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@@ -0,0 +1,184 @@ + + + A Sample Authorization Protocol for X + + +Overview + +The following note describes a very simple mechanism for providing individual +access to an X Window System display. It uses existing core protocol and +library hooks for specifying authorization data in the connection setup block +to restrict use of the display to only those clients that show that they +know a server-specific key called a "magic cookie". This mechanism is *not* +being proposed as an addition to the Xlib standard; among other reasons, a +protocol extension is needed to support more flexible mechanisms. We have +implemented this mechanism already; if you have comments, please send them +to us. + +This scheme involves changes to the following parts of the sample release: + + o xdm + - generate random magic cookie and store in protected file + - pass name of magic cookie file to server + - when user logs in, add magic cookie to user's auth file + - when user logs out, generate a new cookie for server + + o server + - a new command line option to specify cookie file + - check client authorization data against magic cookie + - read in cookie whenever the server resets + - do not add local machine to host list if magic cookie given + + o Xlib + - read in authorization data from file + - find data for appropriate server + - send authorization data if found + + o xauth [new program to manage user auth file] + - add entries to user's auth file + - remove entries from user's auth file + +This mechanism assumes that the superuser and the transport layer between +the client and the server is secure. Organizations that desire stricter +security are encouraged to look at systems such as Kerberos (at Project +Athena). + + +Description + +The sample implementation will use the xdm Display Manager to set up and +control the server's authorization file. Sites that do not run xdm will +need to build their own mechanisms. + +Xdm uses a random key (seeded by the system time and check sum of /dev/kmem) +to generate a unique sequence of characters at 16 bytes long. This sequence +will be written to a file which is made readable only by the server. The +server will then be started with a command line option instructing it to use +the contents of the file as the magic cookie for connections that include +authorization data. This will also disable the server from adding the local +machine's address to the initial host list. Note that the actual cookie must +not be stored on the command line or in an environment variable, to prevent +it from being publicly obtainable by the "ps" command. + +If a client presents an authorization name of "MIT-MAGIC-COOKIE-1" and +authorization data that matches the magic cookie, that client is allowed +access. If the name or data does not match and the host list is empty, +that client will be denied access. Otherwise, the existing host-based access +control will be used. Since any client that is making a connection from a +machine on the host list will be granted access even if their authorization +data is incorrect, sites are strongly urged not to set up any default hosts +using the /etc/X*.hosts files. Granting access to other machines should be +done by the user's session manager instead. + +Assuming the server is configured with an empty host list, the existence of the +cookie is sufficient to ensure there will be no unauthorized access to the +display. However, xdm will (continue to) work to minimize the chances of +spoofing on servers that do not support this authorization mechanism. This +will be done by grabbing the server and the keyboard after opening the display. +This action will be surrounded by a timer which will kill the server if the +grabs cannot be done within several seconds. [This level of security is now +implemented in patches already sent out.] + +After the user logs in, xdm will add authorization entries for each of the +server machine's network addresses to the user's authorization file (the format +of which is described below). This file will usually be named .Xauthority in +the users's home directory; will be owned by the user (as specified by the +pw_uid and pw_gid fields in the user's password entry), and will be accessible +only to the user (no group access). This file will contain authorization data +for all of the displays opened by the user. + +When the session terminates, xdm will generate and store a new magic cookie +for the server. Then, xdm will shutdown its own connection and send a +SIGHUP to the server process, which should cause the server to reset. The +server will then read in the new magic cookie. + +To support accesses (both read and write) from multiple machines (for use in +environments that use distributed file systems), file locking is done using +hard links. This is done by creat'ing (sic) a lock file and then linking it +to another name in the same directory. If the link-target already exists, +the link will fail, indicating failure to obtain the lock. Linking is used +instead of just creating the file read-only since link will fail even for +the superuser. + +Problems and Solutions + +There are a few problems with .Xauthority as described. If no home directory +exists, or if xdm cannot create a file there (disk full), xdm stores the +cookie in a file in a resource-specified back-up directory, and sets an +environment variable in the user's session (called XAUTHORITY) naming this +file. There is also the problem that the locking attempts will need to be +timed out, due to a leftover lock. Xdm, again, creates a file and set an +environment variable. Finally, the back-up directory might be full. Xdm, +as a last resort, provides a function key binding that allows a user to log +in without having the authorization data stored, and with host-based access +control disabled. + +Xlib + +XOpenDisplay in Xlib was enhanced to allow specification of authorization +information. As implied above, Xlib looks for the data in the +.Xauthority file of the home directory, or in the file pointed at by the +XAUTHORITY environment variable instead if that is defined. This required +no programmatic interface change to Xlib. In addition, a new Xlib routine +is provided to explicitly specify authorization. + + XSetAuthorization(name, namelen, data, datalen) + int namelen, datalen; + char *name, *data; + +There are three types of input: + + name NULL, data don't care - use default authorization mechanism. + name non-NULL, data NULL - use the named authorization; get + data from that mechanism's default. + name non-NULL, data non-NULL - use the given authorization and data. + +This interface is used by xdm and might also be used by any other +applications that wish to explicitly set the authorization information. + +Authorization File + +The .Xauthority file is a binary file consisting of a sequence of entries +in the following format: + + 2 bytes Family value (second byte is as in protocol HOST) + 2 bytes address length (always MSB first) + A bytes host address (as in protocol HOST) + 2 bytes display "number" length (always MSB first) + S bytes display "number" string + 2 bytes name length (always MSB first) + N bytes authorization name string + 2 bytes data length (always MSB first) + D bytes authorization data string + +The format is binary for easy processing, since authorization information +usually consists of arbitrary data. Host addresses are used instead of +names to eliminate potentially time-consuming name resolutions in +XOpenDisplay. Programs, such as xdm, that initialize the user's +authorization file will have to do the same work as the server in finding +addresses for all network interfaces. If more than one entry matches the +desired address, the entry that is chosen is implementation-dependent. In +our implementation, it is always the first in the file. + +The Family is specified in two bytes to allow out-of-band values +(i.e. values not in the Protocol) to be used. In particular, +two new values "FamilyLocal" and "FamilyWild" are defined. FamilyLocal +refers to any connections using a non-network method of connetion from the +local machine (Unix domain sockets, shared memory, loopback serial line). +In this case the host address is specified by the data returned from +gethostname() and better be unique in a collection of machines +which share NFS directories. FamilyWild is currently used only +by xdm to communicate authorization data to the server. It matches +any family/host address pair. + +For FamilyInternet, the host address is the 4 byte internet address, for +FamilyDecnet, the host address is the byte decnet address, for FamilyChaos +the address is also two bytes. + +The Display Number is the ascii representation of the display number +portion of the display name. It is in ascii to allow future expansion +to PseudoRoots or anything else that might happen. + +A utility called "xauth" will be provided for editing and viewing the +contents of authorization files. Note that the user's authorization file is +not the same as the server's magic cookie file. |