Commit message: ip: frags: fix crash in ip_do_fragment()

A kernel crash occurrs when defragmented packet is fragmented
in ip_do_fragment().
In defragment routine, skb_orphan() is called and
skb->ip_defrag_offset is set. but skb->sk and
skb->ip_defrag_offset are same union member. so that
frag->sk is not NULL.
Hence crash occurrs in skb->sk check routine in ip_do_fragment() when
defragmented packet is fragmented.

test commands:
   %iptables -t nat -I POSTROUTING -j MASQUERADE
   %hping3 192.168.4.2 -s 1000 -p 2000 -d 60000

splat looks like:
[  261.069429] kernel BUG at net/ipv4/ip_output.c:636!
[  261.075753] invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[  261.083854] CPU: 1 PID: 1349 Comm: hping3 Not tainted 4.19.0-rc2+ #3
[  261.100977] RIP: 0010:ip_do_fragment+0x1613/0x2600
[  261.106945] Code: e8 e2 38 e3 fe 4c 8b 44 24 18 48 8b 74 24 08 e9 92 f6 ff ff 80 3c 02 00 0f 85 da 07 00 00 48 8b b5 d0 00 00 00 e9 25 f6 ff ff <0f> 0b 0f 0b 44 8b 54 24 58 4c 8b 4c 24 18 4c 8b 5c 24 60 4c 8b 6c
[  261.127015] RSP: 0018:ffff8801031cf2c0 EFLAGS: 00010202
[  261.134156] RAX: 1ffff1002297537b RBX: ffffed0020639e6e RCX: 0000000000000004
[  261.142156] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff880114ba9bd8
[  261.150157] RBP: ffff880114ba8a40 R08: ffffed0022975395 R09: ffffed0022975395
[  261.158157] R10: 0000000000000001 R11: ffffed0022975394 R12: ffff880114ba9ca4
[  261.166159] R13: 0000000000000010 R14: ffff880114ba9bc0 R15: dffffc0000000000
[  261.174169] FS:  00007fbae2199700(0000) GS:ffff88011b400000(0000) knlGS:0000000000000000
[  261.183012] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  261.189013] CR2: 00005579244fe000 CR3: 0000000119bf4000 CR4: 00000000001006e0
[  261.198158] Call Trace:
[  261.199018]  ? dst_output+0x180/0x180
[  261.205011]  ? save_trace+0x300/0x300
[  261.209018]  ? ip_copy_metadata+0xb00/0xb00
[  261.213034]  ? sched_clock_local+0xd4/0x140
[  261.218158]  ? kill_l4proto+0x120/0x120 [nf_conntrack]
[  261.223014]  ? rt_cpu_seq_stop+0x10/0x10
[  261.227014]  ? find_held_lock+0x39/0x1c0
[  261.233008]  ip_finish_output+0x51d/0xb50
[  261.237006]  ? ip_fragment.constprop.56+0x220/0x220
[  261.243011]  ? nf_ct_l4proto_register_one+0x5b0/0x5b0 [nf_conntrack]
[  261.250152]  ? rcu_is_watching+0x77/0x120
[  261.255010]  ? nf_nat_ipv4_out+0x1e/0x2b0 [nf_nat_ipv4]
[  261.261033]  ? nf_hook_slow+0xb1/0x160
[  261.265007]  ip_output+0x1c7/0x710
[  261.269005]  ? ip_mc_output+0x13f0/0x13f0
[  261.273002]  ? __local_bh_enable_ip+0xe9/0x1b0
[  261.278152]  ? ip_fragment.constprop.56+0x220/0x220
[  261.282996]  ? nf_hook_slow+0xb1/0x160
[  261.287007]  raw_sendmsg+0x21f9/0x4420
[  261.291008]  ? dst_output+0x180/0x180
[  261.297003]  ? sched_clock_cpu+0x126/0x170
[  261.301003]  ? find_held_lock+0x39/0x1c0
[  261.306155]  ? stop_critical_timings+0x420/0x420
[  261.311004]  ? check_flags.part.36+0x450/0x450
[  261.315005]  ? _raw_spin_unlock_irq+0x29/0x40
[  261.320995]  ? _raw_spin_unlock_irq+0x29/0x40
[  261.326142]  ? cyc2ns_read_end+0x10/0x10
[  261.330139]  ? raw_bind+0x280/0x280
[  261.334138]  ? sched_clock_cpu+0x126/0x170
[  261.338995]  ? check_flags.part.36+0x450/0x450
[  261.342991]  ? __lock_acquire+0x4500/0x4500
[  261.348994]  ? inet_sendmsg+0x11c/0x500
[  261.352989]  ? dst_output+0x180/0x180
[  261.357012]  inet_sendmsg+0x11c/0x500
[ ... ]

v2:
 - clear skb->sk at reassembly routine.(Eric Dumarzet)

Fixes: fa0f527358bd ("ip: use rb trees for IP frag queue.")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
CVE: CVE-2018-14641
CWE: CWE-20
Vulnerability description: A security flaw was found in the ip_frag_reasm() function in net/ipv4/ip_fragment.c in the Linux kernel from 4.19-rc1 to 4.19-rc3 inclusive, which can cause a later system crash in ip_do_fragment(). With certain non-default, but non-rare, configuration of a victim host, an attacker can trigger this crash remotely, thus leading to a remote denial-of-service.

Project: linux
File: ip_fragment.c

static int ip_frag_reasm(struct ipq *qp, struct sk_buff *skb,
			 struct sk_buff *prev_tail, struct net_device *dev)
{
	struct net *net = container_of(qp->q.net, struct net, ipv4.frags);
	struct iphdr *iph;
	struct sk_buff *fp, *head = skb_rb_first(&qp->q.rb_fragments);
	struct sk_buff **nextp; /* To build frag_list. */
	struct rb_node *rbn;
	int len;
	int ihlen;
	int err;
	u8 ecn;

	ipq_kill(qp);

	ecn = ip_frag_ecn_table[qp->ecn];
	if (unlikely(ecn == 0xff)) {
		err = -EINVAL;
		goto out_fail;
	}
	/* Make the one we just received the head. */
	if (head != skb) {
		fp = skb_clone(skb, GFP_ATOMIC);
		if (!fp)
			goto out_nomem;
		FRAG_CB(fp)->next_frag = FRAG_CB(skb)->next_frag;
		if (RB_EMPTY_NODE(&skb->rbnode))
			FRAG_CB(prev_tail)->next_frag = fp;
		else
			rb_replace_node(&skb->rbnode, &fp->rbnode,
					&qp->q.rb_fragments);
		if (qp->q.fragments_tail == skb)
			qp->q.fragments_tail = fp;
		skb_morph(skb, head);
		FRAG_CB(skb)->next_frag = FRAG_CB(head)->next_frag;
		rb_replace_node(&head->rbnode, &skb->rbnode,
				&qp->q.rb_fragments);
		consume_skb(head);
		head = skb;
	}

	WARN_ON(head->ip_defrag_offset != 0);

	/* Allocate a new buffer for the datagram. */
	ihlen = ip_hdrlen(head);
	len = ihlen + qp->q.len;

	err = -E2BIG;
	if (len > 65535)
		goto out_oversize;

	/* Head of list must not be cloned. */
	if (skb_unclone(head, GFP_ATOMIC))
		goto out_nomem;

	/* If the first fragment is fragmented itself, we split
	 * it to two chunks: the first with data and paged part
	 * and the second, holding only fragments. */
	if (skb_has_frag_list(head)) {
		struct sk_buff *clone;
		int i, plen = 0;

		clone = alloc_skb(0, GFP_ATOMIC);
		if (!clone)
			goto out_nomem;
		skb_shinfo(clone)->frag_list = skb_shinfo(head)->frag_list;
		skb_frag_list_init(head);
		for (i = 0; i < skb_shinfo(head)->nr_frags; i++)
			plen += skb_frag_size(&skb_shinfo(head)->frags[i]);
		clone->len = clone->data_len = head->data_len - plen;
		head->truesize += clone->truesize;
		clone->csum = 0;
		clone->ip_summed = head->ip_summed;
		add_frag_mem_limit(qp->q.net, clone->truesize);
		skb_shinfo(head)->frag_list = clone;
		nextp = &clone->next;
	} else {
		nextp = &skb_shinfo(head)->frag_list;
	}

	skb_push(head, head->data - skb_network_header(head));

	/* Traverse the tree in order, to build frag_list. */
	fp = FRAG_CB(head)->next_frag;
	rbn = rb_next(&head->rbnode);
	rb_erase(&head->rbnode, &qp->q.rb_fragments);
	while (rbn || fp) {
		/* fp points to the next sk_buff in the current run;
		 * rbn points to the next run.
		 */
		/* Go through the current run. */
		while (fp) {
			*nextp = fp;
			nextp = &fp->next;
			fp->prev = NULL;
			memset(&fp->rbnode, 0, sizeof(fp->rbnode));
			head->data_len += fp->len;
			head->len += fp->len;
			if (head->ip_summed != fp->ip_summed)
				head->ip_summed = CHECKSUM_NONE;
			else if (head->ip_summed == CHECKSUM_COMPLETE)
				head->csum = csum_add(head->csum, fp->csum);
			head->truesize += fp->truesize;
			fp = FRAG_CB(fp)->next_frag;
		}
		/* Move to the next run. */
		if (rbn) {
			struct rb_node *rbnext = rb_next(rbn);

			fp = rb_to_skb(rbn);
			rb_erase(rbn, &qp->q.rb_fragments);
			rbn = rbnext;
		}
	}
	sub_frag_mem_limit(qp->q.net, head->truesize);

	*nextp = NULL;
	head->next = NULL;
	head->prev = NULL;
	head->dev = dev;
	head->tstamp = qp->q.stamp;
	IPCB(head)->frag_max_size = max(qp->max_df_size, qp->q.max_size);

	iph = ip_hdr(head);
	iph->tot_len = htons(len);
	iph->tos |= ecn;

	/* When we set IP_DF on a refragmented skb we must also force a
	 * call to ip_fragment to avoid forwarding a DF-skb of size s while
	 * original sender only sent fragments of size f (where f < s).
	 *
	 * We only set DF/IPSKB_FRAG_PMTU if such DF fragment was the largest
	 * frag seen to avoid sending tiny DF-fragments in case skb was built
	 * from one very small df-fragment and one large non-df frag.
	 */
	if (qp->max_df_size == qp->q.max_size) {
		IPCB(head)->flags |= IPSKB_FRAG_PMTU;
		iph->frag_off = htons(IP_DF);
	} else {
		iph->frag_off = 0;
	}

	ip_send_check(iph);

	__IP_INC_STATS(net, IPSTATS_MIB_REASMOKS);
	qp->q.fragments = NULL;
	qp->q.rb_fragments = RB_ROOT;
	qp->q.fragments_tail = NULL;
	qp->q.last_run_head = NULL;
	return 0;

out_nomem:
	net_dbg_ratelimited("queue_glue: no memory for gluing queue %p\n", qp);
	err = -ENOMEM;
	goto out_fail;
out_oversize:
	net_info_ratelimited("Oversized IP packet from %pI4\n", &qp->q.key.v4.saddr);
out_fail:
	__IP_INC_STATS(net, IPSTATS_MIB_REASMFAILS);
	return err;
}