Sam Stelfox

Thoughts from a software engineer, systems hacker and Linux gubernāre.

Swap

Security

If at any point a decryption key is needed to access a file, it will be stored in RAM. If an otherwise encrypted file is edited it will get stored in RAM before being saved. Without warning at any time that RAM can be swapped out to disk, at which point there is now a recoverable copy of an encryption key or sections of an encrypted document plain text on disk that can be recovered.

To prevent this I strongly recommend encrypting the swap. During a standard install of Fedora if you want to accomplish this you have to use dirty VolGroups and LVMs. I hate VolGroups and LVMs and on top of that it will be using a set static key that I'm sure will eventually be open to static analysis. While most people won't care about the latter I like to avoid the whole thing if I can do it with a fast simple solution.

My solution is to take swap out of the hands of fstab and mount it myself after generating a one-time key for that session. When the computer turns off the key will be completely lost and a new one will be generated the next time the computer boots. I don't want to do this by hand every time the computer boots so I'll make sure it does this automatically.

The first step is to prevent Linux from mounting the swap on it's own. To do this comment out all the lines in fstab that have to do with swap. They will look like the following:

/dev/sda3       swap                swap        defaults        0 0

Your going to need to know what partitions your swap lives on. In the above example swap lives on /dev/sda3, it is very likely that your's will live on a different device/partition.

First we're going to want to fill the swap space with junk to make sure that there isn't anything useful left on there to be recovered. Your going to want to make sure that your system isn't currently using the swap, so first turn it off before writing over the swap space. The latter command is very dangerous if you do it on the wrong partition! Double check before executing it!

[[email protected] ~]# swapoff -a
[[email protected] ~]# dd if=/dev/urandom of=/dev/sda3

Next we'll want to make one of the boot scripts initialize encryption and mount the swap, there is a specific boot script created explicitly for user modification that other packages won't touch. We're going to use that one which is /etc/rc.d/rc.local. You'll want to add the following lines to it.

cryptsetup -c blowfish -h sha256 -d /dev/urandom create swap /dev/sda3
mkswap -f /dev/mapper/swap
swapon /dev/mapper/swap

The cipher in the above command can be swapped around, blowfish is a strong encryption with a fairly good throughput however it seems that 256bit aes-xts-plain is a bit faster without sacrificing too much security. If you'd prefer to use that instead, replace the cryptsetup invocation with the following one:

cryptsetup -c aes-xts-plain -s 256 -h sha256 -d /dev/urandom create swap /dev/sda3

Additional ciphers that can be used including speed tests can be found on the Encryption page.

Fedora has device mapper support built into the kernel, however if your distribution doesn't have device mapper support active by default, in the line right before the cryptsetup invocation add the line "modprobe dm-mod" to load the kernel module. Otherwise you'll get a "Command failed: Incompatible libdevmapper" error.