Cis GCP Cos 1.1.2

1.1.2 Ensure /tmp is configured (Automated)

Description

The /tmp directory is a world-writable directory used for temporary storage by all users and some applications.

Rationale

Making /tmp its own file system allows an administrator to set the noexec option on the mount, making /tmp useless for an attacker to install executable code. It would also prevent an attacker from establishing a hardlink to a system setuid program and wait for it to be updated. Once the program was updated, the hardlink would be broken and the attacker would have his own copy of the program. If the program happened to have a security vulnerability, the attacker could continue to exploit the known flaw.

This can be accomplished by either mounting tmpfs to /tmp, or creating a separate partition for /tmp.

Impact

Since the /tmp directory is intended to be world-writable, there is a risk of resource exhaustion if it is not bound to a separate partition.

Running out of /tmp space is a problem regardless of what kind of filesystem lies under it, but in a default installation a disk-based /tmp will essentially have the whole disk available, as it only creates a single / partition. On the other hand, a RAM-based /tmp as with tmpfs will almost certainly be much smaller, which can lead to applications filling up the filesystem much more easily.

/tmp utilizing tmpfs can be resized using the size={size} parameter on the Options line on the tmp.mount file.

Audit Procedure

Run the following command and verify output shows /tmp is mounted:

# mount | grep -E '\s/tmp\s'
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noexec,relatime)

Run the following command and verify that output shows static:

# systemctl is-enabled tmp.mount
static

Expected Result

• mount | grep -E '\s/tmp\s' should show tmpfs mounted on /tmp with appropriate options (rw,nosuid,nodev,noexec,relatime)
• systemctl is-enabled tmp.mount should return static

Remediation

Run the following commands to configure mount options:

systemctl edit tmp.mount

Edit the file to define the options that needs to be set. For example:

[Mount]
Options=
Options=mode=1777,strictatime,noexec,nodev,nosuid

Restart the tmp.mount:

systemctl daemon-reload # might be optional
systemctl restart tmp.mount

Note: /etc is stateless on Container-Optimized OS. Therefore, /etc cannot be used to make these changes persistent across reboots. The steps mentioned above needs to be performed after every boot.

References

1. AJ Lewis, "LVM HOWTO", http://tldp.org/HOWTO/LVM-HOWTO/
2. https://www.freedesktop.org/wiki/Software/systemd/APIFileSystems/

CIS Controls

Profile

Level 1 - Server | Automated