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\section*{Handout 4 (Access Control)}

Access control is essentially about deciding whether to grant

turns out that things are not as simple as they seem at first

glance. Let us first look, as a case-study, at how access

control is organised in Unix-like systems (Windows systems

have similar access controls, although the details might be

quite different).

\subsubsection*{Unix-Style Access Control}

Following the Unix-philosophy that everything is considered as

a file, even memory, ports and so on, access control in Unix

is organised around 11 Bits that specify how a file can be

accessed. These Bits are sometimes called the \emph{permission

attributes} of a file. There are typically three modes for

access: \underline{\textbf{r}}ead, \underline{\textbf{w}}rite

and e\underline{\textbf{x}}ecute. Moreover there are three

user groups to which the modes apply: the owner of the file,

the group the file is associated with and everybody else. This

relatively fine granularity seems to cover many useful

scenarios of access control. A typical example of some files

with permission attributes is as follows:

{\small\lstinputlisting[language={}]{../slides/lst}}

\noindent The leading \pcode{d} in Lines 2 and 6 indicate that

the file is a directory, whereby in the Unix-tradition the

\pcode{.} points to the directory itself. The \pcode{..}

points at the directory ``above'', or parent directory. The

second to fourth letter specify how the owner of the file can

access the file. For example Line 3 states that \pcode{ping}

can read and write \pcode{manual.txt}, but cannot execute it.

The next three letters specify how the group members of the

file can access the file. In Line 4, for example, all students

can read and write the file \pcode{report.txt}. Finally the

last three letters specify how everybody else can access a

file. This should all be relatively familiar and

straightforward. No?

There are already some special rules for directories and

links. If the execute attribute of a directory is \emph{not}

set, then one cannot change into the directory and one cannot

access any file inside it. If the write attribute is

\emph{not} set, then one can change existing files (provide

they are changeable), but one cannot create new files. If the

read attribute is \emph{not} set, one cannot search inside the

directory (\pcode{ls -la} does not work) but one can access an

existing file, provided one knows its name. Links to files

never depend on the permission of the link, but the file they

are pointing to. Otherwise one could easily change access

rights to files.

While the above might sound already moderately complicated,

the real complications with Unix-style file permissions

involve the setuid and setgid attributes. For example the file

\pcode{microedit} in Line 5 has the setuid attribute set

(indicated by the \pcode{s} in place of the usual \pcode{x}).

The purpose of setuid and setgid is to solve the following

puzzle: The program \pcode{passwd} allows users to change

their passwords. Therefore \pcode{passwd} needs to have write

access to the file \pcode{/etc/passwd}. But this file cannot

be writable for every user, otherwise anyone can set anyone

else's password. So changing securely passwords cannot be

achieved with the simple Unix access rights discussed so far.

While this situation might look like an anomaly, it is in fact

an often occurring problem. For example looking at current

active processes with \pcode{/bin/ps} requires access to

internal data structures of the operating system, which only

root should be allowed to. In fact any of the following

actions cannot be configured for single users, but need

privileged root access

\begin{itemize}

\item changing system databases (users, groups, routing tables

and so on)

\item opening a network port below 1024

\item interacting with peripheral hardware, such as printers, 

harddisk etc

\item overwriting operating system facilities, like

process scheduling and memory management

\end{itemize}

\noindent This will typically involve quite a lot of programs

on a Unix system. I counted 90 programs with the setuid

attribute set on my bog-standard Mac OSX system (including the

program \pcode{/usr/bin/login} for example). The problem is

that if there is a security problem with only one of them, be

it a buffer overflow for example, then malicious users can

gain root access (and for outside attackers it is much easier

to take over a system). Unfortunately it is rather easy to

cause a security problem since the handling of elevating and

dropping access rights in such programs rests entirely with

the programmer.

The fundamental idea behind the setuid attribute is that a

file will be able to run not with the callers access rights,

but with the rights of the owner of the file. So

\pcode{/usr/bin/login} will always be running with root access

rights, no matter who invokes this program. The problem is

that this entails a rather complicated semantics of what the

identity of a process (that runs the program) is. One would

hope there is only one such ID, but in fact Unix distinguishes

three(!):

\begin{itemize}

\item \emph{real identity}\\ 

This is the ID of the user who creates 

the process; can only be changed to something else by root. 

\item \emph{effective identity}\\ 

This is the ID that is used to 

grant or deny access to a resource; can be changed to either

the real identity or saved identity by users, can be changed 

to anything by root.

\item \emph{saved identity}\\

If the setuid bit set in a file then the process is started

with the real identity of the user who started the program,

and the identity of the owner of the program as effective and

saved identity. If the setuid bit is not set, then the

saved identity will be the real identity. 

\end{itemize}

\noindent As an example consider again the \pcode{passwd}

program. When started by, say the user \pcode{foo}, it has at

the beginning the identities:

\begin{itemize}

\item \emph{real identity}: \pcode{foo}\\

\emph{effective identity}: \pcode{foo}\\ 

\emph{saved identity}: \pcode{root}

\end{itemize}

\noindent It is then allowed to change the effective

identity to the saved identity to have

\begin{itemize}

\item \emph{real identity}: \pcode{foo}\\

\emph{effective identity}: \pcode{root}\\ 

\emph{saved identity}: \pcode{root}

\end{itemize}

\noindent It can now read and write the file

\pcode{/etc/passwd}. After finishing the job it is supposed to

drop the effective identity back to \pcode{foo}. This is the

responsibility of the programmers who wrote \pcode{passwd}.

Notice that the effective identity is not automatically

elevated to \pcode{root}, but the program itself must make

this change. After it has done the work, the effective

identity should go back to the real identity.

Despite this complicated semantics, Unix-style access control

is of no use in a number of situations. For example it cannot

be used to exclude some subset of people, but otherwise have

files readable by everybody else (say you want to restrict

access to a file such that your office mates cannot access 

a file). You could try setting the group of the file to this

subset and then restrict access accordingly. But this does not

help, because users can drop membership in groups. If one

needs such fine-grained control over who can access a file,

one needs more powerful \emph{mandatory access controls}

as described next.

\subsubsection*{Secrecy and Integrity}

Often you need to keep information secret within a system or

\begin{itemize}

\item top secret

\item secret

\item confidential

\item unclassified

\end{itemize}

The idea is that the secrets classified as top-secret are most

closely guarded and only accessible to people who have a

special clearance. The unclassified category is the lowest

level not needing any clearance. While the idea behind these

security levels is quite straightforward, there are some

\begin{itemize}

\end{itemize} 

clearance can read secret documents or lower, but not

documents classified top-secret. The second rule for writing

needs to be the other way around: someone with secret

clearance can write secret or top-secret documents---no

While secrecy is one property you often want to enforce,

\subsubsection*{Further Information}

If you want to know more about the intricacies of the

``simple'' Unix access control system you might find the

relatively readable paper about ``Setuid Demystified'' 

useful.

\begin{center}\small

\url{http://www.cs.berkeley.edu/~daw/papers/setuid-usenix02.pdf}

\end{center}

\end{document}

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