Add nosuid Option to /var/tmp
Description
The nosuid
mount option can be used to prevent
execution of setuid programs in /var/tmp
. The SUID and SGID permissions
should not be required in these world-writable directories.
Add the nosuid
option to the fourth column of
/etc/fstab
for the line which controls mounting of
/var/tmp
.
Rationale
The presence of SUID and SGID executables should be tightly controlled. Users
should not be able to execute SUID or SGID binaries from temporary storage partitions.
Shell script
The following script can be run on the host to remediate the issue.
# Remediation is applicable only in certain platforms
if ( [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ] && findmnt --kernel "/var/tmp" > /dev/null || findmnt --fstab "/var/tmp" > /dev/null ); then
function perform\_remediation {
mount\_point\_match\_regexp="$(printf "[[:space:]]%s[[:space:]]" "/var/tmp")"
grep "$mount\_point\_match\_regexp" -q /etc/fstab \
|| { echo "The mount point '/var/tmp' is not even in /etc/fstab, so we can't set up mount options" >&2;
echo "Not remediating, because there is no record of /var/tmp in /etc/fstab" >&2; return 1; }
mount\_point\_match\_regexp="$(printf "[[:space:]]%s[[:space:]]" /var/tmp)"
# If the mount point is not in /etc/fstab, get previous mount options from /etc/mtab
if ! grep "$mount\_point\_match\_regexp" /etc/fstab; then
# runtime opts without some automatic kernel/userspace-added defaults
previous\_mount\_opts=$(grep "$mount\_point\_match\_regexp" /etc/mtab | head -1 | awk '{print $4}' \
| sed -E "s/(rw|defaults|seclabel|nosuid)(,|$)//g;s/,$//")
[ "$previous\_mount\_opts" ] && previous\_mount\_opts+=","
echo " /var/tmp defaults,${previous\_mount\_opts}nosuid 0 0" >> /etc/fstab
# If the mount\_opt option is not already in the mount point's /etc/fstab entry, add it
elif ! grep "$mount\_point\_match\_regexp" /etc/fstab | grep "nosuid"; then
previous\_mount\_opts=$(grep "$mount\_point\_match\_regexp" /etc/fstab | awk '{print $4}')
sed -i "s|\(${mount\_point\_match\_regexp}.\*${previous\_mount\_opts}\)|\1,nosuid|" /etc/fstab
fi
if mkdir -p "/var/tmp"; then
if mountpoint -q "/var/tmp"; then
mount -o remount --target "/var/tmp"
fi
fi
}
perform\_remediation
else
>&2 echo 'Remediation is not applicable, nothing was done'
fi
Ansible playbook
The following playbook can be run with Ansible to remediate the issue.
- name: 'Add nosuid Option to /var/tmp: Check information associated to mountpoint'
command: findmnt --fstab '/var/tmp'
register: device\_name
failed\_when: device\_name.rc > 1
changed\_when: false
when: ( ansible\_virtualization\_type not in ["docker", "lxc", "openvz", "podman",
"container"] and "/var/tmp" in ansible\_mounts | map(attribute="mount") | list
)
tags:
- configure\_strategy
- high\_disruption
- low\_complexity
- medium\_severity
- mount\_option\_var\_tmp\_nosuid
- no\_reboot\_needed
- name: 'Add nosuid Option to /var/tmp: Create mount\_info dictionary variable'
set\_fact:
mount\_info: '{{ mount\_info|default({})|combine({item.0: item.1}) }}'
with\_together:
- '{{ device\_name.stdout\_lines[0].split() | list | lower }}'
- '{{ device\_name.stdout\_lines[1].split() | list }}'
when:
- ( ansible\_virtualization\_type not in ["docker", "lxc", "openvz", "podman", "container"]
and "/var/tmp" in ansible\_mounts | map(attribute="mount") | list )
- device\_name.stdout is defined and device\_name.stdout\_lines is defined
- (device\_name.stdout | length > 0)
tags:
- configure\_strategy
- high\_disruption
- low\_complexity
- medium\_severity
- mount\_option\_var\_tmp\_nosuid
- no\_reboot\_needed
- name: 'Add nosuid Option to /var/tmp: If /var/tmp not mounted, craft mount\_info
manually'
set\_fact:
mount\_info: '{{ mount\_info|default({})|combine({item.0: item.1}) }}'
with\_together:
- - target
- source
- fstype
- options
- - /var/tmp
- ''
- ''
- defaults
when:
- ( ansible\_virtualization\_type not in ["docker", "lxc", "openvz", "podman", "container"]
and "/var/tmp" in ansible\_mounts | map(attribute="mount") | list )
- ("--fstab" | length == 0)
- (device\_name.stdout | length == 0)
tags:
- configure\_strategy
- high\_disruption
- low\_complexity
- medium\_severity
- mount\_option\_var\_tmp\_nosuid
- no\_reboot\_needed
- name: 'Add nosuid Option to /var/tmp: Make sure nosuid option is part of the to
/var/tmp options'
set\_fact:
mount\_info: '{{ mount\_info | combine( {''options'':''''~mount\_info.options~'',nosuid''
}) }}'
when:
- ( ansible\_virtualization\_type not in ["docker", "lxc", "openvz", "podman", "container"]
and "/var/tmp" in ansible\_mounts | map(attribute="mount") | list )
- mount\_info is defined and "nosuid" not in mount\_info.options
tags:
- configure\_strategy
- high\_disruption
- low\_complexity
- medium\_severity
- mount\_option\_var\_tmp\_nosuid
- no\_reboot\_needed
- name: 'Add nosuid Option to /var/tmp: Ensure /var/tmp is mounted with nosuid option'
mount:
path: /var/tmp
src: '{{ mount\_info.source }}'
opts: '{{ mount\_info.options }}'
state: mounted
fstype: '{{ mount\_info.fstype }}'
when:
- ( ansible\_virtualization\_type not in ["docker", "lxc", "openvz", "podman", "container"]
and "/var/tmp" in ansible\_mounts | map(attribute="mount") | list )
- (device\_name.stdout is defined and (device\_name.stdout | length > 0)) or ("--fstab"
| length == 0)
tags:
- configure\_strategy
- high\_disruption
- low\_complexity
- medium\_severity
- mount\_option\_var\_tmp\_nosuid
- no\_reboot\_needed