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Butler W. Lampson is a very influential American computer scientist...
More than four decades ago this paper introduced the confinement pr...
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#### Artificial Intelligence Confinement Problem If we managed to ...
Operating C. Weissman
Systems Editor
A Note on the
Confinement Problem
Butler W. Lampson
Xerox Palo Alto Research Center
This note explores the problem of confining a
program during its execution so that it cannot transmit
information to any other program except its caller. A
set of examples attempts to stake out the boundaries of
the problem. Necessary conditions for a solution are
stated and informally justified.
Key Words and Phrases: protection, confinement,
proprietary program, privacy, security, leakage of data
CR Categories: 2.11, 4.30
Introduction
Designers of protection systems are usually pre-
occupied with the need to safeguard data from un-
authorized access or modification, or programs from
unauthorized execution. It is known how to solve these
problems well enough so that a program can create
a controlled environment within which another, pos-
sibly untrustworthy program, can be run safely [1, 2].
Adopting terminology appropriate for our particular
case, we will call the first program a customer and the
second a service.
The customer will want to ensure that the service
cannot access (i.e. read or modify) any of his data
except those items to which he explicitly grants access.
If he is cautious, he will only grant access to items which
are needed as input or output for the service program.
In general it is also necessary to provide for smooth
transfers of control, and to handle error conditions.
Furthermore, the service must be protected from
intrusion by the customer, since the service may be a
Copyright (~ 1973, Association for Computing Machinery, Inc.
General
permission to republish, but not for profit, all or part
of this material is granted provided that ACM's copyright notice
is given and that reference is made to the publication, to its date
of issue, and to the fact that reprinting privileges were granted
by permission of the Association for Computing Machinery.
Author's address: Xerox Palo Alto Research Center, 3180 Porter
Drive, Palo Alto, CA 94304.
proprietary program or may have its own private data.
These things, while interesting, will not concern us here.
Even when all unauthorized access has been pre-
vented, there remain two ways in which the customer
may be injured by the service: (1) it may not perform
as advertised; or (2) it may leak, i.e. transmit to its
owner the input data which the customer gives it.
The former problem does not seem to have any general
technical solution short of program certification. It does,
however, have the property that the dissatisfied cus-
tomer is left with evidence, in the form of incorrect
outputs from the service, which he can use to support
his claim for restitution. If, on the other hand, the
service leaks data which the customer regards as con-
fidential, there will generally be no indication that the
security of the data has been compromised.
There is, however, some hope for technical safe-
guards which will prevent such leakages. We will call
the problem of constraining a service in this way the
confinement problem. The purpose of this note is to
characterize the problem more precisely and to de-
scribe methods for blocking some of the subtle paths
by which data can escape from confinement.
The Problem
We want to be able to confine an arbitrary program.
This does not mean that any program which works
when free will still work under confinement, but that
any program, if confined, will be unable to leak data. A
misbehaving program may well be trapped as a result
of an attempt to escape.
A list of possible leaks may help to create some
intuition in preparation for a more abstract description
of confinement rules.
0. If the service has memory, it can collect data, wait
for its owner to call it, and then return the data to him.
1. The service may write into a permanent file in its
owner's directory. The owner can then come around at
his leisure and collect the data.
2. The service may create a temporary file (in itself a
legitimate action which cannot be forbidden without
imposing an unreasonable constraint on the computing
which a service can do) and grant its owner access to
this file. If he tests for its existence at suitable intervals,
he can read out the data before the service completes
its work and the file is destroyed.
3. The service may send a message to a process con-
trolled by its owner, using the system's interprocess
communication facility.
4. More subtly, the information may be encoded in the
bill rendered for the service, since its owner must get a
copy. If the form of bills is suitably restricted, the
amount of information which can be transmitted in this
way can be reduced to a few bits or tens of bits. Re-
ducing it to zero, however, requires more far-reaching
measures.
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If the owner of the service pays for resources con-
sumed by the service, information can also be encoded
in the amount of time used or whatever. This can be
avoided if the customer pays for resources.
5. If the system has interlocks which prevent files
from being open for writing and reading at the same
time, the service can leak data if it is merely allowed to
read files which can be written by its owner. The inter-
locks allow a file to simulate a shared Boolean variable
which one program can set and the other can test.
Given a procedure open (file, error) which does goto
error if the file is already open, the following pro-
cedures will perform this simulation:
procedure settrue
(file);
begin
loop 1: open (file, loop 1)
end;
procedure set false
(file); begin close (file)
end;
Boolean procedure
value
(file); begin
value
:=
true;
open (file, loop 2);
value
:= false; close (file); loop 2:
end
Using these procedures and three files called data,
sendclock, and receiveclock, a service can send a
stream of bits to another concurrently running program.
Referencing the files as though they were variables of
this rather odd kind, then, we can describe the sequence
of events for transmitting a single bit:
sender: data := bit being sent; sendclock :=
true
receiver: wait for sendclock = true; received bit := data;
receive clock :=
true;
sender: wait for receive clock = true; sendclock := false;
receiver: wait for sendclock = false; receiveclock := false;
sender: wait for receiveclock = false;
6. By varying its ratio of computing to input/output or
its paging rate, the service can transmit information
which a concurrently running process can receive by
observing the performance of the system. The com-
munication channel thus established is a noisy one, but
the techniques of information theory can be used to
devise an encoding which will allow the information to
get through reliably no matter how small the effects of
the service on system performance are, provided they
are not zero. The data rate of this channel may be very
low, of course.
Confinement Rules
We begin by observing that a confined program must
be memoryless. In other words, it must not be able to
preserve information within itself from one call to
another. Most existing systems make it quite easy to
enforce this restriction. In ALGOL, for example, a pro-
cedure which has no own variables and which refer-
ences no global variables will be memoryless.
Taking this for granted, it is easy to state a rule
which is sufficient to ensure confinement.
Total isolation:
A confined program shall make no calls
on any other program.
By a "call" we mean any transfer of control which is
caused by the confined program. Unfortunately, this
rule is quite impractical. Ex~Lmple 5 above shows that
supervisor calls must be fl~rbidden, since quite in-
nocuous looking ones can result in leaks. Example 6
shows that even the implicit supervisor calls resulting
from input/output, paging or time-slicing can cause
trouble.
To improve on this situation, we must make a dis-
tinction between programs which are confined and those
which are not. Recall that being confined is not an
intrinsic property of a program but a constraint which
can be imposed on it when it runs. Hence if every
program called by a confined program is also confined,
there can be no leakage. This is still not good enough,
since the supervisor, for example, cannot be confined.
It is at least conceivable, however, that it is
trusted,
i.e.
that the customer believes it will not leak his data or
help any confined program which calls it to do so.
Granting this, we can formulate a new confinement rule.
Transitivity:
If a confined program calls another pro-
gram which is not trusted, the called program must also
be confined.
Examples 5 and 6 show that it is hard to write a trust-
worthy supervisor, since some of the paths by which
information can leak out from a supervisor are quite
subtle and obscure. The remainder of this note argues
that it is possible.
A trustworthy program must guard against any
possible leakage of data. In the case of a supervisor, the
number of possible channels for such leakage is sur-
prisingly large, but certainly not infinite. It is necessary
to enumerate them all and then to block each one.
There is not likely to be any rigorous way of identifying
every channel in any system of even moderate com-
plexity. The six examples above, however, were chosen
to illustrate the full range of possibilities which I have
been able to think of, and for the present plausibility
argument they will be regarded as a sufficient enumera-
tion. The channels used there fall into three categories:
Storage
of various kinds maintained by the super-
visor which can be written by the service and read by
an unconfined program, either shortly after it is written
or at some later time.
Legitimate
channels used by the confined service,
such as the bill.
Covert
channels, i.e. those not intended for informa-
tion transfer at all, such as the service program's effect
on the system load.
All the examples except 4 and 6 use storage channels.
Fairly straightforward techniques can be used by the
supervisor to prevent the misuse of storage as an escape
route for a confined program. (For instance, example 5
can be taken care of by making a copy of a file which is
being read by a confined program if someone else tries
to write it; the confined program can then continue to
read the copy, and the writer can proceed without
receiving any indication that the file was being read.)
The main difficulty, as example 5 illustrates, is to
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