1.\" $OpenBSD: securelevel.7,v 1.30 2017/09/12 02:22:15 schwarze 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: September 12 2017 $ 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 net.inet.ip.sourceroute , 80and 81.Va machdep.kbdreset 82.Xr sysctl 8 83variables may not be changed 84.It 85the 86.Va ddb.console , 87.Va ddb.panic , 88and 89.Va machdep.allowaperture 90.Xr sysctl 8 91variables may not be raised 92.It 93.Xr gpioctl 8 94may only access GPIO pins configured at system startup 95.El 96.It \ 2 Em Highly secure mode 97.Bl -hyphen -compact 98.It 99all effects of securelevel 1 100.It 101raw disk devices are always read-only whether mounted or not 102.It 103.Xr settimeofday 2 104and 105.Xr clock_settime 2 106may not set the time backwards or close to overflow 107.It 108.Xr pf 4 109filter and NAT rules may not be altered 110.El 111.El 112.Pp 113Securelevel provides convenient means of 114.Dq locking down 115a system to a degree suited to its environment. 116It is normally set at boot by 117.Xr rc 8 , 118or the superuser may raise securelevel at any time by modifying the 119.Va kern.securelevel 120.Xr sysctl 8 121variable. 122However, only 123.Xr init 8 124may lower it once the system has entered secure mode. 125.Pp 126Highly secure mode may seem Draconian, but is intended as a last line of 127defence should the superuser account be compromised. 128Its effects preclude 129circumvention of file flags by direct modification of a raw disk device, 130or erasure of a file system by means of 131.Xr newfs 8 . 132Further, it can limit the potential damage of a compromised 133.Dq firewall 134by prohibiting the modification of packet filter rules. 135Preventing 136the system clock from being set backwards aids in post-mortem analysis 137and helps ensure the integrity of logs. 138Precision timekeeping is not 139affected because the clock may still be slowed. 140.Pp 141Because securelevel can be modified with the in-kernel debugger 142.Xr ddb 4 , 143a convenient means of locking it off (if present) is provided 144at securelevels 1 and 2. 145This is accomplished by setting 146.Va ddb.console 147and 148.Va ddb.panic 149to 0 with the 150.Xr sysctl 8 151utility. 152.Sh FILES 153.Bl -tag -width /etc/rc.securelevel -compact 154.It Pa /etc/rc.securelevel 155commands that run before the security level changes 156.El 157.Sh SEE ALSO 158.Xr init 8 , 159.Xr rc 8 , 160.Xr sysctl 8 161.Sh HISTORY 162The 163.Nm 164manual page first appeared in 165.Ox 2.6 . 166.Sh BUGS 167The list of securelevel's effects may not be comprehensive. 168