Smack: File receive for sockets

The existing file receive hook checks for access on
the file inode even for UDS. This is not right, as
the inode is not used by Smack to make access checks
for sockets. This change checks for an appropriate
access relationship between the receiving (current)
process and the socket. If the process can't write
to the socket's send label or the socket's receive
label can't write to the process fail.

This will allow the legitimate cases, where the
socket sender and socket receiver can freely communicate.
Only strangly set socket labels should cause a problem.
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smack_lsm.c

@@ -1860,12 +1860,34 @@ static int smack_file_receive(struct file *file)
 	int may = 0;
 	struct smk_audit_info ad;
 	struct inode *inode = file_inode(file);
+	struct socket *sock;
+	struct task_smack *tsp;
+	struct socket_smack *ssp;
 
 	if (unlikely(IS_PRIVATE(inode)))
 		return 0;
 
 	smk_ad_init(&ad, __func__, LSM_AUDIT_DATA_PATH);
 	smk_ad_setfield_u_fs_path(&ad, file->f_path);
+
+	if (S_ISSOCK(inode->i_mode)) {
+		sock = SOCKET_I(inode);
+		ssp = sock->sk->sk_security;
+		tsp = current_security();
+		/*
+		 * If the receiving process can't write to the
+		 * passed socket or if the passed socket can't
+		 * write to the receiving process don't accept
+		 * the passed socket.
+		 */
+		rc = smk_access(tsp->smk_task, ssp->smk_out, MAY_WRITE, &ad);
+		rc = smk_bu_file(file, may, rc);
+		if (rc < 0)
+			return rc;
+		rc = smk_access(ssp->smk_in, tsp->smk_task, MAY_WRITE, &ad);
+		rc = smk_bu_file(file, may, rc);
+		return rc;
+	}
 	/*
 	 * This code relies on bitmasks.
 	 */