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