Subject: Re: [PATCH net-next-2.6] ipv6: make fragment identifications less predictable

From: Matt Mackall <[email-masked]>
Date: Tue, 19 Jul 2011 15:56:59 -0500

On Tue, 2011-07-19 at 22:47 +0200, Eric Dumazet wrote:
> IPv6 fragment identification generation is way beyond what we use for
> IPv4 : It uses a single generator. Its not scalable and allows DOS
> attacks.
> 
> Now inetpeer is IPv6 aware, we can use it to provide a more secure and
> scalable frag ident generator (per destination, instead of system wide)

This code really needs to get moved out of random.c and into net/. Other
than that, looks fine to me.

> This patch :
> 1) defines a new secure_ipv6_id() helper
> 2) extends inet_getid() to provide 32bit results
> 3) extends ipv6_select_ident() with a new dest parameter
> 
> Reported-by: Fernando Gont <[email-masked]>
> CC: Matt Mackall <[email-masked]>
> CC: Eugene Teo <[email-masked]>
> Signed-off-by: Eric Dumazet <[email-masked]>
> ---
>  drivers/char/random.c  |   15 +++++++++++++++
>  include/linux/random.h |    1 +
>  include/net/inetpeer.h |   13 ++++++++++---
>  include/net/ipv6.h     |   12 +-----------
>  net/ipv4/inetpeer.c    |    7 +++++--
>  net/ipv6/ip6_output.c  |   36 +++++++++++++++++++++++++++++++-----
>  net/ipv6/udp.c         |    2 +-
>  7 files changed, 64 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index d4ddeba..7292819 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1523,6 +1523,21 @@ __u32 secure_ip_id(__be32 daddr)
>  	return half_md4_transform(hash, keyptr->secret);
>  }
>  
> +__u32 secure_ipv6_id(const __be32 daddr[4])
> +{
> +	const struct keydata *keyptr;
> +	__u32 hash[4];
> +
> +	keyptr = get_keyptr();
> +
> +	hash[0] = (__force __u32)daddr[0];
> +	hash[1] = (__force __u32)daddr[1];
> +	hash[2] = (__force __u32)daddr[2];
> +	hash[3] = (__force __u32)daddr[3];
> +
> +	return half_md4_transform(hash, keyptr->secret);
> +}
> +
>  #ifdef CONFIG_INET
>  
>  __u32 secure_tcp_sequence_number(__be32 saddr, __be32 daddr,
> diff --git a/include/linux/random.h b/include/linux/random.h
> index fb7ab9d..ce29a04 100644
> --- a/include/linux/random.h
> +++ b/include/linux/random.h
> @@ -58,6 +58,7 @@ extern void get_random_bytes(void *buf, int nbytes);
>  void generate_random_uuid(unsigned char uuid_out[16]);
>  
>  extern __u32 secure_ip_id(__be32 daddr);
> +extern __u32 secure_ipv6_id(const __be32 daddr[4]);
>  extern u32 secure_ipv4_port_ephemeral(__be32 saddr, __be32 daddr, __be16 dport);
>  extern u32 secure_ipv6_port_ephemeral(const __be32 *saddr, const __be32 *daddr,
>  				      __be16 dport);
> diff --git a/include/net/inetpeer.h b/include/net/inetpeer.h
> index 39d1230..4233e6f 100644
> --- a/include/net/inetpeer.h
> +++ b/include/net/inetpeer.h
> @@ -71,7 +71,7 @@ static inline bool inet_metrics_new(const struct inet_peer *p)
>  }
>  
>  /* can be called with or without local BH being disabled */
> -struct inet_peer	*inet_getpeer(struct inetpeer_addr *daddr, int create);
> +struct inet_peer	*inet_getpeer(const struct inetpeer_addr *daddr, int create);
>  
>  static inline struct inet_peer *inet_getpeer_v4(__be32 v4daddr, int create)
>  {
> @@ -106,11 +106,18 @@ static inline void inet_peer_refcheck(const struct inet_peer *p)
>  
> 
>  /* can be called with or without local BH being disabled */
> -static inline __u16	inet_getid(struct inet_peer *p, int more)
> +static inline int inet_getid(struct inet_peer *p, int more)
>  {
> +	int old, new;
>  	more++;
>  	inet_peer_refcheck(p);
> -	return atomic_add_return(more, &p->ip_id_count) - more;
> +	do {
> +		old = atomic_read(&p->ip_id_count);
> +		new = old + more;
> +		if (!new)
> +			new = 1;
> +	} while (atomic_cmpxchg(&p->ip_id_count, old, new) != old);
> +	return new;
>  }
>  
>  #endif /* _NET_INETPEER_H */
> diff --git a/include/net/ipv6.h b/include/net/ipv6.h
> index c033ed0..3b5ac1f 100644
> --- a/include/net/ipv6.h
> +++ b/include/net/ipv6.h
> @@ -463,17 +463,7 @@ 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
> diff --git a/net/ipv4/inetpeer.c b/net/ipv4/inetpeer.c
> index 90c5f0d..e382138 100644
> --- a/net/ipv4/inetpeer.c
> +++ b/net/ipv4/inetpeer.c
> @@ -391,7 +391,7 @@ static int inet_peer_gc(struct inet_peer_base *base,
>  	return cnt;
>  }
>  
> -struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create)
> +struct inet_peer *inet_getpeer(const struct inetpeer_addr *daddr, int create)
>  {
>  	struct inet_peer __rcu **stack[PEER_MAXDEPTH], ***stackptr;
>  	struct inet_peer_base *base = family_to_base(daddr->family);
> @@ -436,7 +436,10 @@ relookup:
>  		p->daddr = *daddr;
>  		atomic_set(&p->refcnt, 1);
>  		atomic_set(&p->rid, 0);
> -		atomic_set(&p->ip_id_count, secure_ip_id(daddr->addr.a4));
> +		atomic_set(&p->ip_id_count,
> +				(daddr->family == AF_INET) ?
> +					secure_ip_id(daddr->addr.a4) :
> +					secure_ipv6_id(daddr->addr.a6));
>  		p->tcp_ts_stamp = 0;
>  		p->metrics[RTAX_LOCK-1] = INETPEER_METRICS_NEW;
>  		p->rate_tokens = 0;
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 8db0e48..32e5339 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -596,6 +596,31 @@ int ip6_find_1stfragopt(struct sk_buff *skb, u8 **nexthdr)
>  	return offset;
>  }
>  
> +void ipv6_select_ident(struct frag_hdr *fhdr, struct rt6_info *rt)
> +{
> +	static atomic_t ipv6_fragmentation_id;
> +	int old, new;
> +
> +	if (rt) {
> +		struct inet_peer *peer;
> +
> +		if (!rt->rt6i_peer)
> +			rt6_bind_peer(rt, 1);
> +		peer = rt->rt6i_peer;
> +		if (peer) {
> +			fhdr->identification = htonl(inet_getid(peer, 0));
> +			return;
> +		}
> +	}
> +	do {
> +		old = atomic_read(&ipv6_fragmentation_id);
> +		new = old + 1;
> +		if (!new)
> +			new = 1;
> +	} while (atomic_cmpxchg(&ipv6_fragmentation_id, old, new) != old);
> +	fhdr->identification = htonl(new);
> +}
> +
>  int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
>  {
>  	struct sk_buff *frag;
> @@ -680,7 +705,7 @@ 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 +851,7 @@ 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;
> @@ -1076,7 +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;
> @@ -1120,7 +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);
>  
> @@ -1286,7 +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 328985c..29213b5 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()
> 

-- 
Mathematics is the supreme nostalgia of our time.