"Normal file access permissions handle security."
— POSIX.1-1988 rationale, B.188.8.131.52
When a user logs into a Unix system, he is assigned a tty which
echoes and processes his characters, lets him edit command lines, handles
flow control on output, and so on. The user's shell reads and writes from
the tty. Messages sent by the
user communication commands are also sent through the tty. In fact, ttys
(also called terminals, teletypes, and
typewriters) are the focal point of almost all interaction
between a user and the system.
Ttys come in two flavors: hardwired ttys and pseudo-ttys. Hardwired ttys are connected directly to a physical device, such as a modem. Pseudo-ttys may be dynamically connected to any process. For instance, the telnetd program links a pseudo-tty with each network connection.
Unlike pipes, ttys have traditionally been assigned a name in the
/dev/console, for instance, is a hardwired tty,
/dev/ttyp7 is a pseudo-tty. The primary reason for these
files is to support user-to-user communication. The system typically
assigns ownership of tty files to the current user. That way the user can
change his tty modes to control whether
talk messages are allowed.
Occasionally a program (or more often a script) wants to send messages to
the current tty, no matter what redirection has been put on stdout and
stderr. (The author defers judgment on whether this is a sensible thing
for programs to do.) More often, a user wants to know what the current
tty is, to keep track of what he's doing. One obvious solution to both
problems is to keep the current tty in an environment variable, like
TTY=/dev/ttyp7. Then a
tty program can simply
echo $TTY, and programs which want to talk to the user can
Unfortunately, Unix never acquired such a simple user-mode solution. Instead each process was assigned a "controlling tty", something which the kernel kept track of and treated specially. That is where the horror story begins.
Processes start without a controlling terminal. When a process without a
controlling tty (also called "ctty") opens a tty, it is assigned that
ctty, and the ctty is preserved through
exec(). Different Unix systems have various methods of
removing the association between a process and its controlling tty. We
will return to this subject below.
A special file,
/dev/tty, is accessible to all processes at
all times. When a process opens
/dev/tty, it gets a valid
descriptor to its ctty, or
ENODEV if it does not have a ctty.
The protection on the actual tty file is irrelevant to
Under BSD 4.2, the controlling tty has almost no effects on job control. BSD 4.3 added some rules in the name of job control security. POSIX went much farther, and formalized controlling ttys into "sessions." Each process is associated with a session, and various ad-hoc session rules are thrown in to complicate job control and signal processing in general. (For instance, a process can only stop if its parent is in the same session but a different process group.) The problem of supporting job control without losing security became an excuse for controlling terminals and sessions.
At this point we can identify six separate forms of access to a tty
/dev/ttyp7, by a process:
/dev/tty, pointing to
(The names "master" and "slave" are pseudo-tty terminology.) The system
lets a process acquire access in several ways: A process with O access
can gain P access. A process with P access can gain S access. A process
with C access can gain T access. A process with M or S access can gain C
access. A process can gain M access (by opening
if no other process has M access.
Under BSD, to shed C access, a process must gain T access (i.e., open
/dev/tty) and perform the
Unfortunately, if any process applies the
TIOCEXCL ioctl to
the tty, no process will be able to open
until some process does
TIOCNXCL. A program stuck under a
TIOCEXCL set has absolutely no reliable way
to dissociate itself from that tty. (Perhaps this is not a surprise given
that there is also absolutely no reliable way for a process to figure out
what tty it is associated with in the first place.)
TIOCEXCL problem pales beside the number of twists
and turns introduced by POSIX. Obviously this is not a simple system to
implement, use, understand, or make secure.
Steve Bellovin pointed out years ago (), as did this author (), that an attacker could abuse controlling ttys to take almost complete control of the system. Put simply, it is very difficult to detect, let alone revoke, whether another user has S, T, or C access to a tty.
BSD 4.2 and its descendants have a
vhangup() system call
meant to revoke access to a tty. Unfortunately, it doesn't work on most
systems, and only revokes S access on the rest. (This author considers
vhangup() a joke in very poor taste.)
Several vendors have attempted to fix this problem. The Convex Unix 8.0 documentation, for instance, says that the holes are gone. They're not. Convex insisted that its system was secure until the author sent explicit code showing how to break tty security.
Similarly, after a series of Internet breakins using tty security holes,
Sun distributed a patch to their SunOS 4.1
rlogind supposedly fixing the problems (). (It is worth
noting that essentially the same fix appeared in version 3.0 of the
author's pty package  several months before.) Although the patch was
enough to temporarily stop the attacks, it did not close C access, and
hence was not enough.
Version 4.0 of the author's pty package () includes security tests which detect all possible forms of tty access, and which work on any BSD system. It is obscene that such measures have proven necessary. In the meantime, crackers from the Netherlands and elsewhere have broken into literally thousands of Internet accounts, taking advantage of tty security holes at will.
Under SunOS 4.1, BSD 4.4, and a few other systems, T access has been
removed. A process which opens
/dev/tty will get a
full-fledged descriptor to
/dev/ttyp7; there is no longer any
/dev/tty and the real tty file it refers
to, except that
/dev/tty is always completely unprotected.
Any excuse for controlling terminals or POSIX sessions on the basis of
job control security is completely invalid. As pointed out by the author
How to add job control to a Unix system,
the job control system can be made much simpler, easier to use, and
simultaneously completely secure, with absolutely no help from cttys or
POSIX sessions. As suggested above, the controlling tty can be passed in
TTY environment variable. C (and T) access, as well as
POSIX sessions, can simply disappear.
(Apparently the disappearance of cttys and POSIX sessions would remove
absolutely no functionality, from either the user's or the application's
point of view; and of course it would make a much cleaner system and
standard if it were officially adopted. The author would greatly love to
hear from any proponents of POSIX sessions who can explain what features
they provide, especially in light of his new, simple, secure job control
system, when in fact vendors, programmers, and users wouldn't care if
setsid() became a no-op.)
For backwards compatibility it would be useful to keep a
/dev/tty device so that programs do not have to be changed to
$TTY. Applications running under the author's pty
program can normally access the tty through descriptor 3, so an easy
solution here is to let
/dev/tty be a simple driver which
dup()s fd 3.
The only other useful function of a controlling terminal — viz., to
help the user categorize processes — is easily taken care of at user
ps can use
$TTY for its output.
Accounting could be by pid (as it should be) instead of by the marginally
We're still left with O and P access — ttys are still in the
filesystem. Given fd 3 support, programs which want to talk to the user
don't need to
open() ttys. The other use of ttys in the
filesystem is user-to-user communication, but there are several
replacement communication systems which depend on users running their own
daemons rather than having a writable tty file. So tty files can
disappear. Rather than opening tty files, programs can create ttys with a
new system call. Then ttys will be much more like pipes, which may be
accessed only through a pair of descriptors.
If these simplifications had been made when Unix was young, before it was used by millions of people, then maybe the controlling terminal horror story would not have lasted so long or caused so much damage. Fortunately, the story will end soon, as in October 1992 the author will distribute code which anyone can use to exploit these holes. Any vendor which wants to stay in business will fix its systems long before then.
© Copyright 1991 Daniel J. Bernstein. Presumably.