netfilter: xt_osf: Add missing permission checks

CVE-2017-17450

The capability check in nfnetlink_rcv() verifies that the caller
has CAP_NET_ADMIN in the namespace that "owns" the netlink socket.
However, xt_osf_fingers is shared by all net namespaces on the
system.  An unprivileged user can create user and net namespaces
in which he holds CAP_NET_ADMIN to bypass the netlink_net_capable()
check:

    vpnns -- nfnl_osf -f /tmp/pf.os

    vpnns -- nfnl_osf -f /tmp/pf.os -d

These non-root operations successfully modify the systemwide OS
fingerprint list.  Add new capable() checks so that they can't.
Signed-off-by: Kevin Cernekee <cernekee@chromium.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
(cherry picked from commit 916a2790)
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

net/netfilter/xt_osf.c

@@ -19,6 +19,7 @@
 #include <linux/module.h>
 #include <linux/kernel.h>
 
+#include <linux/capability.h>
 #include <linux/if.h>
 #include <linux/inetdevice.h>
 #include <linux/ip.h>
@@ -69,6 +70,9 @@ static int xt_osf_add_callback(struct sock *ctnl, struct sk_buff *skb,
 	struct xt_osf_finger *kf = NULL, *sf;
 	int err = 0;
 
+	if (!capable(CAP_NET_ADMIN))
+		return -EPERM;
+
 	if (!osf_attrs[OSF_ATTR_FINGER])
 		return -EINVAL;
 
@@ -112,6 +116,9 @@ static int xt_osf_remove_callback(struct sock *ctnl, struct sk_buff *skb,
 	struct xt_osf_finger *sf;
 	int err = -ENOENT;
 
+	if (!capable(CAP_NET_ADMIN))
+		return -EPERM;
+
 	if (!osf_attrs[OSF_ATTR_FINGER])
 		return -EINVAL;