[PATCH RFC] Smack: More sanity in the use of Netlabel

[Piotr Sawicki]

Hi,

My name is Piotr. Currently I'm involved in maintaining the Nether 
service (a user-space firewall used in Tizen). I have a few remarks 
about this patch.

On 06/09/2017 04:41 AM, Casey Schaufler wrote:
> Subject: [PATCH RFC] Smack: More sanity in the use of Netlabel
>
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
>
> @@ -4042,15 +4094,19 @@ static int smack_socket_sock_rcv_skb(struct 
> sock *sk, struct sk_buff *skb)
>           rc = smk_access(skp, ssp->smk_in, MAY_WRITE, &ad);
>           rc = smk_bu_note("IPv4 delivery", skp, ssp->smk_in,
>                       MAY_WRITE, rc);
> -        if (rc != 0)
> +        if (rc == 0)
> +            break;
> +        if (by_host)
> +            icmp_send(skb, ICMP_DEST_UNREACH, ICMP_HOST_ANO, 0);
> +        else
>               netlbl_skbuff_err(skb, sk->sk_family, rc, 0);
>           break;
>   #if IS_ENABLED(CONFIG_IPV6)
>       case PF_INET6:
> -        proto = smk_skb_to_addr_ipv6(skb, &sadd);
> -        if (proto != IPPROTO_UDP && proto != IPPROTO_TCP)
> +        rc = smk_skb_to_addr_ipv6(skb, &sadd);
> +        if (rc != IPPROTO_UDP && rc != IPPROTO_TCP)
>               break;

The PF_INET6 socket may receive IPv4 packets too. In this case 
smk_skb_to_addr_ipv6() returns -EINVAL or some rubbish value. 
Furthermore, the smk_skb_to_addr_ipv6() function returns a detected 
protocol type (e.g. DCCP). If it is neither TCP nor UDP, then the packet 
will be blocked.

I wonder why are the other protocols not handled here (e.g. UDP Lite, 
DCCP)?

> -#ifdef SMACK_IPV6_SECMARK_LABELING
> +#ifdef CONFIG_SECURITY_SMACK_NETFILTER
>           if (skb && skb->secmark != 0)
>               skp = smack_from_secid(skb->secmark);
>           else
> @@ -4066,10 +4122,9 @@ static int smack_socket_sock_rcv_skb(struct 
> sock *sk, struct sk_buff *skb)
>           rc = smk_access(skp, ssp->smk_in, MAY_WRITE, &ad);
>           rc = smk_bu_note("IPv6 delivery", skp, ssp->smk_in,
>                       MAY_WRITE, rc);
> -#endif /* SMACK_IPV6_SECMARK_LABELING */
> -#ifdef SMACK_IPV6_PORT_LABELING
> +#else
>           rc = smk_ipv6_port_check(sk, &sadd, SMK_RECEIVING);
> -#endif /* SMACK_IPV6_PORT_LABELING */
> +#endif /* CONFIG_SECURITY_SMACK_NETFILTER */
>           break;
>   #endif /* CONFIG_IPV6 */
>       }
> @@ -4149,11 +4204,14 @@ static int 
> smack_socket_getpeersec_dgram(struct socket *sock,
>           s = ssp->smk_out->smk_secid;
>           break;
>       case PF_INET:
> -#ifdef CONFIG_SECURITY_SMACK_NETFILTER
> +        skp = smack_ipv4_skb_host_label(skb);
> +        if (skp) {
> +            s = skp->smk_secid;
> +            break;
> +        }

There are three functions which have very similar fragments of code. 
They deduce a Smack label from an incoming socket buffer. I've noticed 
some inconsistencies:
- In the smack_socket_sock_recv_skb() function skp defaults to 
smack_net_ambient.
- In smack_socket_getpeersec_dgram() the secid variable defaults to 0, 
which means the invalid secid.
- In the smack_inet_conn_request() function the default value is 
smack_known_huh.

Is it intentional?

> diff --git a/security/smack/smack_netfilter.c 
> b/security/smack/smack_netfilter.c
> index 205b785..9904f37 100644
> --- a/security/smack/smack_netfilter.c
> +++ b/security/smack/smack_netfilter.c
> @@ -51,7 +51,9 @@ static unsigned int smack_ipv4_output(void *priv,
>       if (sk && sk->sk_security) {
>           ssp = sk->sk_security;
>           skp = ssp->smk_out;
> -        skb->secmark = skp->smk_secid;
> +        if (ssp->smk_state == SMK_SOCK_DEFERRED &&
> +            netlbl_skbuff_setattr(skb, PF_INET, &skp->smk_netlabel))
> +            return NF_DROP;
>       }
>         return NF_ACCEPT;

The above change will affect the NFQUEUE mechanism. The secmark field of 
a socket buffer is used by the nfqnl_get_sk_secctx() function 
(net/netfilter/nfnetlink_queue.c) to retrieve a Smack label (a security 
context). Please take a look at this commit regarding 
libnetfilter_queue: 
https://git.netfilter.org/libnetfilter_queue/commit/?id=46912f1c18e01b63660a56ea7d9c572741e06117 
The Nether service (https://wiki.tizen.org/Security:Nether) uses 
libnetfilter_queue to implement a software firewall. It utilizes the 
security context and UDI/GID fields of a netlink message to make a 
decision about what to do with an outgoing packet.

Also, I've noticed an inconsistency of handling the secmark field for 
IPv4 and IPv6 protocols. In smack_ipv6_output function() the 
skp->smk_secid field is copied to skb->secmark.



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