1.\" $OpenBSD: securelevel.7,v 1.27 2016/09/18 15:05:02 jmc 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 18 2016 $ 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 69may not be written to 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.Xr sysctl 8 78variable may not be changed 79.It 80the 81.Va hw.allowpowerdown 82.Xr sysctl 8 83variable may not be changed 84.It 85the 86.Va net.inet.ip.sourceroute 87.Xr sysctl 8 88variable may not be changed 89.It 90the 91.Va machdep.kbdreset 92.Xr sysctl 8 93variable may not be changed 94.It 95the 96.Va ddb.console 97and 98.Va ddb.panic 99.Xr sysctl 8 100variables may not be raised 101.It 102the 103.Va machdep.allowaperture 104.Xr sysctl 8 105variable may not be raised 106.It 107.Xr gpioctl 8 108may only access GPIO pins configured at system startup 109.El 110.It \ 2 Em Highly secure mode 111.Bl -hyphen -compact 112.It 113all effects of securelevel 1 114.It 115raw disk devices are always read-only whether mounted or not 116.It 117.Xr settimeofday 2 118and 119.Xr clock_settime 2 120may not set the time backwards or close to overflow 121.It 122.Xr pf 4 123filter and NAT rules may not be altered 124.El 125.El 126.Pp 127Securelevel provides convenient means of 128.Dq locking down 129a system to a degree suited to its environment. 130It is normally set at boot by 131.Xr rc 8 , 132or the superuser may raise securelevel at any time by modifying the 133.Va kern.securelevel 134.Xr sysctl 8 135variable. 136However, only 137.Xr init 8 138may lower it once the system has entered secure mode. 139.Pp 140Highly secure mode may seem Draconian, but is intended as a last line of 141defence should the superuser account be compromised. 142Its effects preclude 143circumvention of file flags by direct modification of a raw disk device, 144or erasure of a file system by means of 145.Xr newfs 8 . 146Further, it can limit the potential damage of a compromised 147.Dq firewall 148by prohibiting the modification of packet filter rules. 149Preventing 150the system clock from being set backwards aids in post-mortem analysis 151and helps ensure the integrity of logs. 152Precision timekeeping is not 153affected because the clock may still be slowed. 154.Pp 155Because securelevel can be modified with the in-kernel debugger 156.Xr ddb 4 , 157a convenient means of locking it off (if present) is provided 158at securelevels 1 and 2. 159This is accomplished by setting 160.Va ddb.console 161and 162.Va ddb.panic 163to 0 with the 164.Xr sysctl 8 165utility. 166.Sh FILES 167.Bl -tag -width /etc/rc.securelevel -compact 168.It Pa /etc/rc.securelevel 169commands that run before the security level changes 170.El 171.Sh SEE ALSO 172.Xr init 8 , 173.Xr rc 8 , 174.Xr sysctl 8 175.Sh HISTORY 176The 177.Nm 178manual page first appeared in 179.Ox 2.6 . 180.Sh BUGS 181The list of securelevel's effects may not be comprehensive. 182