1.\" $OpenBSD: securelevel.7,v 1.31 2019/08/21 20:44:09 cheloha Exp $ 2.\" 3.\" Copyright (c) 2000 Hugh Graham 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED 15.\" WARRANTIES, INCLUDING, BUT NOT LIMITED TO, IMPLIED WARRANTIES OF 16.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 17.\" IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, 18.\" INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 19.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 20.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 22.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING 23.\" IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 24.\" POSSIBILITY OF SUCH DAMAGE. 25.\" 26.Dd $Mdocdate: August 21 2019 $ 27.Dt SECURELEVEL 7 28.Os 29.Sh NAME 30.Nm securelevel 31.Nd securelevel and its effects 32.Sh DESCRIPTION 33The 34.Ox 35kernel provides four levels of system security: 36.Bl -tag -width flag 37.It \&-1 Em Permanently insecure mode 38.Bl -hyphen -compact 39.It 40.Xr init 8 41will not attempt to raise the securelevel 42.It 43may only be set with 44.Xr sysctl 8 45while the system is insecure 46.It 47otherwise identical to securelevel 0 48.El 49.It \ 0 Em Insecure mode 50.Bl -hyphen -compact 51.It 52used during bootstrapping and while the system is single-user 53.It 54all devices may be read or written subject to their permissions 55.It 56system file flags may be cleared with 57.Xr chflags 2 58.El 59.It \ 1 Em Secure mode 60.Bl -hyphen -compact 61.It 62default mode when system is multi-user 63.It 64securelevel may no longer be lowered except by init 65.It 66.Pa /dev/mem 67and 68.Pa /dev/kmem 69cannot be opened 70.It 71raw disk devices of mounted file systems are read-only 72.It 73system immutable and append-only file flags may not be removed 74.It 75the 76.Va fs.posix.setuid , 77.Va hw.allowpowerdown , 78.Va kern.allowkmem , 79.Va kern.utc_offset , 80.Va net.inet.ip.sourceroute , 81and 82.Va machdep.kbdreset 83.Xr sysctl 8 84variables may not be changed 85.It 86the 87.Va ddb.console , 88.Va ddb.panic , 89and 90.Va machdep.allowaperture 91.Xr sysctl 8 92variables may not be raised 93.It 94.Xr gpioctl 8 95may only access GPIO pins configured at system startup 96.El 97.It \ 2 Em Highly secure mode 98.Bl -hyphen -compact 99.It 100all effects of securelevel 1 101.It 102raw disk devices are always read-only whether mounted or not 103.It 104.Xr settimeofday 2 105and 106.Xr clock_settime 2 107may not set the time backwards or close to overflow 108.It 109.Xr pf 4 110filter and NAT rules may not be altered 111.El 112.El 113.Pp 114Securelevel provides convenient means of 115.Dq locking down 116a system to a degree suited to its environment. 117It is normally set at boot by 118.Xr rc 8 , 119or the superuser may raise securelevel at any time by modifying the 120.Va kern.securelevel 121.Xr sysctl 8 122variable. 123However, only 124.Xr init 8 125may lower it once the system has entered secure mode. 126.Pp 127Highly secure mode may seem Draconian, but is intended as a last line of 128defence should the superuser account be compromised. 129Its effects preclude 130circumvention of file flags by direct modification of a raw disk device, 131or erasure of a file system by means of 132.Xr newfs 8 . 133Further, it can limit the potential damage of a compromised 134.Dq firewall 135by prohibiting the modification of packet filter rules. 136Preventing 137the system clock from being set backwards aids in post-mortem analysis 138and helps ensure the integrity of logs. 139Precision timekeeping is not 140affected because the clock may still be slowed. 141.Pp 142Because securelevel can be modified with the in-kernel debugger 143.Xr ddb 4 , 144a convenient means of locking it off (if present) is provided 145at securelevels 1 and 2. 146This is accomplished by setting 147.Va ddb.console 148and 149.Va ddb.panic 150to 0 with the 151.Xr sysctl 8 152utility. 153.Sh FILES 154.Bl -tag -width /etc/rc.securelevel -compact 155.It Pa /etc/rc.securelevel 156commands that run before the security level changes 157.El 158.Sh SEE ALSO 159.Xr init 8 , 160.Xr rc 8 , 161.Xr sysctl 8 162.Sh HISTORY 163The 164.Nm 165manual page first appeared in 166.Ox 2.6 . 167.Sh BUGS 168The list of securelevel's effects may not be comprehensive. 169