executable_stack.5 (2010 09)

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executable_stack(5) executable_stack(5)
(Tunable Kernel Parameters)
NAME
executable_stack - controls whether program stacks are executable by default
VALUES
Failsafe
1
Default
0
Allowed values
0-2
Recommended values
0-2
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 per-
formance. 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 combina-
tion 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 informa-
tion 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 set-
ting 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 per-
mits 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 exe-
cute 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.
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1