How to create a dm-crypt block device in /dev/mapper without wiping it?

Question

I can create a dm-crypt filesystem with:

root@smarcimx8mq4g:~# cat /data/caam/randomkey | keyctl padd logon logkey: @s
731358804

root@smarcimx8mq4g:~# dmsetup -v create encrypted --table "0 $(blockdev --getsz /dev/mmcblk1p3) crypt capi:tk(cbc(aes))-plain :36:logon:logkey: 0 /dev/mmcblk1p3 0 1 sector_size:512"

(I'm using the imx8's CAAM's implementation of tk(cbc(aes)).)

Unfortunately this wipes (or loses the ability to decode) the data in /dev/mmcblk1p3. I can use /dev/mapper/encrypted, but only once I've run mkfs.ext4 and mounted it.

I've also tried:

root@smarcimx8mq4g:~# dmsetup -v load encrypted --table "0 $(blockdev --getsz /dev/mmcblk1p3) crypt capi:tk(cbc(aes))-plain :36:logon:logkey: 0 /dev/mmcblk1p3 0 1 sector_size:512"
device-mapper: reload ioctl on encrypted  failed: No such device or address
Command failed.

(I expect that nothing other than "create" will create the /dev/mapper/encrypted node.)

How do I use dm-crypt to get access to the original data?

Answer 1

What I hadn't realised was that you MUST import the key from the black-blob (.bb) file after every reboot as the keys are session specific. The initial "dmsetup -v create" command should be used on all subsequent reboots, with a newly imported key.

A full, working log:

root@smarcimx8mq4g:~# caam-keygen create randomkey ecb -s 16

root@smarcimx8mq4g:~# cd /data/caam/

root@smarcimx8mq4g:/data/caam# ls -l
total 8
-rw-r--r-- 1 root root 36 Apr  4 13:32 randomkey
-rw-r--r-- 1 root root 96 Apr  4 13:32 randomkey.bb

root@smarcimx8mq4g:/data/caam# cat /data/caam/randomkey | keyctl padd logon logkey: @s
600708898

root@smarcimx8mq4g:/data/caam# keyctl list @s
2 keys in keyring:
 63045264: --alswrv     0 65534 keyring: _uid.0
600708898: --alsw-v     0     0 logon: logkey:

root@smarcimx8mq4g:/data/caam# dmsetup -v create encrypted --table "0 $(blockdev --getsz /dev/mmcblk1p3) crypt capi:tk(cbc(aes))-plain :36:logon:logkey: 0 /dev/mmcblk1p3 0 1 sector_size:512"
Name:              encrypted
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        0
Event number:      0
Major, minor:      252, 0
Number of targets: 1

root@smarcimx8mq4g:/data/caam# dmsetup table --showkey encrypted
0 53933055 crypt capi:tk(cbc(aes))-plain :36:logon:logkey: 0 179:99 0
root@smarcimx8mq4g:/data/caam# mkfs.ext4 /dev/mapper/encrypted
mke2fs 1.46.2 (28-Feb-2021)
Creating filesystem with 6741631 4k blocks and 1687552 inodes
Filesystem UUID: f28e329b-6f40-47e1-9ef3-893dc2646339
Superblock backups stored on blocks:
    32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
    4096000

Allocating group tables: done                            
Writing inode tables: done                            
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done   

root@smarcimx8mq4g:/data/caam# mkdir /mnt/encrypted

root@smarcimx8mq4g:/data/caam# mount -t ext4 /dev/mapper/encrypted /mnt/encrypted/

root@smarcimx8mq4g:/data/caam# echo "This is a test of full disk encryption on i.MX" > /mnt/encrypted/readme.txt

root@smarcimx8mq4g:/data/caam# umount /mnt/encrypted/

root@smarcimx8mq4g:/data/caam# dmsetup remove encrypted

root@smarcimx8mq4g:/data/caam# reboot
The system is going down for reboot NOW!g (pts/0) (Thu Apr  4 13:37:13 2024):
Connection to 10.1.2.3 closed by remote host.
Connection to 10.1.2.3 closed.

fadedbee@box ~ $ ssh [email protected]
...

root@smarcimx8mq4g:~# cd /data/caam/
root@smarcimx8mq4g:/data/caam# caam-keygen import /data/caam/randomkey.bb importKey

root@smarcimx8mq4g:/data/caam# sha1sum *

1873e20436126910ea83bcb2bb5229d7d94237ba  importKey
5c14092a3be806551df9e1c8bba4dae638bd82d1  randomkey
784a04eef33b21f12c1c9d9c0cdfd754febe34f7  randomkey.bb

root@smarcimx8mq4g:/data/caam# cat /data/caam/importKey | keyctl padd logon logkey: @s
941979697

root@smarcimx8mq4g:/data/caam# dmsetup -v create encrypted --table "0 $(blockdev --getsz /dev/mmcblk1p3) crypt capi:tk(cbc(aes))-plain :36:logon:logkey: 0 /dev/mmcblk1p3 0 1 sector_size:512"
Name:              encrypted
State:             ACTIVE
Read Ahead:        256
Tables present:    LIVE
Open count:        0
Event number:      0
Major, minor:      252, 0
Number of targets: 1

root@smarcimx8mq4g:/data/caam# mount /dev/mapper/encrypted /mnt/encrypted/

root@smarcimx8mq4g:/data/caam# cat /mnt/encrypted/readme.txt
This is a test of full disk encryption on i.MX