ipv6: make fragment identifications less predictable

[ Backport of upstream commit 87c48fa3 ]

Fernando Gont reported current IPv6 fragment identification generation
was not secure, because using a very predictable system-wide generator,
allowing various attacks.

IPv4 uses inetpeer cache to address this problem and to get good
performance. We'll use this mechanism when IPv6 inetpeer is stable
enough in linux-3.1

For the time being, we use jhash on destination address to provide less
predictable identifications. Also remove a spinlock and use cmpxchg() to
get better SMP performance.
Reported-by: Fernando Gont <fernando@gont.com.ar>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
[bwh: Backport further to 2.6.32]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Willy Tarreau <w@1wt.eu>

include/net/ipv6.h

@@ -449,17 +449,7 @@ static inline int ipv6_addr_diff(const struct in6_addr *a1, const struct in6_add
 	return __ipv6_addr_diff(a1, a2, sizeof(struct in6_addr));
 }
 
-static __inline__ void ipv6_select_ident(struct frag_hdr *fhdr)
-{
-	static u32 ipv6_fragmentation_id = 1;
-	static DEFINE_SPINLOCK(ip6_id_lock);
-
-	spin_lock_bh(&ip6_id_lock);
-	fhdr->identification = htonl(ipv6_fragmentation_id);
-	if (++ipv6_fragmentation_id == 0)
-		ipv6_fragmentation_id = 1;
-	spin_unlock_bh(&ip6_id_lock);
-}
+extern void ipv6_select_ident(struct frag_hdr *fhdr, struct rt6_info *rt);
 
 /*
  *	Prototypes exported by ipv6

include/net/transp_v6.h

@@ -16,6 +16,8 @@ extern struct proto tcpv6_prot;
 
 struct flowi;
 
+extern void initialize_hashidentrnd(void);
+
 /* extention headers */
 extern int				ipv6_exthdrs_init(void);
 extern void				ipv6_exthdrs_exit(void);

net/ipv6/af_inet6.c

@@ -1073,6 +1073,8 @@ static int __init inet6_init(void)
 		goto out;
 	}
 
+	initialize_hashidentrnd();
+
 	err = proto_register(&tcpv6_prot, 1);
 	if (err)
 		goto out;

net/ipv6/ip6_output.c

@@ -604,6 +604,35 @@ int ip6_find_1stfragopt(struct sk_buff *skb, u8 **nexthdr)
 	return offset;
 }
 
+static u32 hashidentrnd __read_mostly;
+#define FID_HASH_SZ 16
+static u32 ipv6_fragmentation_id[FID_HASH_SZ];
+
+void __init initialize_hashidentrnd(void)
+{
+	get_random_bytes(&hashidentrnd, sizeof(hashidentrnd));
+}
+
+static u32 __ipv6_select_ident(const struct in6_addr *addr)
+{
+	u32 newid, oldid, hash = jhash2((u32 *)addr, 4, hashidentrnd);
+	u32 *pid = &ipv6_fragmentation_id[hash % FID_HASH_SZ];
+
+	do {
+		oldid = *pid;
+		newid = oldid + 1;
+		if (!(hash + newid))
+			newid++;
+	} while (cmpxchg(pid, oldid, newid) != oldid);
+
+	return hash + newid;
+}
+
+void ipv6_select_ident(struct frag_hdr *fhdr, struct rt6_info *rt)
+{
+	fhdr->identification = htonl(__ipv6_select_ident(&rt->rt6i_dst.addr));
+}
+
 static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 {
 	struct sk_buff *frag;
@@ -689,7 +718,7 @@ static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 		skb_reset_network_header(skb);
 		memcpy(skb_network_header(skb), tmp_hdr, hlen);
 
-		ipv6_select_ident(fh);
+		ipv6_select_ident(fh, rt);
 		fh->nexthdr = nexthdr;
 		fh->reserved = 0;
 		fh->frag_off = htons(IP6_MF);
@@ -835,7 +864,7 @@ static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 		fh->nexthdr = nexthdr;
 		fh->reserved = 0;
 		if (!frag_id) {
-			ipv6_select_ident(fh);
+			ipv6_select_ident(fh, rt);
 			frag_id = fh->identification;
 		} else
 			fh->identification = frag_id;
@@ -1039,7 +1068,8 @@ static inline int ip6_ufo_append_data(struct sock *sk,
 			int getfrag(void *from, char *to, int offset, int len,
 			int odd, struct sk_buff *skb),
 			void *from, int length, int hh_len, int fragheaderlen,
-			int transhdrlen, int mtu,unsigned int flags)
+			int transhdrlen, int mtu,unsigned int flags,
+			struct rt6_info *rt)
 
 {
 	struct sk_buff *skb;
@@ -1084,7 +1114,7 @@ static inline int ip6_ufo_append_data(struct sock *sk,
 		skb_shinfo(skb)->gso_size = (mtu - fragheaderlen -
 					     sizeof(struct frag_hdr)) & ~7;
 		skb_shinfo(skb)->gso_type = SKB_GSO_UDP;
-		ipv6_select_ident(&fhdr);
+		ipv6_select_ident(&fhdr, rt);
 		skb_shinfo(skb)->ip6_frag_id = fhdr.identification;
 		__skb_queue_tail(&sk->sk_write_queue, skb);
 
@@ -1233,7 +1263,7 @@ int ip6_append_data(struct sock *sk, int getfrag(void *from, char *to,
 
 		err = ip6_ufo_append_data(sk, getfrag, from, length, hh_len,
 					  fragheaderlen, transhdrlen, mtu,
-					  flags);
+					  flags, rt);
 		if (err)
 			goto error;
 		return 0;

net/ipv6/udp.c

@@ -1162,7 +1162,7 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb, int features)
 	fptr = (struct frag_hdr *)(skb_network_header(skb) + unfrag_ip6hlen);
 	fptr->nexthdr = nexthdr;
 	fptr->reserved = 0;
-	ipv6_select_ident(fptr);
+	ipv6_select_ident(fptr, (struct rt6_info *)skb_dst(skb));
 
 	/* Fragment the skb. ipv6 header and the remaining fields of the
 	 * fragment header are updated in ipv6_gso_segment()