SECURITY(7) Miscellaneous Information Manual SECURITY(7)
security -- NetBSD security features
NetBSD supports a variety of security features. Below is a brief
description of them with some quick usage examples that will help you get
- Veriexec (file integrity)
- Exploit mitigation
- Per-user /tmp directory
- Information filtering
- Administrative security
Veriexec is a file integrity subsystem.
For more information about it, and a quick guide on how to use it, please
In a nutshell, once enabled, Veriexec can be started as follows:
# veriexecgen && veriexecctl load
NetBSD incorporates some exploit mitigation features. The purpose of
exploit mitigation features is to interfere with the way exploits work,
in order to prevent them from succeeding. Due to that, some features may
have other impacts on the system, so be sure to fully understand the
implications of each feature.
NetBSD provides the following exploit mitigation features:
- PaX ASLR (Address Space Layout Randomization).
- PaX MPROTECT (mprotect(2) restrictions)
- PaX SegvGuard
- gcc(1) stack-smashing protection (SSP)
- bounds checked libc functions (FORTIFY_SOURCE)
- Protections against NULL pointer dereferences
PaX ASLR implements Address Space Layout Randomization (ASLR), meant to
complement non-executable mappings. Its purpose is to harden prediction
of the address space layout, namely location of library and application
functions that can be used by an attacker to circumvent non-executable
mappings by using a technique called ``return to library'' to bypass the
need to write new code to (potentially executable) regions of memory.
When PaX ASLR is used, it is more likely the attacker will fail to
predict the addresses of such functions, causing the application to
segfault. To detect cases where an attacker might try and brute-force
the return address of respawning services, PaX Segvguard can be used (see
For non-PIE (Position Independent Executable) executables, the NetBSD PaX
ASLR implementation introduces randomization to the following memory
1. The data segment
2. The stack
For PIE executables:
1. The program itself (exec base)
2. All shared libraries
3. The data segment
4. The stack
While it can be enabled globally, NetBSD provides a tool, paxctl(8), to
enable PaX ASLR on a per-program basis.
# paxctl +A /usr/sbin/sshd
Enabling PaX ASLR globally:
# sysctl -w security.pax.aslr.global=1
PaX MPROTECT implements memory protection restrictions, meant to
complement non-executable mappings. The purpose is to prevent situations
where malicious code attempts to mark writable memory regions as
executable, often by trashing arguments to an mprotect(2) call.
While it can be enabled globally, NetBSD provides a tool, paxctl(8), to
enable PaX MPROTECT on a per-program basis.
# paxctl +M /usr/sbin/sshd
Enabling PaX MPROTECT globally:
# sysctl -w security.pax.mprotect.global=1
PaX Segvguard monitors the number of segmentation faults in a program on
a per-user basis, in an attempt to detect on-going exploitation attempts
and possibly prevent them. For instance, PaX Segvguard can help detect
when an attacker tries to brute-force a function return address, when
attempting to perform a return-to-lib attack.
PaX Segvguard consumes kernel memory, so use it wisely. While it
provides rate-limiting protections, records are tracked for all users on
a per-program basis, meaning that irresponsible use may result in
tracking all segmentation faults in the system, possibly consuming all
For this reason, it is highly recommended to have PaX Segvguard enabled
explicitly only for network services or other processes deemed as
critical to system security. Enabling PaX Segvguard explicitly works
# paxctl +G /usr/sbin/sshd
However, a global knob is still provided, for use in strict environments
with no local users (for example, some network appliances, embedded
devices, and firewalls)
# sysctl -w security.pax.segvguard.global=1
Explicitly disabling PaX Segvguard is also possible:
# paxctl +g /bin/ls
In addition, PaX Segvguard provides several tunable options. For
example, to limit a program to 5 segmentation faults from the same user
in a 60 second timeframe:
# sysctl -w security.pax.segvguard.max_crashes=5
# sysctl -w security.pax.segvguard.expiry_timeout=60
The number of seconds a user will be suspended from running the culprit
program is also configurable. For example, 10 minutes seem like a sane
# sysctl -w security.pax.segvguard.suspend_timeout=600
GCC Stack Smashing Protection (SSP)
As of NetBSD 4.0, gcc(1) includes SSP, a set of compiler extensions to
raise the bar on exploitation attempts by detecting corruption of
variables and buffer overruns, which may be used to affect program
Upon detection of a buffer overrun, SSP will immediately abort execution
of the program and send a log message to syslog(3).
The system (userland and kernel) can be built with SSP by using the
``USE_SSP'' flag in /etc/mk.conf:
You are encouraged to use SSP for software you build, by providing one of
the -fstack-protector or -fstack-protector-all flags to gcc(1). Keep in
mind, however, that SSP will not work for functions that make use of
alloca(3), as the latter modifies the stack size during run-time, while
SSP relies on it being a compile-time static.
Use of SSP is especially encouraged on platforms without per-page execute
bit granularity such as i386. As of NetBSD 6.0, SSP is used by default
on i386 and amd64 architectures.
The so-called FORTIFY_SOURCE is a relatively simple technique to detect a
subset of buffer overflows before these can do damage. It is integrated
to gcc(1) together with some common memory and string functions in the
standard C library of NetBSD.
The underlying idea builds on the observation that there are cases where
the compiler knows the size of a buffer. If a buffer overflow is
suspected in a function that does little or no bounds checking, either a
compile time warning can be issued or a safer substitute function can be
used at runtime. Refer to ssp(3) for additional details.
The FORTIFY_SOURCE is enabled by default in some parts of the NetBSD
source tree. It is also possible to explicitly enable it by defining the
following in mk.conf(5):
Protections against NULL pointer dereferences
A certain class of attacks rely on kernel bugs that dereference NULL
pointers. If user processes are allowed to map the virtual address 0
with mmap(2) or by other means, there is a risk that code or data can be
injected into the kernel address space.
In NetBSD it is possible to restrict whether user processes are allowed
to make mappings at the zero address. By default, address 0 mappings are
restricted on the i386 and amd64 architectures. It is however known that
some third-party programs may not function properly with the restriction.
Such mappings can be allowed either by using the USER_VA0_DISABLE_DEFAULT
kernel configuration option or by changing the following variable at
# sysctl -w vm.user_va0_disable=0
Note that if securelevel (see secmodel_securelevel(9)) is greater than
zero, it is not possible to change the sysctl(8) variable.
Per-user temporary storage
It is possible to configure per-user temporary storage to avoid potential
security issues (race conditions, etc.) in programs that do not make
secure usage of /tmp.
To enable per-user temporary storage, add the following line to
If /tmp is a mount point, you will also need to update its fstab(5) entry
to use ``/private/tmp'' (or whatever directory you want, if you override
the default using the ``per_user_tmp_dir'' rc.conf(5) keyword) instead of
Following that, run:
# /etc/rc.d/perusertmp start
The per-user temporary storage is implemented by using ``magic
symlinks''. These are further described in symlink(7).
NetBSD provides administrators the ability to restrict information passed
from the kernel to userland so that users can only view information they
The hooks that manage this restriction are located in various parts of
the system and affect programs such as ps(1), fstat(1), and netstat(1).
Information filtering is enabled as follows:
# sysctl -w security.curtain=1
Also certain administrative tasks are related to security. For instance,
the daily maintenance script includes some basic consistency checks; see
security.conf(5) for more details. In particular, it is possible to
configure NetBSD to automatically audit all third-party packages
installed via pkgsrc(7). To audit for any known vulnerabilities on daily
basis, set the following in /etc/daily.conf:
ssp(3), options(4), paxctl(8), sysctl(8), veriexec(8), kauth(9)
Joseph Kong, Designing BSD Rootkits: An Introduction to Kernel Hacking,
No Starch Press, 2007.
Enrico Perla and Massimiliano Oldani, A Guide to Kernel Exploitation:
Attacking the Core, Elsevier, 2010.
Erik Buchanan, Ryan Roemer, Hovav Shacham, and Stefan Savage, When Good
Instructions Go Bad: Generalizing Return-Oriented Programming to RISC,
ACM Press, 27-38, October 27-31, 2008, CCS '08: Proceedings of the 15th
ACM Conference on Computer and Communications Security,
Sebastian Krahmer, x86-64 Buffer Overflow Exploits and the Borrowed Code
Chunks Exploitation Technique, September 28, 2005,
Many of the security features were pioneered by Elad Efrat
NetBSD 6.1.5 March 30, 2011 NetBSD 6.1.5