dm-crypt + dm-integrity + dm-raid = awesome!

cryptsetup-with-luks2-and-integrity-demo.sh

#!/usr/bin/env bash
# 
#    Author: Markus (MawKKe) ekkwam@gmail.com
#    Date: 2018-03-19
#
#
# What?
#
#    Linux dm-crypt + dm-integrity + dm-raid (RAID1) 
#
#    = Secure, redundant array with data integrity protection
#
# Why? 
#
#   You see, RAID1 is dead simple tool for disk redundancy, 
#   but it does NOT protect you from bit rot. There is no way
#   for RAID1 to distinguish which drive has the correct data if rot occurs.
#   This is a silent killer.
#
#   But with dm-integrity, you can now have error detection
#   at the block level. But it alone does not provide error correction
#   and is pretty useless with just one disk (disks fail, shit happens).
#
#   But if you use dm-integrity *below* RAID1, now you have disk redundancy,
#   AND error checking AND error correction. Invalid data received from
#   a drive will cause a checksum error which the RAID array notices and 
#   replaces with correct data.
#
#   If you throw encryption into the mix, you'll have secure,
#   redundant array. Oh, and the data integrity can be protected with 
#   authenticated encryption, so no-one can tamper your data maliciously. 
#   
#   How cool is that?
#
#   Also: If you use RAID1 arrays as LVM physical volumes, the overall 
#   architecture is quite similar to ZFS! All with native Linux tools, 
#   and no hacky solaris compatibility layers or licencing issues!
#
#   (I guess you can use whatever RAID level you want, but RAID1 is the 
#    simplest and fastest to set up)
# 
#
#   Let's try it out!
#
#   ---
#   NOTE: The dm-integrity target is available since Linux kernel version 4.12.
#   NOTE: This example requires LUKS2 which is only recently released (2018-03)
#   NOTE: The authenticated encryption is still experimental (2018-03)
#   ---

set -eux

# 1) Make dummy disks

cd /tmp

truncate -s 500M disk1.img
truncate -s 500M disk2.img

# Format the disk with luksFormat:

dd if=/dev/urandom of=key.bin bs=512 count=1

cryptsetup luksFormat -q --type luks2 --integrity hmac-sha256 disk1.img key.bin
cryptsetup luksFormat -q --type luks2 --integrity hmac-sha256 disk2.img key.bin

# The luksFormat's might take a while since the --integrity causes the disks to be wiped.
# dm-integrity is usually configured with 'integritysetup' (see below), but as 
# it happens, cryptsetup can do all the integrity configuration automatically if
# the --integrity flag is specified.

# Open/attach the encrypted disks

cryptsetup luksOpen disk1.img disk1luks --key-file key.bin
cryptsetup luksOpen disk2.img disk2luks --key-file key.bin


# Create raid1:

mdadm \
  --create \
  --verbose --level 1 \
  --metadata=1.2 \
  --raid-devices=2 \
  /dev/md/mdtest \
  /dev/mapper/disk1luks \
  /dev/mapper/disk2luks


# Create a filesystem, add to LVM volume group, etc...

mkfs.ext4 /dev/md/mdtest

# Cool! Now you can 'scrub' the raid setup, which verifies
# the contents of each drive. Ordinarily detecting an error would
# be problematic, but since we are now using dm-integrity, the raid1
# *knows* which one has the correct data, and is able to fix it automatically. 
#
# To scrub the array:
#
#    $ echo check > /sys/block/md127/md/sync_action
#
# ... wait a while
#
#    $ dmesg | tail -n 30
#
# You should see 
#
#    [957578.661711] md: data-check of RAID array md127
#    [957586.932826] md: md127: data-check done.
#
#
# Let's simulate disk corruption:
# 
#    $ dd if=/dev/urandom of=disk2.img seek=30000 count=30 bs=1k conv=notrunc
#
# (this writes 30kB of random data into disk2.img)
#
#
# Run scrub again:
#
#    $ echo check > /sys/block/md127/md/sync_action
#
# ... wait a while
#
#    $ dmesg | tail -n 30
#
# Now you should see 
#	...
#    [959146.618086] md: data-check of RAID array md127
#    [959146.962543] device-mapper: crypt: INTEGRITY AEAD ERROR, sector 39784
#    [959146.963086] device-mapper: crypt: INTEGRITY AEAD ERROR, sector 39840
#    [959154.932650] md: md127: data-check done.
#
# But now if you run scrub yet again:
#    ...
#    [959212.329473] md: data-check of RAID array md127
#    [959220.566150] md: md127: data-check done.
#
# And since we didn't get any errors a second time, we can deduce that the invalid 
# data was repaired automatically.
#
# Great! We are done.
#
# --------
#
# If you don't need encryption, then you can use 'integritysetup' instead of cryptsetup. 
# It works in similar fashion: 
#
#    $ integritysetup format --integrity sha256 disk1.img
#    $ integritysetup format --integrity sha256 disk2.img
#    $ integritysetup open --integrity sha256 disk1.img disk1int
#    $ integritysetup open --integrity sha256 disk2.img disk2int
#    $ mdadm --create ...
#
# ...and so on. Though now you can detect and repair disk errors but have no protection
# against malicious cold-storage attacks. Data is also readable by anybody.
#
#	2018-03 NOTE: 
#
#       if you override the default --integrity value (whatever it is) during formatting, 
#       then you must specify it again when opening, like in the example above. For some 
#       reason the algorithm is not autodetected. I guess there is no header written onto 
#       disk like is with LUKS ?
#
# ----------
#
#  Read more:
#   https://fosdem.org/2018/schedule/event/cryptsetup/
#   https://gitlab.com/cryptsetup/cryptsetup/wikis/DMCrypt
#   https://gitlab.com/cryptsetup/cryptsetup/wikis/DMIntegrity
#   https://mirrors.edge.kernel.org/pub/linux/utils/cryptsetup/v2.0/v2.0.0-rc0-ReleaseNotes

Given it's likely that both copies will be written at once, there's high chance that both copies will be mismatched with the checksums. So, no, it's definitely not safe in general.

This is a nice example to get ZFS-like error detection with regular filesystems like EXT4 or XFS, but what about alignment issues and write amplification? Do we have to correct mdadm or the filesystem chunk/stride calculation for alignment, or is it now taken care of by default?

The linked article https://securitypitfalls.wordpress.com/2020/09/27/making-raid-work-dm-integrity-with-md-raid/ suggest:

• avoiding read-modify-write by passing --chunk 2048 to mdadm
• having a /sys/block/md0/md/stripe_cache_size of 4096
• using stripe unit and stripe width when doing mkfs to reflect the hardware realities as the mdadm device wrongly looks like 4kn

Alignment issues are no worse than with any RAID: for a file system it looks like regular MD RAID.
There is journaling in integrity level so write amplification is on the horrible side. May be better if you have a device with configurable sector size (like 520 instead of 512 bytes).

If this is for the root drive, one potential advantage of doing LUKS over mdadm RAID1 instead of the reverse is that LUKS over a degraded RAID will boot normally since this patch, whereas a degraded RAID over LUKS array may fail the boot process and drop into an initramfs shell due to a missing encrypted root disk. That may require typing exit to continue, although this may not be a big deal to some.