selinux: Add netlink xperm support

Reuse the existing extended permissions infrastructure to support
policies based on the netlink message types.

A new policy capability "netlink_xperm" is introduced. When disabled,
the previous behaviour is preserved. That is, netlink_send will rely on
the permission mappings defined in nlmsgtab.c (e.g, nlmsg_read for
RTM_GETADDR on NETLINK_ROUTE). When enabled, the mappings are ignored
and the generic "nlmsg" permission is used instead.

The new "nlmsg" permission is an extended permission. The 16 bits of the
extended permission are mapped to the nlmsg_type field.

Example policy on Android, preventing regular apps from accessing the
device's MAC address and ARP table, but allowing this access to
privileged apps, looks as follows:

allow netdomain self:netlink_route_socket {
	create read getattr write setattr lock append connect getopt
	setopt shutdown nlmsg
};
allowxperm netdomain self:netlink_route_socket nlmsg ~{
	RTM_GETLINK RTM_GETNEIGH RTM_GETNEIGHTBL
};
allowxperm priv_app self:netlink_route_socket nlmsg {
	RTM_GETLINK RTM_GETNEIGH RTM_GETNEIGHTBL
};

The constants in the example above (e.g., RTM_GETLINK) are explicitly
defined in the policy.

It is possible to generate policies to support kernels that may or
may not have the capability enabled by generating a rule for each
scenario. For instance:

allow domain self:netlink_audit_socket nlmsg_read;
allow domain self:netlink_audit_socket nlmsg;
allowxperm domain self:netlink_audit_socket nlmsg { AUDIT_GET };

The approach of defining a new permission ("nlmsg") instead of relying
on the existing permissions (e.g., "nlmsg_read", "nlmsg_readpriv" or
"nlmsg_tty_audit") has been preferred because:
  1. This is similar to the other extended permission ("ioctl");
  2. With the new extended permission, the coarse-grained mapping is not
     necessary anymore. It could eventually be removed, which would be
     impossible if the extended permission was defined below these.
  3. Having a single extra extended permission considerably simplifies
     the implementation here and in libselinux.

Signed-off-by: Thiébaud Weksteen <tweek@google.com>
Signed-off-by: Bram Bonné <brambonne@google.com>
[PM: manual merge fixes for sock_skip_has_perm()]
Signed-off-by: Paul Moore <paul@paul-moore.com>

1 files changed, 42 insertions, 9 deletions

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index fc926d3cac6e..ad3abd48eed1 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -4590,14 +4590,10 @@ static int socket_sockcreate_sid(const struct task_security_struct *tsec,
 				       secclass, NULL, socksid);
 }
 
-static int sock_has_perm(struct sock *sk, u32 perms)
+static bool sock_skip_has_perm(u32 sid)
 {
-	struct sk_security_struct *sksec = selinux_sock(sk);
-	struct common_audit_data ad;
-	struct lsm_network_audit net;
-
-	if (sksec->sid == SECINITSID_KERNEL)
-		return 0;
+	if (sid == SECINITSID_KERNEL)
+		return true;
 
 	/*
 	 * Before POLICYDB_CAP_USERSPACE_INITIAL_CONTEXT, sockets that
@@ -4611,7 +4607,19 @@ static int sock_has_perm(struct sock *sk, u32 perms)
 	 * setting.
 	 */
 	if (!selinux_policycap_userspace_initial_context() &&
-	    sksec->sid == SECINITSID_INIT)
+	    sid == SECINITSID_INIT)
+		return true;
+	return false;
+}
+
+
+static int sock_has_perm(struct sock *sk, u32 perms)
+{
+	struct sk_security_struct *sksec = sk->sk_security;
+	struct common_audit_data ad;
+	struct lsm_network_audit net;
+
+	if (sock_skip_has_perm(sksec->sid))
 		return 0;
 
 	ad_net_init_from_sk(&ad, &net, sk);
@@ -5920,6 +5928,26 @@ static unsigned int selinux_ip_postroute(void *priv,
 }
 #endif	/* CONFIG_NETFILTER */
 
+static int nlmsg_sock_has_extended_perms(struct sock *sk, u32 perms, u16 nlmsg_type)
+{
+	struct sk_security_struct *sksec = sk->sk_security;
+	struct common_audit_data ad;
+	struct lsm_network_audit net;
+	u8 driver;
+	u8 xperm;
+
+	if (sock_skip_has_perm(sksec->sid))
+		return 0;
+
+	ad_net_init_from_sk(&ad, &net, sk);
+
+	driver = nlmsg_type >> 8;
+	xperm = nlmsg_type & 0xff;
+
+	return avc_has_extended_perms(current_sid(), sksec->sid, sksec->sclass,
+			perms, driver, xperm, &ad);
+}
+
 static int selinux_netlink_send(struct sock *sk, struct sk_buff *skb)
 {
 	int rc = 0;
@@ -5945,7 +5973,12 @@ static int selinux_netlink_send(struct sock *sk, struct sk_buff *skb)
 
 		rc = selinux_nlmsg_lookup(sclass, nlh->nlmsg_type, &perm);
 		if (rc == 0) {
-			rc = sock_has_perm(sk, perm);
+			if (selinux_policycap_netlink_xperm()) {
+				rc = nlmsg_sock_has_extended_perms(
+					sk, perm, nlh->nlmsg_type);
+			} else {
+				rc = sock_has_perm(sk, perm);
+			}
 			if (rc)
 				return rc;
 		} else if (rc == -EINVAL) {