security

unix v7是最后一个广泛发布的研究型UNIX版本

.ND June 10, 1977
.TL
On the Security of UNIX
.AU
Dennis M. Ritchie
.AI
.MH
.PP
Recently there has been much interest in the security
aspects of operating systems and software.
At issue is the ability
to prevent undesired
disclosure of information, destruction of information,
and harm to the functioning of the system.
This paper discusses the degree of security which can be provided
under the
.UX
system and offers a number of hints
on how to improve security.
.PP
The first fact to face is that
.UX
was not developed with
security, in any realistic sense, in mind;
this fact alone guarantees a vast number of holes.
(Actually the same statement can be made with respect
to most systems.)
The area of security in which
.UX
is theoretically weakest is
in protecting against crashing or at least crippling
the operation of the system.
The problem here is not mainly in uncritical
acceptance of bad parameters to system calls\(em
there may be bugs in this area, but none are known\(em
but rather in lack of checks for excessive
consumption of resources.
Most notably, there is no limit on the amount of disk
storage used, either in total space allocated or in
the number of files or directories.
Here is a particularly ghastly shell sequence guaranteed
to stop the system:
.DS
while : ; do
	mkdir x
	cd x
done
.DE
Either a panic will occur because all the i-nodes
on the device are used up, or all the disk blocks will
be consumed, thus preventing anyone from writing files
on the device.
.PP
In this version of the system,
users are prevented from creating more than
a set number of processes simultaneously,
so unless users are in collusion it is unlikely that any one
can stop the system altogether.
However, creation of 20 or so CPU or disk-bound jobs
leaves few resources available for others.
Also, if many large jobs are run simultaneously,
swap space may run out, causing a panic.
.PP
It should be evident that excessive consumption of disk
space, files, swap space, and processes can easily occur
accidentally in malfunctioning programs
as well as at command level.
In fact
.UX
is essentially defenseless against this kind of
abuse,
nor is there any easy fix.
The best that can be said is that it is generally
fairly
easy to detect what has happened when disaster
strikes,
to identify the user responsible,
and take appropriate action.
In practice,
we have found that difficulties
in this area are rather rare,
but we have not been faced with malicious users,
and enjoy a fairly generous supply of
resources which have served to cushion us against
accidental overconsumption.
.PP
The picture is considerably brighter
in the area of protection of information
from unauthorized perusal and destruction.
Here the degree of security seems (almost)
adequate theoretically, and the problems lie
more in the necessity for care in the actual use of
the system.
.PP
Each
.UX
file has associated with it
eleven bits of protection information
together with a user identification number and a user-group
identification number
(UID and GID).
Nine of the protection bits
are used to specify independently
permission to read, to write, and to execute the file
to the user himself, to members of the user's
group, and to all other users.
Each process generated
by or for a user has associated with
it an effective UID and a real UID, and an effective and real GID.
When an attempt is made
to access the file for reading, writing, or execution,
the user process's effective UID is compared
against the file's UID; if a match is obtained,
access is granted provided the read, write, or execute
bit respectively for the user himself is
present.
If the UID for the file and for the process fail to match,
but the GID's do match, the group bits are used; if the GID's
do not match, the bits for other users are tested.
The last two bits of each file's protection information,
called the set-UID and set-GID bits,
are used only when the
file is executed as a program.
If, in this case, the set-UID bit is on for the file,
the effective UID for the process is changed to the UID
associated with the file; the change persists
until the process terminates or until the UID
changed again by another execution of a set-UID file.
Similarly the effective group ID of a process is changed
to the GID associated with a file
when that file is executed and has the set-GID bit set.
The real UID and GID of a process do not change
when any file is executed,
but only as the result of a privileged system
call.
.PP
The basic notion of the set-UID and set-GID
bits is that one may write a program which is executable
by others and which maintains files accessible to others only
by that program.
The classical example is the game-playing program which
maintains records of the scores of its players.
The program itself has to read and write the score file,
but
no one but the game's sponsor can be allowed
unrestricted access to the file lest they manipulate
the game to their own advantage.
The solution is to
turn on the set-UID bit of the
game
program.
When, and only when, it is invoked
by players of the game, it may update the score file
but ordinary programs executed by others cannot
access the score.
.PP
There are a number of special cases involved
in determining access permissions.
Since executing a directory as a program is a meaningless
operation, the execute-permission
bit, for directories, is taken instead to mean
permission to search the directory for a given file
during the scanning of a path name;
thus if a directory has execute permission but no read
permission for a given user, he may access files
with known names in the directory,
but may not read (list) the entire contents of the
directory.
Write permission on a directory is interpreted to
mean that the user may create and delete
files in that directory;
it is impossible
for any user to write directly into any directory.
.PP
Another, and from the point of view of security, much
more serious special case is that there is a ``super user''
who is able to read any file and write any non-directory.
The super-user is also able to change the protection
mode and the owner UID and GID of any file
and to invoke privileged system calls.
It must be recognized that the mere notion of
a super-user is a theoretical, and usually
practical, blemish on any protection scheme.
.PP
The first necessity for a secure system
is of course arranging that all files and directories
have the proper protection modes.
Traditionally,
.UX
software has been exceedingly
permissive in this regard;
essentially all commands create files
readable and writable by everyone.
In the current version,
this policy may be easily adjusted to suit the needs of
the installation or the individual user.
Associated with each process and its descendants
is a mask, which is in effect
.I and\fR\|-ed
with the mode of every file and directory created by
that process.
In this way, users can arrange that, by default,
all their files are no more accessible than they wish.
The standard mask, set by
.I login,
allows all permissions to the user himself and to his group,
but disallows writing by others.
.PP
To maintain both data privacy and
data integrity,
it is necessary, and largely sufficient,
to make one's files inaccessible to others.
The lack of sufficiency could follow
from the existence of set-UID programs
created by the user
and the possibility of total
breach of system security
in one of the ways discussed below
(or one of the ways not discussed below).
For greater protection,
an encryption scheme is available.
Since the editor is able to create encrypted
documents, and the
.I crypt
command can be used to pipe such documents into
the other text-processing programs,
the length of time during which cleartext versions
need be available is strictly limited.
The encryption scheme used is not one of the strongest
known, but it is judged adequate, in the sense that
cryptanalysis
is likely to require considerably more effort than more direct
methods of reading the encrypted files.
For example, a user who stores data that he regards as truly secret
should be aware that he is implicitly trusting the system
administrator not to install a version of the crypt command
that stores every typed password in a file.
.PP
Needless to say, the system administrators
must be at least as careful as their most
demanding user to place the correct
protection mode on the files under their
control.
In particular,
it is necessary that special files be protected
from writing, and probably reading, by ordinary
users when
they store sensitive files belonging to other
users.
It is easy to write programs that examine and change
files by accessing the device
on which the files live.
.PP
On the issue of password security,
.UX
is probably better than most systems.
Passwords are stored in an encrypted form
which, in the absence of serious attention
from specialists in the field,
appears reasonably secure,
provided its limitations are understood.
In the current version, it is based on a slightly
defective version of the Federal DES;
it is purposely defective so that
easily-available hardware is useless for attempts at exhaustive
key-search.
Since both the encryption algorithm and the encrypted passwords
are available,
exhaustive enumeration of potential passwords
is still feasible up to a point.
We have observed that users choose passwords that are easy to guess:
they are short, or from a limited alphabet, or
in a dictionary.
Passwords should be
at least six characters long and randomly chosen from an alphabet
which includes digits and special characters.
.PP
Of course there also exist
feasible non-cryptanalytic
ways of finding out passwords.
For example: write a program which types out ``login:\|''
on the typewriter and copies whatever is typed
to a file of your own.
Then invoke the command and go away until the victim arrives.
.PP
The set-UID (set-GID)
notion must be used carefully if any security is to be maintained.
The first thing to keep in mind is that
a writable set-UID file can have another program copied onto it.
For example, if the super-user
.I (su)
command is writable,
anyone can copy the shell
onto it and get a password-free version
of
.I su.
A more subtle problem
can come from
set-UID programs which are not sufficiently
careful of what is fed into them.
To take an obsolete example,
the previous version of the
.I mail
command was set-UID and owned by the super-user.
This version sent mail to the recipient's own directory.
The notion was that one should be able to send
mail to anyone even if they want to protect
their directories from writing.
The trouble was that
.I mail
was rather dumb:
anyone could mail someone else's private file to himself.
Much more serious
is the following scenario:
make a file with a line like one in the password file
which allows one to log in as the super-user.
Then make a link named ``.mail'' to the password file
in some writable
directory on the same device as the password file (say /tmp).
Finally mail the bogus login line to /tmp/.mail;
You can then login as the super-user,
clean up the incriminating evidence,
and have your will.
.PP
The fact that users can mount their own disks and tapes
as file systems
can be another way of gaining super-user status.
Once a disk pack is mounted, the system believes
what is on it.
Thus one can take a blank disk pack,
put on it anything desired,
and mount it.
There are obvious and unfortunate consequences.
For example:
a mounted disk with garbage on it will crash the
system;
one of the files on the mounted disk can easily be
a password-free version of
.I su;
other files can be unprotected entries for special files.
The only easy fix for this problem is to
forbid the use of
.I mount
to unprivileged users.
A partial solution, not so restrictive,
would be to have the
.I mount
command examine the special file for bad data,
set-UID programs owned by others, and accessible
special files,
and balk at unprivileged invokers.