nixpkgs/nixos/modules/security/wrappers/default.nix
Jörg Thalheim eadffd9154
nixos/wrappers: fix applying capabilities
With libcap 2.41 the output of cap_to_text changed, also the original
author of code hoped that this would never happen.
To counter this now the security-wrapper only relies on the syscall
ABI, which is more stable and robust than string parsing. If new
breakages occur this will be more obvious because version numbers will
be incremented.
Furthermore all errors no make execution explicitly fail instead of
hiding errors behind debug environment variables and the code style was
more consistent with no goto fail; goto fail; vulnerabilities (https://gotofail.com/)
2021-01-14 08:46:57 +01:00

207 lines
6.6 KiB
Nix

{ config, lib, pkgs, ... }:
let
inherit (config.security) wrapperDir wrappers;
parentWrapperDir = dirOf wrapperDir;
programs =
(lib.mapAttrsToList
(n: v: (if v ? program then v else v // {program=n;}))
wrappers);
securityWrapper = pkgs.callPackage ./wrapper.nix {
inherit parentWrapperDir;
};
###### Activation script for the setcap wrappers
mkSetcapProgram =
{ program
, capabilities
, source
, owner ? "nobody"
, group ? "nogroup"
, permissions ? "u+rx,g+x,o+x"
, ...
}:
assert (lib.versionAtLeast (lib.getVersion config.boot.kernelPackages.kernel) "4.3");
''
cp ${securityWrapper}/bin/security-wrapper $wrapperDir/${program}
echo -n "${source}" > $wrapperDir/${program}.real
# Prevent races
chmod 0000 $wrapperDir/${program}
chown ${owner}.${group} $wrapperDir/${program}
# Set desired capabilities on the file plus cap_setpcap so
# the wrapper program can elevate the capabilities set on
# its file into the Ambient set.
${pkgs.libcap.out}/bin/setcap "cap_setpcap,${capabilities}" $wrapperDir/${program}
# Set the executable bit
chmod ${permissions} $wrapperDir/${program}
'';
###### Activation script for the setuid wrappers
mkSetuidProgram =
{ program
, source
, owner ? "nobody"
, group ? "nogroup"
, setuid ? false
, setgid ? false
, permissions ? "u+rx,g+x,o+x"
, ...
}:
''
cp ${securityWrapper}/bin/security-wrapper $wrapperDir/${program}
echo -n "${source}" > $wrapperDir/${program}.real
# Prevent races
chmod 0000 $wrapperDir/${program}
chown ${owner}.${group} $wrapperDir/${program}
chmod "u${if setuid then "+" else "-"}s,g${if setgid then "+" else "-"}s,${permissions}" $wrapperDir/${program}
'';
mkWrappedPrograms =
builtins.map
(s: if (s ? capabilities)
then mkSetcapProgram
({ owner = "root";
group = "root";
} // s)
else if
(s ? setuid && s.setuid) ||
(s ? setgid && s.setgid) ||
(s ? permissions)
then mkSetuidProgram s
else mkSetuidProgram
({ owner = "root";
group = "root";
setuid = true;
setgid = false;
permissions = "u+rx,g+x,o+x";
} // s)
) programs;
in
{
imports = [
(lib.mkRemovedOptionModule [ "security" "setuidOwners" ] "Use security.wrappers instead")
(lib.mkRemovedOptionModule [ "security" "setuidPrograms" ] "Use security.wrappers instead")
];
###### interface
options = {
security.wrappers = lib.mkOption {
type = lib.types.attrs;
default = {};
example = lib.literalExample
''
{ sendmail.source = "/nix/store/.../bin/sendmail";
ping = {
source = "${pkgs.iputils.out}/bin/ping";
owner = "nobody";
group = "nogroup";
capabilities = "cap_net_raw+ep";
};
}
'';
description = ''
This option allows the ownership and permissions on the setuid
wrappers for specific programs to be overridden from the
default (setuid root, but not setgid root).
<note>
<para>The sub-attribute <literal>source</literal> is mandatory,
it must be the absolute path to the program to be wrapped.
</para>
<para>The sub-attribute <literal>program</literal> is optional and
can give the wrapper program a new name. The default name is the same
as the attribute name itself.</para>
<para>Additionally, this option can set capabilities on a
wrapper program that propagates those capabilities down to the
wrapped, real program.</para>
<para>NOTE: cap_setpcap, which is required for the wrapper
program to be able to raise caps into the Ambient set is NOT
raised to the Ambient set so that the real program cannot
modify its own capabilities!! This may be too restrictive for
cases in which the real program needs cap_setpcap but it at
least leans on the side security paranoid vs. too
relaxed.</para>
</note>
'';
};
security.wrapperDir = lib.mkOption {
type = lib.types.path;
default = "/run/wrappers/bin";
internal = true;
description = ''
This option defines the path to the wrapper programs. It
should not be overriden.
'';
};
};
###### implementation
config = {
security.wrappers = {
# These are mount related wrappers that require the +s permission.
fusermount.source = "${pkgs.fuse}/bin/fusermount";
fusermount3.source = "${pkgs.fuse3}/bin/fusermount3";
mount.source = "${lib.getBin pkgs.util-linux}/bin/mount";
umount.source = "${lib.getBin pkgs.util-linux}/bin/umount";
};
boot.specialFileSystems.${parentWrapperDir} = {
fsType = "tmpfs";
options = [ "nodev" "mode=755" ];
};
# Make sure our wrapperDir exports to the PATH env variable when
# initializing the shell
environment.extraInit = ''
# Wrappers override other bin directories.
export PATH="${wrapperDir}:$PATH"
'';
###### setcap activation script
system.activationScripts.wrappers =
lib.stringAfter [ "specialfs" "users" ]
''
# Look in the system path and in the default profile for
# programs to be wrapped.
WRAPPER_PATH=${config.system.path}/bin:${config.system.path}/sbin
chmod 755 "${parentWrapperDir}"
# We want to place the tmpdirs for the wrappers to the parent dir.
wrapperDir=$(mktemp --directory --tmpdir="${parentWrapperDir}" wrappers.XXXXXXXXXX)
chmod a+rx $wrapperDir
${lib.concatStringsSep "\n" mkWrappedPrograms}
if [ -L ${wrapperDir} ]; then
# Atomically replace the symlink
# See https://axialcorps.com/2013/07/03/atomically-replacing-files-and-directories/
old=$(readlink -f ${wrapperDir})
if [ -e ${wrapperDir}-tmp ]; then
rm --force --recursive ${wrapperDir}-tmp
fi
ln --symbolic --force --no-dereference $wrapperDir ${wrapperDir}-tmp
mv --no-target-directory ${wrapperDir}-tmp ${wrapperDir}
rm --force --recursive $old
else
# For initial setup
ln --symbolic $wrapperDir ${wrapperDir}
fi
'';
};
}