	mutex_lock(&cfg80211_mutex);
 	res = nlmsg_parse(cb->nlh, GENL_HDRLEN + nl80211_fam.hdrsize,
-			  tb, nl80211_fam.maxattr, nl80211_policy);
+			  tb, NL80211_ATTR_MAX, nl80211_policy);
 	if (res == 0) {
 		split = tb[NL80211_ATTR_SPLIT_WIPHY_DUMP];
 		if (tb[NL80211_ATTR_WIPHY])
@@ -1583,6 +1588,7 @@ static int nl80211_dump_wiphy(struct sk_buff *skb, struct netlink_callback *cb)
 			netdev = dev_get_by_index(sock_net(skb->sk), ifidx);
 			if (!netdev) {
 				mutex_unlock(&cfg80211_mutex);
+				kfree(tb);
 				return -ENODEV;
 			}
 			if (netdev->ieee80211_ptr) {
@@ -1593,6 +1599,7 @@ static int nl80211_dump_wiphy(struct sk_buff *skb, struct netlink_callback *cb)
 			dev_put(netdev);
 		}
 	}
+	kfree(tb);
 
 	list_for_each_entry(dev, &cfg80211_rdev_list, list) {
 		if (!net_eq(wiphy_net(&dev->wiphy), sock_net(skb->sk)))