securelevel and its effects
The OpenBSD kernel provides four levels of system security:
- -1 Permanently insecure mode
- 0 Insecure mode
- used during bootstrapping and while the system is single-user
- all devices may be read or written subject to their permissions
- system file flags may be cleared with chflags(2)
- 1 Secure mode
- default mode when system is multi-user
- securelevel may no longer be lowered except by init
- /dev/mem and /dev/kmem cannot be opened
- raw disk devices of mounted file systems are read-only
- system immutable and append-only file flags may not be removed
- the fs.posix.setuid, hw.allowpowerdown, kern.allowkmem, kern.utc_offset, net.inet.ip.sourceroute, and machdep.kbdreset sysctl(8) variables may not be changed
- the ddb.console, ddb.panic, and machdep.allowaperture sysctl(8) variables may not be raised
- gpioctl(8) may only access GPIO pins configured at system startup
- 2 Highly secure mode
Securelevel provides convenient means of “locking down” a system to a degree suited to its environment. It is normally set at boot by rc(8), or the superuser may raise securelevel at any time by modifying the kern.securelevel sysctl(8) variable. However, only init(8) may lower it once the system has entered secure mode.
Highly secure mode may seem Draconian, but is intended as a last line of defence should the superuser account be compromised. Its effects preclude circumvention of file flags by direct modification of a raw disk device, or erasure of a file system by means of newfs(8). Further, it can limit the potential damage of a compromised “firewall” by prohibiting the modification of packet filter rules. Preventing the system clock from being set backwards aids in post-mortem analysis and helps ensure the integrity of logs. Precision timekeeping is not affected because the clock may still be slowed.
Because securelevel can be modified with the in-kernel debugger ddb(4), a convenient means of locking it off (if present) is provided at securelevels 1 and 2. This is accomplished by setting ddb.console and ddb.panic to 0 with the sysctl(8) utility.
- commands that run before the security level changes
init(8), rc(8), sysctl(8)
securelevel manual page first appeared
in OpenBSD 2.6.
The list of securelevel's effects may not be comprehensive.