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<info><title>Secure Programming</title>
<authorgroup>
<author><personname><firstname>Murray</firstname><surname>Stokely</surname></personname><contrib>Contributed by </contrib></author>
</authorgroup>
</info>
<sect1 xml:id="secure-synopsis"><title>Synopsis</title>
<para>This chapter describes some of the security issues that
have plagued &unix; programmers for decades and some of the new
tools available to help programmers avoid writing exploitable
code.</para>
</sect1>
<sect1 xml:id="secure-philosophy"><title>Secure Design
Methodology</title>
<para>Writing secure applications takes a very scrutinous and
pessimistic outlook on life. Applications should be run with
the principle of <quote>least privilege</quote> so that no
process is ever running with more than the bare minimum access
that it needs to accomplish its function. Previously tested
code should be reused whenever possible to avoid common
mistakes that others may have already fixed.</para>
<para>One of the pitfalls of the &unix; environment is how easy it
is to make assumptions about the sanity of the environment.
Applications should never trust user input (in all its forms),
system resources, inter-process communication, or the timing of
events. &unix; processes do not execute synchronously so logical
operations are rarely atomic.</para>
</sect1>
<sect1 xml:id="secure-bufferov"><title>Buffer Overflows</title>
<para>Buffer Overflows have been around since the very
beginnings of the von Neumann <xref linkend="COD"/> architecture.
<indexterm><primary>buffer overflow</primary></indexterm>
<indexterm><primary>von Neumann</primary></indexterm>
They first gained widespread notoriety in 1988 with the Morris
Internet worm. Unfortunately, the same basic attack remains
<indexterm><primary>Morris Internet worm</primary></indexterm>
effective today.
By far the most common type of buffer overflow attack is based
on corrupting the stack.</para>
<indexterm><primary>stack</primary></indexterm>
<indexterm><primary>arguments</primary></indexterm>
<para>Most modern computer systems use a stack to pass arguments
to procedures and to store local variables. A stack is a last
in first out (LIFO) buffer in the high memory area of a process
image. When a program invokes a function a new "stack frame" is
<indexterm><primary>LIFO</primary></indexterm>
<indexterm>
<primary>process image</primary>
<secondary>stack pointer</secondary>
</indexterm>
created. This stack frame consists of the arguments passed to
the function as well as a dynamic amount of local variable
space. The "stack pointer" is a register that holds the current
<indexterm><primary>stack frame</primary></indexterm>
<indexterm><primary>stack pointer</primary></indexterm>
location of the top of the stack. Since this value is
constantly changing as new values are pushed onto the top of the
stack, many implementations also provide a "frame pointer" that
is located near the beginning of a stack frame so that local
variables can more easily be addressed relative to this
value. <xref linkend="COD"/> The return address for function
<indexterm><primary>frame pointer</primary></indexterm>
<indexterm>
<primary>process image</primary>
<secondary>frame pointer</secondary>
</indexterm>
<indexterm><primary>return address</primary></indexterm>
<indexterm><primary>stack-overflow</primary></indexterm>
calls is also stored on the stack, and this is the cause of
stack-overflow exploits since overflowing a local variable in a
function can overwrite the return address of that function,
potentially allowing a malicious user to execute any code he or
she wants.</para>
<para>Although stack-based attacks are by far the most common,
it would also be possible to overrun the stack with a heap-based
(malloc/free) attack.</para>
<para>The C programming language does not perform automatic
bounds checking on arrays or pointers as many other languages
do. In addition, the standard C library is filled with a
handful of very dangerous functions.</para>
<informaltable frame="none" pgwide="1">
<tgroup cols="2">
<tbody>
<row><entry><function>strcpy</function>(char *dest, const char
*src)</entry>
<entry><simpara>May overflow the dest buffer</simpara></entry>
</row>
<row><entry><function>strcat</function>(char *dest, const char
*src)</entry>
<entry><simpara>May overflow the dest buffer</simpara></entry>
</row>
<row><entry><function>getwd</function>(char *buf)</entry>
<entry><simpara>May overflow the buf buffer</simpara></entry>
</row>
<row><entry><function>gets</function>(char *s)</entry>
<entry><simpara>May overflow the s buffer</simpara></entry>
</row>
<row><entry><function>[vf]scanf</function>(const char *format,
...)</entry>
<entry><simpara>May overflow its arguments.</simpara></entry>
</row>
<row><entry><function>realpath</function>(char *path, char
resolved_path[])</entry>
<entry><simpara>May overflow the path buffer</simpara></entry>
</row>
<row><entry><function>[v]sprintf</function>(char *str, const char
*format, ...)</entry>
<entry><simpara>May overflow the str buffer.</simpara></entry>
</row>
</tbody>
</tgroup>
</informaltable>
<sect2><title>Example Buffer Overflow</title>
<para>The following example code contains a buffer overflow
designed to overwrite the return address and skip the
instruction immediately following the function call. (Inspired
by <xref linkend="Phrack"/>)</para>
<programlisting>#include <stdio.h>
void manipulate(char *buffer) {
char newbuffer[80];
strcpy(newbuffer,buffer);
}
int main() {
char ch,buffer[4096];
int i=0;
while ((buffer[i++] = getchar()) != '\n') {};
i=1;
manipulate(buffer);
i=2;
printf("The value of i is : %d\n",i);
return 0;
}</programlisting>
<para>Let us examine what the memory image of this process would
look like if we were to input 160 spaces into our little program
before hitting return.</para>
<para>[XXX figure here!]</para>
<para>Obviously more malicious input can be devised to execute
actual compiled instructions (such as exec(/bin/sh)).</para>
</sect2>
<sect2><title>Avoiding Buffer Overflows</title>
<para>The most straightforward solution to the problem of
stack-overflows is to always use length restricted memory and
string copy functions. <function>strncpy</function> and
<function>strncat</function> are part of the standard C library.
<indexterm>
<primary>string copy functions</primary>
<secondary>strncpy</secondary>
</indexterm>
<indexterm>
<primary>string copy functions</primary>
<secondary>strncat</secondary>
</indexterm>
These functions accept a length value as a parameter which
should be no larger than the size of the destination buffer.
These functions will then copy up to `length' bytes from the
source to the destination. However there are a number of
problems with these functions. Neither function guarantees NUL
termination if the size of the input buffer is as large as the
<indexterm><primary>NUL termination</primary></indexterm>
destination. The length parameter is also used inconsistently
between strncpy and strncat so it is easy for programmers to get
confused as to their proper usage. There is also a significant
performance loss compared to <function>strcpy</function> when
copying a short string into a large buffer since
<function>strncpy</function> NUL fills up the size
specified.</para>
<para>Another memory copy implementation exists
to get around these problems. The
<function>strlcpy</function> and <function>strlcat</function>
functions guarantee that they will always null terminate the
destination string when given a non-zero length argument.</para>
<indexterm>
<primary>string copy functions</primary>
<secondary>strlcpy</secondary>
</indexterm>
<indexterm>
<primary>string copy functions</primary>
<secondary>strlcat</secondary>
</indexterm>
<sect3><title>Compiler based run-time bounds checking</title>
<indexterm><primary>bounds checking</primary>
<secondary>compiler-based</secondary></indexterm>
<para>Unfortunately there is still a very large assortment of
code in public use which blindly copies memory around without
using any of the bounded copy routines we just discussed.
Fortunately, there is a way to help prevent such attacks —
run-time bounds checking, which is implemented by several
C/C++ compilers.</para>
<indexterm><primary>ProPolice</primary></indexterm>
<indexterm><primary>StackGuard</primary></indexterm>
<indexterm><primary>gcc</primary></indexterm>
<para>ProPolice is one such compiler feature, and is integrated
into &man.gcc.1; versions 4.1 and later. It replaces and
extends the earlier StackGuard &man.gcc.1; extension.</para>
<para>ProPolice helps to protect against stack-based buffer
overflows and other attacks by laying pseudo-random numbers in
key areas of the stack before calling any function. When a
function returns, these <quote>canaries</quote> are checked
and if they are found to have been changed the executable is
immediately aborted. Thus any attempt to modify the return
address or other variable stored on the stack in an attempt to
get malicious code to run is unlikely to succeed, as the
attacker would have to also manage to leave the pseudo-random
canaries untouched.</para>
<indexterm><primary>buffer overflow</primary></indexterm>
<para>Recompiling your application with ProPolice is an
effective means of stopping most buffer-overflow attacks, but
it can still be compromised.</para>
</sect3>
<sect3><title>Library based run-time bounds checking</title>
<indexterm>
<primary>bounds checking</primary>
<secondary>library-based</secondary>
</indexterm>
<para>Compiler-based mechanisms are completely useless for
binary-only software for which you cannot recompile. For
these situations there are a number of libraries which
re-implement the unsafe functions of the C-library
(<function>strcpy</function>, <function>fscanf</function>,
<function>getwd</function>, etc..) and ensure that these
functions can never write past the stack pointer.</para>
<itemizedlist>
<listitem><simpara>libsafe</simpara></listitem>
<listitem><simpara>libverify</simpara></listitem>
<listitem><simpara>libparanoia</simpara></listitem>
</itemizedlist>
<para>Unfortunately these library-based defenses have a number
of shortcomings. These libraries only protect against a very
small set of security related issues and they neglect to fix
the actual problem. These defenses may fail if the
application was compiled with -fomit-frame-pointer. Also, the
LD_PRELOAD and LD_LIBRARY_PATH environment variables can be
overwritten/unset by the user.</para>
</sect3>
</sect2>
</sect1>
<sect1 xml:id="secure-setuid"><title>SetUID issues</title>
<indexterm><primary>seteuid</primary></indexterm>
<para>There are at least 6 different IDs associated with any
given process, and you must therefore be very careful with
the access that your process has at any given time. In
particular, all seteuid applications should give up their
privileges as soon as it is no longer required.</para>
<indexterm>
<primary>user IDs</primary>
<secondary>real user ID</secondary>
</indexterm>
<indexterm>
<primary>user IDs</primary>
<secondary>effective user ID</secondary>
</indexterm>
<para>The real user ID can only be changed by a superuser
process. The <application>login</application> program sets this
when a user initially logs in and it is seldom changed.</para>
<para>The effective user ID is set by the
<function>exec()</function> functions if a program has its
seteuid bit set. An application can call
<function>seteuid()</function> at any time to set the effective
user ID to either the real user ID or the saved set-user-ID.
When the effective user ID is set by <function>exec()</function>
functions, the previous value is saved in the saved set-user-ID.</para>
</sect1>
<sect1 xml:id="secure-chroot"><title>Limiting your program's environment</title>
<indexterm><primary>chroot()</primary></indexterm>
<para>The traditional method of restricting a process
is with the <function>chroot()</function> system call. This
system call changes the root directory from which all other
paths are referenced for a process and any child processes. For
this call to succeed the process must have execute (search)
permission on the directory being referenced. The new
environment does not actually take effect until you
<function>chdir()</function> into your new environment. It
should also be noted that a process can easily break out of a
chroot environment if it has root privilege. This could be
accomplished by creating device nodes to read kernel memory,
attaching a debugger to a process outside of the &man.chroot.8;
environment, or in
many other creative ways.</para>
<para>The behavior of the <function>chroot()</function> system
call can be controlled somewhat with the
kern.chroot_allow_open_directories <command>sysctl</command>
variable. When this value is set to 0,
<function>chroot()</function> will fail with EPERM if there are
any directories open. If set to the default value of 1, then
<function>chroot()</function> will fail with EPERM if there are
any directories open and the process is already subject to a
<function>chroot()</function> call. For any other value, the
check for open directories will be bypassed completely.</para>
<sect2><title>FreeBSD's jail functionality</title>
<indexterm><primary>jail</primary></indexterm>
<para>The concept of a Jail extends upon the
<function>chroot()</function> by limiting the powers of the
superuser to create a true `virtual server'. Once a prison is
set up all network communication must take place through the
specified IP address, and the power of "root privilege" in this
jail is severely constrained.</para>
<para>While in a prison, any tests of superuser power within the
kernel using the <function>suser()</function> call will fail.
However, some calls to <function>suser()</function> have been
changed to a new interface <function>suser_xxx()</function>.
This function is responsible for recognizing or denying access
to superuser power for imprisoned processes.</para>
<para>A superuser process within a jailed environment has the
power to:</para>
<itemizedlist>
<listitem><simpara>Manipulate credential with
<function>setuid</function>, <function>seteuid</function>,
<function>setgid</function>, <function>setegid</function>,
<function>setgroups</function>, <function>setreuid</function>,
<function>setregid</function>, <function>setlogin</function></simpara></listitem>
<listitem><simpara>Set resource limits with <function>setrlimit</function></simpara></listitem>
<listitem><simpara>Modify some sysctl nodes
(kern.hostname)</simpara></listitem>
<listitem><simpara><function>chroot()</function></simpara></listitem>
<listitem><simpara>Set flags on a vnode:
<function>chflags</function>,
<function>fchflags</function></simpara></listitem>
<listitem><simpara>Set attributes of a vnode such as file
permission, owner, group, size, access time, and modification
time.</simpara></listitem>
<listitem><simpara>Bind to privileged ports in the Internet
domain (ports < 1024)</simpara></listitem>
</itemizedlist>
<para><function>Jail</function> is a very useful tool for
running applications in a secure environment but it does have
some shortcomings. Currently, the IPC mechanisms have not been
converted to the <function>suser_xxx</function> so applications
such as MySQL cannot be run within a jail. Superuser access
may have a very limited meaning within a jail, but there is
no way to specify exactly what "very limited" means.</para>
</sect2>
<sect2><title>&posix;.1e Process Capabilities</title>
<indexterm><primary>POSIX.1e Process Capabilities</primary></indexterm>
<indexterm><primary>TrustedBSD</primary></indexterm>
<para>&posix; has released a working draft that adds event
auditing, access control lists, fine grained privileges,
information labeling, and mandatory access control.</para>
<para>This is a work in progress and is the focus of the <link xlink:href="http://www.trustedbsd.org/">TrustedBSD</link> project. Some
of the initial work has been committed to &os.current;
(cap_set_proc(3)).</para>
</sect2>
</sect1>
<sect1 xml:id="secure-trust"><title>Trust</title>
<para>An application should never assume that anything about the
users environment is sane. This includes (but is certainly not
limited to): user input, signals, environment variables,
resources, IPC, mmaps, the filesystem working directory, file
descriptors, the # of open files, etc.</para>
<indexterm><primary>positive filtering</primary></indexterm>
<indexterm><primary>data validation</primary></indexterm>
<para>You should never assume that you can catch all forms of
invalid input that a user might supply. Instead, your
application should use positive filtering to only allow a
specific subset of inputs that you deem safe. Improper data
validation has been the cause of many exploits, especially with
CGI scripts on the world wide web. For filenames you need to be
extra careful about paths ("../", "/"), symbolic links, and
shell escape characters.</para>
<indexterm><primary>Perl Taint mode</primary></indexterm>
<para>Perl has a really cool feature called "Taint" mode which
can be used to prevent scripts from using data derived outside
the program in an unsafe way. This mode will check command line
arguments, environment variables, locale information, the
results of certain syscalls (<function>readdir()</function>,
<function>readlink()</function>,
<function>getpwxxx()</function>), and all file input.</para>
</sect1>
<sect1 xml:id="secure-race-conditions">
<title>Race Conditions</title>
<para>A race condition is anomalous behavior caused by the
unexpected dependence on the relative timing of events. In
other words, a programmer incorrectly assumed that a particular
event would always happen before another.</para>
<indexterm><primary>race conditions</primary>
<secondary>signals</secondary></indexterm>
<indexterm><primary>race conditions</primary>
<secondary>access checks</secondary></indexterm>
<indexterm><primary>race conditions</primary>
<secondary>file opens</secondary></indexterm>
<para>Some of the common causes of race conditions are signals,
access checks, and file opens. Signals are asynchronous events
by nature so special care must be taken in dealing with them.
Checking access with <function>access(2)</function> then
<function>open(2)</function> is clearly non-atomic. Users can
move files in between the two calls. Instead, privileged
applications should <function>seteuid()</function> and then call
<function>open()</function> directly. Along the same lines, an
application should always set a proper umask before
<function>open()</function> to obviate the need for spurious
<function>chmod()</function> calls.</para>
</sect1>
</chapter>
