inet: update the IP ID generation algorithm to higher standards.

Commit 355b98553789 ("netns: provide pure entropy for net_hash_mix()")
makes net_hash_mix() return a true 32 bits of entropy.  When used in the
IP ID generation algorithm, this has the effect of extending the IP ID
generation key from 32 bits to 64 bits.

However, net_hash_mix() is only used for IP ID generation starting with
kernel version 4.1.  Therefore, earlier kernels remain with 32-bit key
no matter what the net_hash_mix() return value is.

This change addresses the issue by explicitly extending the key to 64
bits for kernels older than 4.1.

Signed-off-by: Amit Klein <aksecurity@gmail.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
{
	struct sk_buff *frag;
	struct rt6_info *rt = (struct rt6_info *)skb_dst(skb);
	struct ipv6_pinfo *np = skb->sk && !dev_recursion_level() ?
				inet6_sk(skb->sk) : NULL;
	struct ipv6hdr *tmp_hdr;
	struct frag_hdr *fh;
	unsigned int mtu, hlen, left, len;
	int hroom, troom;
	__be32 frag_id = 0;
	int ptr, offset = 0, err = 0;
	u8 *prevhdr, nexthdr = 0;
	struct net *net = dev_net(skb_dst(skb)->dev);

	err = ip6_find_1stfragopt(skb, &prevhdr);
	if (err < 0)
		goto fail;
	hlen = err;
	nexthdr = *prevhdr;

	mtu = ip6_skb_dst_mtu(skb);

	/* We must not fragment if the socket is set to force MTU discovery
	 * or if the skb it not generated by a local socket.
	 */
	if (unlikely(!skb->ignore_df && skb->len > mtu) ||
		     (IP6CB(skb)->frag_max_size &&
		      IP6CB(skb)->frag_max_size > mtu)) {
		if (skb->sk && dst_allfrag(skb_dst(skb)))
			sk_nocaps_add(skb->sk, NETIF_F_GSO_MASK);

		skb->dev = skb_dst(skb)->dev;
		icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
		IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
			      IPSTATS_MIB_FRAGFAILS);
		kfree_skb(skb);
		return -EMSGSIZE;
	}

	if (np && np->frag_size < mtu) {
		if (np->frag_size)
			mtu = np->frag_size;
	}
	mtu -= hlen + sizeof(struct frag_hdr);

	if (skb_has_frag_list(skb)) {
		int first_len = skb_pagelen(skb);
		struct sk_buff *frag2;

		if (first_len - hlen > mtu ||
		    ((first_len - hlen) & 7) ||
		    skb_cloned(skb))
			goto slow_path;

		skb_walk_frags(skb, frag) {
			/* Correct geometry. */
			if (frag->len > mtu ||
			    ((frag->len & 7) && frag->next) ||
			    skb_headroom(frag) < hlen)
				goto slow_path_clean;

			/* Partially cloned skb? */
			if (skb_shared(frag))
				goto slow_path_clean;

			BUG_ON(frag->sk);
			if (skb->sk) {
				frag->sk = skb->sk;
				frag->destructor = sock_wfree;
			}
			skb->truesize -= frag->truesize;
		}

		err = 0;
		offset = 0;
		frag = skb_shinfo(skb)->frag_list;
		skb_frag_list_init(skb);
		/* BUILD HEADER */

		*prevhdr = NEXTHDR_FRAGMENT;
		tmp_hdr = kmemdup(skb_network_header(skb), hlen, GFP_ATOMIC);
		if (!tmp_hdr) {
			IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
				      IPSTATS_MIB_FRAGFAILS);
			return -ENOMEM;
		}

		__skb_pull(skb, hlen);
		fh = (struct frag_hdr *)__skb_push(skb, sizeof(struct frag_hdr));
		__skb_push(skb, hlen);
		skb_reset_network_header(skb);
		memcpy(skb_network_header(skb), tmp_hdr, hlen);

		ipv6_select_ident(fh, rt);
		fh->nexthdr = nexthdr;
		fh->reserved = 0;
		fh->frag_off = htons(IP6_MF);
		frag_id = fh->identification;

		first_len = skb_pagelen(skb);
		skb->data_len = first_len - skb_headlen(skb);
		skb->len = first_len;
		ipv6_hdr(skb)->payload_len = htons(first_len -
						   sizeof(struct ipv6hdr));

		dst_hold(&rt->dst);

		for (;;) {
			/* Prepare header of the next frame,
			 * before previous one went down. */
			if (frag) {
				frag->ip_summed = CHECKSUM_NONE;
				skb_reset_transport_header(frag);
				fh = (struct frag_hdr *)__skb_push(frag, sizeof(struct frag_hdr));
				__skb_push(frag, hlen);
				skb_reset_network_header(frag);
				memcpy(skb_network_header(frag), tmp_hdr,
				       hlen);
				offset += skb->len - hlen - sizeof(struct frag_hdr);
				fh->nexthdr = nexthdr;
				fh->reserved = 0;
				fh->frag_off = htons(offset);
				if (frag->next != NULL)
					fh->frag_off |= htons(IP6_MF);
				fh->identification = frag_id;
				ipv6_hdr(frag)->payload_len =
						htons(frag->len -
						      sizeof(struct ipv6hdr));
				ip6_copy_metadata(frag, skb);
			}

			err = output(skb);
			if (!err)
				IP6_INC_STATS(net, ip6_dst_idev(&rt->dst),
					      IPSTATS_MIB_FRAGCREATES);

			if (err || !frag)
				break;

			skb = frag;
			frag = skb->next;
			skb->next = NULL;
		}

		kfree(tmp_hdr);

		if (err == 0) {
			IP6_INC_STATS(net, ip6_dst_idev(&rt->dst),
				      IPSTATS_MIB_FRAGOKS);
			ip6_rt_put(rt);
			return 0;
		}

		kfree_skb_list(frag);

		IP6_INC_STATS(net, ip6_dst_idev(&rt->dst),
			      IPSTATS_MIB_FRAGFAILS);
		ip6_rt_put(rt);
		return err;

slow_path_clean:
		skb_walk_frags(skb, frag2) {
			if (frag2 == frag)
				break;
			frag2->sk = NULL;
			frag2->destructor = NULL;
			skb->truesize += frag2->truesize;
		}
	}

slow_path:
	if ((skb->ip_summed == CHECKSUM_PARTIAL) &&
	    skb_checksum_help(skb))
		goto fail;

	left = skb->len - hlen;		/* Space per frame */
	ptr = hlen;			/* Where to start from */

	/*
	 *	Fragment the datagram.
	 */

	*prevhdr = NEXTHDR_FRAGMENT;
	hroom = LL_RESERVED_SPACE(rt->dst.dev);
	troom = rt->dst.dev->needed_tailroom;

	/*
	 *	Keep copying data until we run out.
	 */
	while (left > 0)	{
		len = left;
		/* IF: it doesn't fit, use 'mtu' - the data space left */
		if (len > mtu)
			len = mtu;
		/* IF: we are not sending up to and including the packet end
		   then align the next start on an eight byte boundary */
		if (len < left)	{
			len &= ~7;
		}
		/*
		 *	Allocate buffer.
		 */

		if ((frag = alloc_skb(len + hlen + sizeof(struct frag_hdr) +
				      hroom + troom, GFP_ATOMIC)) == NULL) {
			NETDEBUG(KERN_INFO "IPv6: frag: no memory for new fragment!\n");
			IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
				      IPSTATS_MIB_FRAGFAILS);
			err = -ENOMEM;
			goto fail;
		}

		/*
		 *	Set up data on packet
		 */

		ip6_copy_metadata(frag, skb);
		skb_reserve(frag, hroom);
		skb_put(frag, len + hlen + sizeof(struct frag_hdr));
		skb_reset_network_header(frag);
		fh = (struct frag_hdr *)(skb_network_header(frag) + hlen);
		frag->transport_header = (frag->network_header + hlen +
					  sizeof(struct frag_hdr));

		/*
		 *	Charge the memory for the fragment to any owner
		 *	it might possess
		 */
		if (skb->sk)
			skb_set_owner_w(frag, skb->sk);

		/*
		 *	Copy the packet header into the new buffer.
		 */
		skb_copy_from_linear_data(skb, skb_network_header(frag), hlen);

		/*
		 *	Build fragment header.
		 */
		fh->nexthdr = nexthdr;
		fh->reserved = 0;
		if (!frag_id) {
			ipv6_select_ident(fh, rt);
			frag_id = fh->identification;
		} else
			fh->identification = frag_id;

		/*
		 *	Copy a block of the IP datagram.
		 */
		BUG_ON(skb_copy_bits(skb, ptr, skb_transport_header(frag),
				     len));
		left -= len;

		fh->frag_off = htons(offset);
		if (left > 0)
			fh->frag_off |= htons(IP6_MF);
		ipv6_hdr(frag)->payload_len = htons(frag->len -
						    sizeof(struct ipv6hdr));

		ptr += len;
		offset += len;

		/*
		 *	Put this fragment into the sending queue.
		 */
		err = output(frag);
		if (err)
			goto fail;

		IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
			      IPSTATS_MIB_FRAGCREATES);
	}
	IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
		      IPSTATS_MIB_FRAGOKS);
	consume_skb(skb);
	return err;

fail:
	IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
		      IPSTATS_MIB_FRAGFAILS);
	kfree_skb(skb);
	return err;
}

int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
{
	struct sk_buff *frag;
	struct rt6_info *rt = (struct rt6_info *)skb_dst(skb);
	struct ipv6_pinfo *np = skb->sk && !dev_recursion_level() ?
				inet6_sk(skb->sk) : NULL;
	struct ipv6hdr *tmp_hdr;
	struct frag_hdr *fh;
	unsigned int mtu, hlen, left, len;
	int hroom, troom;
	__be32 frag_id = 0;
	int ptr, offset = 0, err = 0;
	u8 *prevhdr, nexthdr = 0;
	struct net *net = dev_net(skb_dst(skb)->dev);

	err = ip6_find_1stfragopt(skb, &prevhdr);
	if (err < 0)
		goto fail;
	hlen = err;
	nexthdr = *prevhdr;

	mtu = ip6_skb_dst_mtu(skb);

	/* We must not fragment if the socket is set to force MTU discovery
	 * or if the skb it not generated by a local socket.
	 */
	if (unlikely(!skb->ignore_df && skb->len > mtu) ||
		     (IP6CB(skb)->frag_max_size &&
		      IP6CB(skb)->frag_max_size > mtu)) {
		if (skb->sk && dst_allfrag(skb_dst(skb)))
			sk_nocaps_add(skb->sk, NETIF_F_GSO_MASK);

		skb->dev = skb_dst(skb)->dev;
		icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
		IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
			      IPSTATS_MIB_FRAGFAILS);
		kfree_skb(skb);
		return -EMSGSIZE;
	}

	if (np && np->frag_size < mtu) {
		if (np->frag_size)
			mtu = np->frag_size;
	}
	mtu -= hlen + sizeof(struct frag_hdr);

	if (skb_has_frag_list(skb)) {
		int first_len = skb_pagelen(skb);
		struct sk_buff *frag2;

		if (first_len - hlen > mtu ||
		    ((first_len - hlen) & 7) ||
		    skb_cloned(skb))
			goto slow_path;

		skb_walk_frags(skb, frag) {
			/* Correct geometry. */
			if (frag->len > mtu ||
			    ((frag->len & 7) && frag->next) ||
			    skb_headroom(frag) < hlen)
				goto slow_path_clean;

			/* Partially cloned skb? */
			if (skb_shared(frag))
				goto slow_path_clean;

			BUG_ON(frag->sk);
			if (skb->sk) {
				frag->sk = skb->sk;
				frag->destructor = sock_wfree;
			}
			skb->truesize -= frag->truesize;
		}

		err = 0;
		offset = 0;
		frag = skb_shinfo(skb)->frag_list;
		skb_frag_list_init(skb);
		/* BUILD HEADER */

		*prevhdr = NEXTHDR_FRAGMENT;
		tmp_hdr = kmemdup(skb_network_header(skb), hlen, GFP_ATOMIC);
		if (!tmp_hdr) {
			IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
				      IPSTATS_MIB_FRAGFAILS);
			return -ENOMEM;
		}

		__skb_pull(skb, hlen);
		fh = (struct frag_hdr *)__skb_push(skb, sizeof(struct frag_hdr));
		__skb_push(skb, hlen);
		skb_reset_network_header(skb);
		memcpy(skb_network_header(skb), tmp_hdr, hlen);

		ipv6_select_ident(fh, rt);
		fh->nexthdr = nexthdr;
		fh->reserved = 0;
		fh->frag_off = htons(IP6_MF);
		frag_id = fh->identification;

		first_len = skb_pagelen(skb);
		skb->data_len = first_len - skb_headlen(skb);
		skb->len = first_len;
		ipv6_hdr(skb)->payload_len = htons(first_len -
						   sizeof(struct ipv6hdr));

		dst_hold(&rt->dst);

		for (;;) {
			/* Prepare header of the next frame,
			 * before previous one went down. */
			if (frag) {
				frag->ip_summed = CHECKSUM_NONE;
				skb_reset_transport_header(frag);
				fh = (struct frag_hdr *)__skb_push(frag, sizeof(struct frag_hdr));
				__skb_push(frag, hlen);
				skb_reset_network_header(frag);
				memcpy(skb_network_header(frag), tmp_hdr,
				       hlen);
				offset += skb->len - hlen - sizeof(struct frag_hdr);
				fh->nexthdr = nexthdr;
				fh->reserved = 0;
				fh->frag_off = htons(offset);
				if (frag->next != NULL)
					fh->frag_off |= htons(IP6_MF);
				fh->identification = frag_id;
				ipv6_hdr(frag)->payload_len =
						htons(frag->len -
						      sizeof(struct ipv6hdr));
				ip6_copy_metadata(frag, skb);
			}

			err = output(skb);
			if (!err)
				IP6_INC_STATS(net, ip6_dst_idev(&rt->dst),
					      IPSTATS_MIB_FRAGCREATES);

			if (err || !frag)
				break;

			skb = frag;
			frag = skb->next;
			skb->next = NULL;
		}

		kfree(tmp_hdr);

		if (err == 0) {
			IP6_INC_STATS(net, ip6_dst_idev(&rt->dst),
				      IPSTATS_MIB_FRAGOKS);
			ip6_rt_put(rt);
			return 0;
		}

		kfree_skb_list(frag);

		IP6_INC_STATS(net, ip6_dst_idev(&rt->dst),
			      IPSTATS_MIB_FRAGFAILS);
		ip6_rt_put(rt);
		return err;

slow_path_clean:
		skb_walk_frags(skb, frag2) {
			if (frag2 == frag)
				break;
			frag2->sk = NULL;
			frag2->destructor = NULL;
			skb->truesize += frag2->truesize;
		}
	}

slow_path:
	if ((skb->ip_summed == CHECKSUM_PARTIAL) &&
	    skb_checksum_help(skb))
		goto fail;

	left = skb->len - hlen;		/* Space per frame */
	ptr = hlen;			/* Where to start from */

	/*
	 *	Fragment the datagram.
	 */

	*prevhdr = NEXTHDR_FRAGMENT;
	hroom = LL_RESERVED_SPACE(rt->dst.dev);
	troom = rt->dst.dev->needed_tailroom;

	/*
	 *	Keep copying data until we run out.
	 */
	while (left > 0)	{
		len = left;
		/* IF: it doesn't fit, use 'mtu' - the data space left */
		if (len > mtu)
			len = mtu;
		/* IF: we are not sending up to and including the packet end
		   then align the next start on an eight byte boundary */
		if (len < left)	{
			len &= ~7;
		}
		/*
		 *	Allocate buffer.
		 */

		if ((frag = alloc_skb(len + hlen + sizeof(struct frag_hdr) +
				      hroom + troom, GFP_ATOMIC)) == NULL) {
			NETDEBUG(KERN_INFO "IPv6: frag: no memory for new fragment!\n");
			IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
				      IPSTATS_MIB_FRAGFAILS);
			err = -ENOMEM;
			goto fail;
		}

		/*
		 *	Set up data on packet
		 */

		ip6_copy_metadata(frag, skb);
		skb_reserve(frag, hroom);
		skb_put(frag, len + hlen + sizeof(struct frag_hdr));
		skb_reset_network_header(frag);
		fh = (struct frag_hdr *)(skb_network_header(frag) + hlen);
		frag->transport_header = (frag->network_header + hlen +
					  sizeof(struct frag_hdr));

		/*
		 *	Charge the memory for the fragment to any owner
		 *	it might possess
		 */
		if (skb->sk)
			skb_set_owner_w(frag, skb->sk);

		/*
		 *	Copy the packet header into the new buffer.
		 */
		skb_copy_from_linear_data(skb, skb_network_header(frag), hlen);

		/*
		 *	Build fragment header.
		 */
		fh->nexthdr = nexthdr;
		fh->reserved = 0;
		if (!frag_id) {
			ipv6_select_ident(fh, rt);
			frag_id = fh->identification;
		} else
			fh->identification = frag_id;

		/*
		 *	Copy a block of the IP datagram.
		 */
		BUG_ON(skb_copy_bits(skb, ptr, skb_transport_header(frag),
				     len));
		left -= len;

		fh->frag_off = htons(offset);
		if (left > 0)
			fh->frag_off |= htons(IP6_MF);
		ipv6_hdr(frag)->payload_len = htons(frag->len -
						    sizeof(struct ipv6hdr));

		ptr += len;
		offset += len;

		/*
		 *	Put this fragment into the sending queue.
		 */
		err = output(frag);
		if (err)
			goto fail;

		IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
			      IPSTATS_MIB_FRAGCREATES);
	}
	IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
		      IPSTATS_MIB_FRAGOKS);
	consume_skb(skb);
	return err;

fail:
	IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
		      IPSTATS_MIB_FRAGFAILS);
	kfree_skb(skb);
	return err;
}
