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1.1.2.1.1 Ensure /tmp is tmpfs or a separate partition

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Information

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

Note: If an entry for /tmp exists in /etc/fstab it will take precedence over entries in a systemd unit file. There is an exception to this when a system is diskless and connected to iSCSI, entries in /etc/fstab may not take precedence over a systemd unit file.

Making /tmp its own file system allows an administrator to set additional mount options such as 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 hard link to a system setuid program and wait for it to be updated. Once the program was updated, the hard link 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

Solution

First ensure that systemd is correctly configured to ensure that /tmp will be mounted at boot time.

# systemctl unmask tmp.mount

For specific configuration requirements of the /tmp mount for your environment, modify /etc/fstab

Example of using tmpfs with specific mount options:

tmpfs/tmptmpfs defaults,rw,nosuid,nodev,noexec,relatime,size=2G 0 0

Note: the size=2G is an example of setting a specific size for tmpfs

Example of using a volume or disk with specific mount options. The source location of the volume or disk will vary depending on your environment:

<device> /tmp <fstype> defaults,nodev,nosuid,noexec 0 0

Impact:

By design files saved to /tmp should have no expectation of surviving a reboot of the system. tmpfs is ram based and all files stored to tmpfs will be lost when the system is rebooted.

If files need to be persistent through a reboot, they should be saved to /var/tmp not /tmp

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

Running out of /tmp space is a problem regardless of what kind of filesystem lies under it, but in a configuration where /tmp is not a separate file system it will essentially have the whole disk available, as the default installation 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. Another alternative is to create a dedicated partition for /tmp from a separate volume or disk. One of the downsides of a disk-based dedicated partition is that it will be slower than tmpfs which is RAM-based.

See Also

https://workbench.cisecurity.org/benchmarks/23593

Item Details

Category: CONFIGURATION MANAGEMENT

References: 800-53|CM-6b., CSCv7|14.6

Plugin: Unix

Control ID: 9508d03c8dcca7875f9490973bdcff7ded75031c68847805ade8504e02c43edc