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.
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.
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.
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.
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.
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.
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.