NAME
executable_stack — controls whether program stacks are executable by default
DESCRIPTION
This tunable parameter controls whether program stacks are executable
by default.
It allows systems to be configured to have extra protection from stack
buffer overflow attacks without sacrificing system performance.
This class of attack very commonly attempts to trick privileged
programs into performing unauthorized actions or giving unauthorized
access.
Background information on this type of attack is available on the web by
searching for 'Smashing the Stack for Fun and Profit.'
The majority of programs that run on HP-UX do not need to execute code
located on their stacks.
A few programs, notably some simulators, interpreters and older versions
of Java, may have a legitimate reason to execute code from their stacks.
These programs typically have self-modifying code.
Using a combination of this tunable and the
+es
option of the
chatr
command permits such executables to function without sacrificing protection
for the rest of the system.
Refer to the 'Restricting Execute Permission on Stacks' section of
the
chatr(1)
manpage for more information before changing this tunable.
Who is Expected to Change This Tunable?
Anyone.
Restrictions on Changing
Changes to this tunable take effect for new processes started after
the change.
When Should the Value of This Tunable Be Changed?
This tunable controls operational modes rather than data structure
sizes and limits.
The appropriate setting for a system depends on whether
you consider security or compatibility to be most important.
A value of
1
is compatible with previous releases of HP-UX, but it is the
least secure.
This setting permits the execution of potentially malicious
code located on a program's stack.
A value of
2
provides warnings about any program attempting to execute code
on its stacks, but does not alter the program's behavior.
Suspicious activity is logged in the kernel's message buffers.
(See
dmesg(1M).)
This is a 'trial mode' setting intended to allow you to safely
determine whether a tunable value of
0
would affect any legitimate application.
A tunable value of
0
is the recommended setting on systems where a higher level of security
is important.
This is essentially the same as a setting of
2,
but it will also terminate any process that attempts to execute code on
its stacks.
The process will be terminated before the potentially malicious code
is executed.
What Are the Side Effects of Changing the Value
This tunable has no effect on system behavior unless an application
attempts to execute instructions located on its stacks.
The majority of HP-UX applications are not programmed to do this.
What Other Tunable Values Should Be Changed at the Same Time?
None.
WARNINGS
All HP-UX kernel tunable parameters are release specific.
This parameter may be removed or have its meaning changed in
future releases of HP-UX.
Installation of optional kernel software, from HP or other vendors,
may cause changes to tunable parameter values. After installation,
some tunable parameters may no longer be at the default or
recommended values. For information about the effects of
installation on tunable values, consult the documentation
for the kernel software being installed.
For information about optional kernel software that was
factory installed on your system, see
HP-UX Release Notes
at
http://docs.hp.com.
AUTHOR
executable_stack
was developed by HP.
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