[PATCH net-next] nfnetlink: do not ack malformed messages

[Jiri Benc <jbenc@redhat.com>]

Commit 0628b123c96d ("netfilter: nfnetlink: add batch support and use it
from nf_tables") introduced a bug leading to various crashes in netlink_ack
when netlink message with invalid nlmsg_len was sent by an unprivileged
user.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
---
 net/netfilter/nfnetlink.c |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/netfilter/nfnetlink.c b/net/netfilter/nfnetlink.c
index 027f16a..046aa13 100644
--- a/net/netfilter/nfnetlink.c
+++ b/net/netfilter/nfnetlink.c
@@ -363,13 +363,15 @@ static void nfnetlink_rcv(struct sk_buff *skb)
 	struct net *net = sock_net(skb->sk);
 	int msglen;
 
-	if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
-		return netlink_ack(skb, nlh, -EPERM);
-
 	if (nlh->nlmsg_len < NLMSG_HDRLEN ||
 	    skb->len < nlh->nlmsg_len)
 		return;
 
+	if (!ns_capable(net->user_ns, CAP_NET_ADMIN)) {
+		netlink_ack(skb, nlh, -EPERM);
+		return;
+	}
+
 	if (nlh->nlmsg_type == NFNL_MSG_BATCH_BEGIN) {
 		struct nfgenmsg *nfgenmsg;
 
-- 
1.7.6.5