Commit message: ipv6: use prandom_u32() for ID generation

This is a complement to commit aa6dd211e4b1 ("inet: use bigger hash
table for IP ID generation"), but focusing on some specific aspects
of IPv6.

Contary to IPv4, IPv6 only uses packet IDs with fragments, and with a
minimum MTU of 1280, it's much less easy to force a remote peer to
produce many fragments to explore its ID sequence. In addition packet
IDs are 32-bit in IPv6, which further complicates their analysis. On
the other hand, it is often easier to choose among plenty of possible
source addresses and partially work around the bigger hash table the
commit above permits, which leaves IPv6 partially exposed to some
possibilities of remote analysis at the risk of weakening some
protocols like DNS if some IDs can be predicted with a good enough
probability.

Given the wide range of permitted IDs, the risk of collision is extremely
low so there's no need to rely on the positive increment algorithm that
is shared with the IPv4 code via ip_idents_reserve(). We have a fast
PRNG, so let's simply call prandom_u32() and be done with it.

Performance measurements at 10 Gbps couldn't show any difference with
the previous code, even when using a single core, because due to the
large fragments, we're limited to only ~930 kpps at 10 Gbps and the cost
of the random generation is completely offset by other operations and by
the network transfer time. In addition, this change removes the need to
update a shared entry in the idents table so it may even end up being
slightly faster on large scale systems where this matters.

The risk of at least one collision here is about 1/80 million among
10 IDs, 1/850k among 100 IDs, and still only 1/8.5k among 1000 IDs,
which remains very low compared to IPv4 where all IDs are reused
every 4 to 80ms on a 10 Gbps flow depending on packet sizes.

Reported-by: Amit Klein <aksecurity@gmail.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20210529110746.6796-1-w@1wt.eu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
CVE: CVE-2021-45485
CWE: CWE-327
Vulnerability description: In the IPv6 implementation in the Linux kernel before 5.13.3, net/ipv6/output_core.c has an information leak because of certain use of a hash table which, although big, doesn't properly consider that IPv6-based attackers can typically choose among many IPv6 source addresses.

Project: linux
File: output_core.c

int ip6_find_1stfragopt(struct sk_buff *skb, u8 **nexthdr)
{
	unsigned int offset = sizeof(struct ipv6hdr);
	unsigned int packet_len = skb_tail_pointer(skb) -
		skb_network_header(skb);
	int found_rhdr = 0;
	*nexthdr = &ipv6_hdr(skb)->nexthdr;

	while (offset <= packet_len) {
		struct ipv6_opt_hdr *exthdr;

		switch (**nexthdr) {

		case NEXTHDR_HOP:
			break;
		case NEXTHDR_ROUTING:
			found_rhdr = 1;
			break;
		case NEXTHDR_DEST:
#if IS_ENABLED(CONFIG_IPV6_MIP6)
			if (ipv6_find_tlv(skb, offset, IPV6_TLV_HAO) >= 0)
				break;
#endif
			if (found_rhdr)
				return offset;
			break;
		default:
			return offset;
		}

		if (offset + sizeof(struct ipv6_opt_hdr) > packet_len)
			return -EINVAL;

		exthdr = (struct ipv6_opt_hdr *)(skb_network_header(skb) +
						 offset);
		offset += ipv6_optlen(exthdr);
		if (offset > IPV6_MAXPLEN)
			return -EINVAL;
		*nexthdr = &exthdr->nexthdr;
	}

	return -EINVAL;
}

Project: linux
File: output_core.c

int __ip6_local_out(struct net *net, struct sock *sk, struct sk_buff *skb)
{
	int len;

	len = skb->len - sizeof(struct ipv6hdr);
	if (len > IPV6_MAXPLEN)
		len = 0;
	ipv6_hdr(skb)->payload_len = htons(len);
	IP6CB(skb)->nhoff = offsetof(struct ipv6hdr, nexthdr);

	/* if egress device is enslaved to an L3 master device pass the
	 * skb to its handler for processing
	 */
	skb = l3mdev_ip6_out(sk, skb);
	if (unlikely(!skb))
		return 0;

	skb->protocol = htons(ETH_P_IPV6);

	return nf_hook(NFPROTO_IPV6, NF_INET_LOCAL_OUT,
		       net, sk, skb, NULL, skb_dst(skb)->dev,
		       dst_output);
}

Project: linux
File: output_core.c

int ip6_local_out(struct net *net, struct sock *sk, struct sk_buff *skb)
{
	int err;

	err = __ip6_local_out(net, sk, skb);
	if (likely(err == 1))
		err = dst_output(net, sk, skb);

	return err;
}

Project: linux
File: output_core.c

__be32 ipv6_select_ident(struct net *net,
			 const struct in6_addr *daddr,
			 const struct in6_addr *saddr)
{
	u32 id;

	id = __ipv6_select_ident(net, daddr, saddr);
	return htonl(id);
}

Project: linux
File: output_core.c

static u32 __ipv6_select_ident(struct net *net,
			       const struct in6_addr *dst,
			       const struct in6_addr *src)
{
	u32 id;

	do {
		id = prandom_u32();
	} while (!id);

	return id;
}

Project: linux
File: output_core.c

__be32 ipv6_proxy_select_ident(struct net *net, struct sk_buff *skb)
{
	struct in6_addr buf[2];
	struct in6_addr *addrs;
	u32 id;

	addrs = skb_header_pointer(skb,
				   skb_network_offset(skb) +
				   offsetof(struct ipv6hdr, saddr),
				   sizeof(buf), buf);
	if (!addrs)
		return 0;

	id = __ipv6_select_ident(net, &addrs[1], &addrs[0]);
	return htonl(id);
}