Aug 08, 2013

Encrypted root on a remote vds

Most advice wrt encryption on a remote hosts (VPS, VDS) don't seem to involve full-disk encryption as such, but is rather limited to encrypting /var and /home, so that machine will boot from non-crypted / and you'll be able to ssh to it, decrypt these parts manually, then start services that use data there.

That seem to be in contrast with what's generally used on local machines - make LUKS container right on top of physical disk device, except for /boot (if it's not on USB key) and don't let that encryption layer bother you anymore.

Two policies seem to differ in that former one is opt-in - you have to actively think which data to put onto encrypted part (e.g. /etc/ssl has private keys? move to /var, shred from /etc), while the latter is opt-out - everything is encrypted, period.

So, in spirit of that opt-out way, I thought it'd be a drag to go double-think wrt which data should be stored where and it'd be better to just go ahead and put everything possible to encrypted container for a remote host as well, leaving only /boot with kernel and initramfs in the clear.

Naturally, to enter encryption password and not have it stored alongside LUKS header, some remote login from the network is in order, and sshd seem to be the secure and easy way to go about it.
Initramfs in question should then also be able to setup network, which luckily dracut can. Openssh sshd is a bit too heavy for it though, but there are much lighter sshd's like dropbear.

Searching around for someone to tie the two things up, found a bit incomplete and non-packaged solutions like this RH enhancement proposal and a set of hacky scripts and instructions in dracut-crypt-wait repo on bitbucket.

Approach outlined in RH bugzilla is to make dracut "crypt" module to operate normally and let cryptsetup query for password in linux console, but also start sshd in the background, where one can login and use a simple tool to echo password to that console (without having it echoed).
dracut-crypt-wait does a clever hack of removing "crypt" module hook instead and basically creates "rescure" console on sshd, where user have to manually do all the decryption necessary and then signal initramfs to proceed with the boot.

I thought first way was rather more elegant and clever, allowing dracut to figure out which device to decrypt and start cryptsetup with all the necessary, configured and documented parameters, also still allowing to type passphrase into console - best of both worlds, so went along with that one, creating dracut-crypt-sshd project.

As README there explains, using it is as easy as adding it into dracut.conf (or passing to dracut on command line) and adding networking to grub.cfg, e.g.:

menuentry "My Linux" {
        linux /vmlinuz ro root=LABEL=root
                rd.luks.uuid=7a476ea0 rd.lvm.vg=lvmcrypt rd.neednet=1
                ip=88.195.61.177::88.195.61.161:255.255.255.224:myhost:enp0s9:off
        initrd /dracut.xz
}

("ip=dhcp" might be simplier way to go, but doesn't yield default route in my case)

And there, you'll have sshd on that IP port 2222 (configurable), with pre-generated (during dracut build) keys, which might be a good idea to put into "known_hosts" for that ip/port somewhere. "authorized_keys" is taken from /root/.ssh by default, but also configurable via dracut.conf, if necessary.

Apart from sshd, that module includes two tools for interaction with console - console_peek and console_auth (derived from auth.c in the bugzilla link above).

Logging in to that sshd then yields sequence like this:

[214] Aug 08 13:29:54 lastlog_perform_login: Couldn't stat /var/log/lastlog: No such file or directory
[214] Aug 08 13:29:54 lastlog_openseek: /var/log/lastlog is not a file or directory!

# console_peek
[    1.711778] Write protecting the kernel text: 4208k
[    1.711875] Write protecting the kernel read-only data: 1116k
[    1.735488] dracut: dracut-031
[    1.756132] systemd-udevd[137]: starting version 206
[    1.760022] tsc: Refined TSC clocksource calibration: 2199.749 MHz
[    1.760109] Switching to clocksource tsc
[    1.809905] systemd-udevd[145]: renamed network interface eth0 to enp0s9
[    1.974202] 8139too 0000:00:09.0 enp0s9: link up, 100Mbps, full-duplex, lpa 0x45E1
[    1.983151] dracut: sshd port: 2222
[    1.983254] dracut: sshd key fingerprint: 2048 0e:14:...:36:f9  root@congo (RSA)
[    1.983392] dracut: sshd key bubblebabble: 2048 xikak-...-poxix  root@congo (RSA)
[185] Aug 08 13:29:29 Failed reading '-', disabling DSS
[186] Aug 08 13:29:29 Running in background
[    2.093869] dracut: luksOpen /dev/sda3 luks-...
Enter passphrase for /dev/sda3:
[213] Aug 08 13:29:50 Child connection from 188.226.62.174:46309
[213] Aug 08 13:29:54 Pubkey auth succeeded for 'root' with key md5 0b:97:bb:...

# console_auth
Passphrase:

#
First command - "console_peek" - allows to see which password is requested (if any) and second one allows to login.
Note that fingerprints of host keys are also echoed to console on sshd start, in case one has access to console but still needs sshd later.
I quickly found out that such initramfs with sshd is also a great and robust rescue tool, especially if "debug" and/or "rescue" dracut modules are enabled.
And as it includes fairly comprehensive network-setup options, might be a good way to boot multiple different OS'es with same (machine-specific) network parameters,

Probably obligatory disclaimer for such post should mention that crypto above won't save you from malicious hoster or whatever three-letter-agency that will coerce it into cooperation, should it take interest in your poor machine - it'll just extract keys from RAM image (especially if it's a virtualized VPS) or backdoor kernel/initramfs and force a reboot.

Threat model here is more trivial - be able to turn off and decomission host without fear of disks/images then falling into some other party's hands, which might also happen if hoster eventually goes bust or sells/scraps disks due to age or bad blocks.

Also, even minor inconvenience like forcing to extract keys like outlined above might be helpful in case of quite well-known "we came fishing to a datacenter, shut everything down, give us all the hardware in these racks" tactic employed by some agencies.

Absolute security is a myth, but these measures are fairly trivial and practical to be employed casually to cut off at least some number of basic threats.

So, yay for dracut, the amazingly cool and hackable initramfs project, which made it that easy.

Code link: https://github.com/mk-fg/dracut-crypt-sshd

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