287d467c5d
Generally, access to the kernel debugger is considered to be unsafe from a security perspective since it presents an unrestricted interface to inspect or modify the system state, including sensitive data such as signing keys. However, having some access to debugger functionality on production systems may be useful in determining the cause of a panic or hang. Therefore, it is desirable to have an optional policy which allows limited use of ddb(4) while disabling the functionality which could reveal system secrets. This loadable MAC module allows for the use of some ddb(4) commands while preventing the execution of others. The commands have been broadly grouped into three categories: - Those which are 'safe' and will not emit sensitive data (e.g. trace). Generally, these commands are deterministic and don't accept arguments. - Those which are definitively unsafe (e.g. examine <addr>, search <addr> <value>) - Commands which may be safe to execute depending on the arguments provided (e.g. show thread <addr>). Safe commands have been flagged as such with the DB_CMD_MEMSAFE flag. Commands requiring extra validation can provide a function to do so. For example, 'show thread <addr>' can be used as long as addr can be checked against the system's list of process structures. The policy also prevents debugger backends other than ddb(4) from executing, for example gdb(4). Reviewed by: markj, pauamma_gundo.com (manpages) Sponsored by: Juniper Networks, Inc. Sponsored by: Klara, Inc. Differential Revision: https://reviews.freebsd.org/D35371
109 lines
3.4 KiB
Groff
109 lines
3.4 KiB
Groff
.\" Copyright (c) 2022 Klara Systems
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.\"
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.\" This software was developed by Mitchell Horne <mhorne@FreeBSD.org>
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.\" under sponsorship from Juniper Networks and Klara Systems.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.Dd June 29, 2022
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.Dt MAC_DDB 4
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.Os
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.Sh NAME
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.Nm mac_ddb
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.Nd "Restricted kernel debugger interface policy"
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.Sh SYNOPSIS
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To compile the ddb policy
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into your kernel, place the following lines in your kernel
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configuration file:
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.Bd -ragged -offset indent
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.Cd "options MAC"
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.Cd "options MAC_DDB"
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.Ed
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.Pp
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Alternately, to load the ddb module at boot time, place the following line
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in your kernel configuration file:
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.Bd -ragged -offset indent
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.Cd "options MAC"
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.Ed
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.Pp
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and in
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.Xr loader.conf 5 :
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.Bd -literal -offset indent
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mac_ddb_load="YES"
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.Ed
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.Sh DESCRIPTION
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The
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.Nm
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policy module implements a MAC policy which restricts the set of commands that
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can be used at the
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.Xr ddb 4
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command prompt.
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The subset of permitted commands is limited to those which do not read or write
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to arbitrary memory locations.
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This is done to deter the possible extraction of system secrets while still
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allowing enough debugger functionality to diagnose a kernel panic.
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For example, the
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.Ic trace
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or
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.Ic show registers
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commands are allowed by this policy, but
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.Ic show Cm buffer Ar addr
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is not.
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.Pp
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All debugger commands that are declared with the
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.Va DB_CMD_MEMSAFE
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flag are allowed by
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.Nm .
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The policy provides validation functions to conditionally allow some additional
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commands, based on the user provided arguments.
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.Pp
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When loaded, the
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.Nm
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policy also ensures that only the
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.Xr ddb 4
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debugger backend may be executed;
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.Xr gdb 4
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may not.
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.Ss Label Format
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No labels are defined for
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.Nm .
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.Sh SEE ALSO
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.Xr ddb 4 ,
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.Xr mac 4 ,
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.Xr mac_biba 4 ,
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.Xr mac_bsdextended 4 ,
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.Xr mac_ifoff 4 ,
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.Xr mac_lomac 4 ,
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.Xr mac_mls 4 ,
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.Xr mac_none 4 ,
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.Xr mac_partition 4 ,
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.Xr mac_portacl 4 ,
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.Xr mac_seeotheruids 4 ,
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.Xr mac_test 4 ,
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.Xr mac 9
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.Sh BUGS
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While the MAC Framework design is intended to support the containment of
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the root user, not all attack channels are currently protected by entry
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point checks.
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As such, MAC Framework policies should not be relied on, in isolation,
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to protect against a malicious privileged user.
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