ipv6: make fragment identifications less predictable

[ Backport of upstream commit 87c48fa3b4630905f98268dde838ee43626a060c ]

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>

3 files changed, 38 insertions, 6 deletions

diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
index 3b5669a2582..559123644e5 100644
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -1078,6 +1078,8 @@ static int __init inet6_init(void)
                 goto out;
         }
 
+        initialize_hashidentrnd();
+
         err = proto_register(&tcpv6_prot, 1);
         if (err)
                 goto out;
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 9d4b165837d..16612968aaa 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -596,6 +596,35 @@ 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));
+}
+
 int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 {
         struct sk_buff *frag;
@@ -680,7 +709,7 @@ 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);
@@ -826,7 +855,7 @@ slow_path:
                 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;
@@ -1072,7 +1101,8 @@ 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;
@@ -1116,7 +1146,7 @@ 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);
 
@@ -1282,7 +1312,7 @@ 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);
+                                                  transhdrlen, mtu, flags, rt);
                         if (err)
                                 goto error;
                         return 0;
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 328985c4088..29213b51c49 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1359,7 +1359,7 @@ static struct sk_buff *udp6_ufo_fragment(struct sk_buff *skb, u32 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()