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User service daemon and client specification
Chapter 6 - Applications and notes on use

6.1 Examples

The companion package, userv-utils, contains a selection of example services, some of which are useful tools in their own right. See the README in its top-level directory for details.

6.2 Standard services and directory management

In later versions of this specification standard service names and interfaces for common services such as mail delivery and WWW CGI scripts may be specified.

userv-using applications and system services which hide userv behind wrapper scripts may need to store information in the user's filespace to preserve the correct placement of the security perimiters. Such applications should usually do so in a directory (created by them) ~/.userv/service, where service is the service name or application in question.

If desired, a dot-directory inside ~/.userv may be used to avoid the user becoming confused by finding parts of a semi-privileged application's internal state in their filespace, and/or discourage them from fiddling with and thus corrupting it.

However, userv applications should of course not rely for their global integrity and security on the integrity of the data on the user's side of the security boundary.

6.3 Reducing the number of absolutely privileged subsystems

Currently most Unix systems have many components which need to run as root, even though most of their activity does not strictly require it. This gives rise to a large and complex body of code which must be trusted with the security of the system.

If they were to use userv, many of these subsystems would no longer need any unusual privilege.

cron and at, lpr and the system's mail transfer agent (sendmail, smail, exim or the like) all fall into this category, though userv-based versions of these programs are not currently available.

6.4 Do not give away excessive privilege to userv-using facilities

There is a danger that people reimplementing the facilities I mention above using userv will discard much of the security benefit by using a naive implementation technique. This will become clearer with an example:

Consider the lpr program. In current systems this needs to have an absolutely privileged component in order to support delayed printing without copying: when the user queues a file to be printed the filename is stored in the print queue, rather than a copy of it, and the printer daemon accesses the file directly when it is ready to print the job. In order that the user can print files which are not world-readable the daemon is given root privilege so that it can open the file in the context of the user, rather than its own.

A simple-minded approach to converting this scheme to use userv might involve giving the printer daemon (the lp user) the ability to read the file by allowing them to run cat (or a special-purpose file-reading program) as any user. The lpr program would use a userv service to store the filename in the printer daemon's queues, and the daemon would read the file later when it felt like it.

However, this would allow the printer daemon to read any file on the system, whether or not someone had asked for it to be printed. Since many files will contain passwords and other security-critical information this is nearly as bad as giving the daemon root access in the first place. Any security holes in the print server which allow a user to execute commands as the lp user will give the user the ability to read any file on the system.

Instead, it is necessary to keep a record of which files the daemon has been asked to print outside the control of the print daemon. This record could be kept by a new root-privileged component, but this is not necessary: the record of which files a user has asked to be printed can be kept under the control of the user in question. The submission program lpr will make a record in an area under the user's control before communicating with the print server, and the print server would be given the ability to run a special file-reading program which would only allow files to be read which were listed in the user's file of things they'd asked to print.

Now security holes in most of the printing system do not critically affect the security of the entire system: they only allow the attacker to read and interfere with print jobs. Bugs in the programs run by the print server to read users' files (and to remove entries from the list of files when it has done with them) will still be serious, but this program can be quite simple.

Similar considerations apply to many userv-based versions of facilities which currently run as root.

It is debatable whether the user-controlled state should be kept in the user's filespace (in dotfiles, say) or kept in a separate area set aside for the purpose; however, using the user's home directory (and possibly creating a separate subdirectory of it as a dotfile to contain subsystem state) has fewer implications for the rest of the system and makes it entirely clear where the security boundaries lie.

6.5 userv can often replace sudo, but not really

userv is not intended as a general-purpose system administration tool with which system administrators can execute arbitrary programs like text editors as root (or other system users) when they need to. It is unsuitable for this purpose precisely because it enforces a strong separation between the calling and the called program, which is undesirable in this context.

However, its use when restricted to running particular programs in particular ways is very similar to many common uses of sudo[2]. userv is generally much better than restricted sudo, because it protects the called program much more strongly from bad environmental conditions set up by the caller. Most programs that one might want to run via restricted sudo, have not been designed to run in a partially hostile environment. userv allows these programs to be run in a safer environment and should be used instead.

6.6 Error handling and input streams (eg stdin)

When the service program is reading from a file descriptor connected to the calling side, the fd that the service program refers to a pipe set up by userv and not to the same object as was presented by the caller.

Therefore if there is some kind of error it is possible for the service-side fd to give premature end of file. If it is important to tell whether all of the intended data has been received by the service program, the datastream must contain an explicit end-of-file indication of some kind.

For example, consider a userv service for submitting a mail message, where message is supplied on the service's stdin. However, if the calling process is interrupted before it has written all of the message, the service program will get EOF on the message data. In a naive arrangement this would cause a half-complete message to be sent. To prevent this, it is necessary to adopt some kind of explicit end indication; for example, the end of the message could be signalled by a dot on a line by itself, and dots doubled, as in SMTP. Then the service program would know when the entire message had been received, and could avoid queueing incomplete messages.

6.7 Don't give access to general-purpose utilities

Do not specify general purpose programs like mv or cat in execute- directives without careful thought about their arguments, and certainly not if no-suppress-args is specified. If you do so it will give the caller much more privilige than you probably intend.

It is a shame that I have to say this here, but inexperienced administrators have made similar mistakes with programs like sudo.

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User service daemon and client specification


Ian Jackson mailto:ian@davenant.greenend.org.uk