Commit message: netfilter: nf_nat_redirect: add missing NULL pointer check

Commit 8b13eddfdf04cbfa561725cfc42d6868fe896f56 ("netfilter: refactor NAT
redirect IPv4 to use it from nf_tables") has introduced a trivial logic
change which can result in the following crash.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000030
IP: [<ffffffffa033002d>] nf_nat_redirect_ipv4+0x2d/0xa0 [nf_nat_redirect]
PGD 3ba662067 PUD 3ba661067 PMD 0
Oops: 0000 [#1] SMP
Modules linked in: ipv6(E) xt_REDIRECT(E) nf_nat_redirect(E) xt_tcpudp(E) iptable_nat(E) nf_conntrack_ipv4(E) nf_defrag_ipv4(E) nf_nat_ipv4(E) nf_nat(E) nf_conntrack(E) ip_tables(E) x_tables(E) binfmt_misc(E) xfs(E) libcrc32c(E) evbug(E) evdev(E) psmouse(E) i2c_piix4(E) i2c_core(E) acpi_cpufreq(E) button(E) ext4(E) crc16(E) jbd2(E) mbcache(E) dm_mirror(E) dm_region_hash(E) dm_log(E) dm_mod(E)
CPU: 0 PID: 2536 Comm: ip Tainted: G            E   4.1.7-15.23.amzn1.x86_64 #1
Hardware name: Xen HVM domU, BIOS 4.2.amazon 05/06/2015
task: ffff8800eb438000 ti: ffff8803ba664000 task.ti: ffff8803ba664000
[...]
Call Trace:
 <IRQ>
 [<ffffffffa0334065>] redirect_tg4+0x15/0x20 [xt_REDIRECT]
 [<ffffffffa02e2e99>] ipt_do_table+0x2b9/0x5e1 [ip_tables]
 [<ffffffffa0328045>] iptable_nat_do_chain+0x25/0x30 [iptable_nat]
 [<ffffffffa031777d>] nf_nat_ipv4_fn+0x13d/0x1f0 [nf_nat_ipv4]
 [<ffffffffa0328020>] ? iptable_nat_ipv4_fn+0x20/0x20 [iptable_nat]
 [<ffffffffa031785e>] nf_nat_ipv4_in+0x2e/0x90 [nf_nat_ipv4]
 [<ffffffffa03280a5>] iptable_nat_ipv4_in+0x15/0x20 [iptable_nat]
 [<ffffffff81449137>] nf_iterate+0x57/0x80
 [<ffffffff814491f7>] nf_hook_slow+0x97/0x100
 [<ffffffff814504d4>] ip_rcv+0x314/0x400

unsigned int
nf_nat_redirect_ipv4(struct sk_buff *skb,
...
{
...
		rcu_read_lock();
		indev = __in_dev_get_rcu(skb->dev);
		if (indev != NULL) {
			ifa = indev->ifa_list;
			newdst = ifa->ifa_local; <---
		}
		rcu_read_unlock();
...
}

Before the commit, 'ifa' had been always checked before access. After the
commit, however, it could be accessed even if it's NULL. Interestingly,
this was once fixed in 2003.

http://marc.info/?l=netfilter-devel&m=106668497403047&w=2

In addition to the original one, we have seen the crash when packets that
need to be redirected somehow arrive on an interface which hasn't been
yet fully configured.

This change just reverts the logic to the old behavior to avoid the crash.

Fixes: 8b13eddfdf04 ("netfilter: refactor NAT redirect IPv4 to use it from nf_tables")
Signed-off-by: Munehisa Kamata <kamatam@amazon.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
CVE: CVE-2015-8787
CWE: CWE-703
Vulnerability description: The nf_nat_redirect_ipv4 function in net/netfilter/nf_nat_redirect.c in the Linux kernel before 4.4 allows remote attackers to cause a denial of service (NULL pointer dereference and system crash) or possibly have unspecified other impact by sending certain IPv4 packets to an incompletely configured interface, a related issue to CVE-2003-1604.

Project: linux
File: nf_nat_redirect.c

nf_nat_redirect_ipv6(struct sk_buff *skb, const struct nf_nat_range *range,
		     unsigned int hooknum)
{
	struct nf_nat_range newrange;
	struct in6_addr newdst;
	enum ip_conntrack_info ctinfo;
	struct nf_conn *ct;

	ct = nf_ct_get(skb, &ctinfo);
	if (hooknum == NF_INET_LOCAL_OUT) {
		newdst = loopback_addr;
	} else {
		struct inet6_dev *idev;
		struct inet6_ifaddr *ifa;
		bool addr = false;

		rcu_read_lock();
		idev = __in6_dev_get(skb->dev);
		if (idev != NULL) {
			list_for_each_entry(ifa, &idev->addr_list, if_list) {
				newdst = ifa->addr;
				addr = true;
				break;
			}
		}
		rcu_read_unlock();

		if (!addr)
			return NF_DROP;
	}

	newrange.flags		= range->flags | NF_NAT_RANGE_MAP_IPS;
	newrange.min_addr.in6	= newdst;
	newrange.max_addr.in6	= newdst;
	newrange.min_proto	= range->min_proto;
	newrange.max_proto	= range->max_proto;

	return nf_nat_setup_info(ct, &newrange, NF_NAT_MANIP_DST);
}