Veriexec

Veriexec is a file integrity subsystem.

For more information about it, and a quick guide on how to use it, please see veriexec(8).

In a nutshell, once enabled, Veriexec can be started as follows:

# veriexecgen && veriexecctl load

Exploit mitigation

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

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 below).

For non-PIE (Position Independent Executable) executables, the NetBSD PaX ASLR implementation introduces randomization to the following memory regions:

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.

Example usage:

# paxctl +A /usr/sbin/sshd

Enabling PaX ASLR globally:

# sysctl -w security.pax.aslr.global=1

PaX MPROTECT

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.

Example usage:

# paxctl +M /usr/sbin/sshd

Enabling PaX MPROTECT globally:

# sysctl -w security.pax.mprotect.global=1

PaX Segvguard

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 kernel memory.

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 like this:

# 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 setting:

# 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 control flow.

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:

USE_SSP=yes

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.

FORTIFY_SOURCE

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):

USE_FORT=yes

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 runtime:

# 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 rc.conf(5):

per_user_tmp=YES

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 “/tmp”.

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).

Information filtering

NetBSD provides administrators the ability to restrict information passed from the kernel to userland so that users can only view information they “own”.

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

Administrative security

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:

fetch_pkg_vulnerabilities=YES