Question about fscrypt on ubifs compared with ext4

[Ivan Barr]

Hi there,

This is my first time writing to this mail list and I hope this is the
right place for my question.

I am working on an embedded Linux project that can run on multiple
platforms. One uses e.MMC for storage and another NAND flash. I want
to encrypt all the filesystems (mainly to protect against someone
unsoldering the flash chips and putting them in a reader). I want to
try and maintain a common approach across both hardware types as far
as possible. One big difference between the two is the wear levelling
is in the hardware for the e,MMC whereas for NAND I'll be using UBI.

For the root filesystem I am thinking of using squashfs which is
protected using dm-crypt. For the NAND device I have tried this out
and I can layer dm-crypt on top of ubiblock then use the device mapper
to load the squashfs. This maps nicely to the e.MMC world with the
only difference being that the device mapper is on a gpt partition
rather than a ubiblock device.

My challenge is for other read/ write filesystems. I want to mount an
overlay filesystem on top of the read-only root and a data partition.
I want both of these to also be encrypted. I have been investigating
how fscrypt can help me. (I believe dm-crypt won't work with ubifs).

For filesystems on the e.MMC I will be using ext4 and for NAND
ubifs.Documentation says both of these support fscrypt. I've struggled
a bit to find detailed documentation about how to use this with ubi
(there is a lot more for the ext4) but I think that there are some
differences between how this has been implemented on each and I'd like
those who know more to confirm this.

On the NAND side I have only been able to get it to work by using the
fscryptctl tool (https://github.com/google/fscryptctl
) as opposed to the fuller featured fscrypt tool
(https://github.com/google/fscrypt). This was following instructions I
found in a patch to add fscrypt support to mkfs.ubifs here:

https://patchwork.ozlabs.org/project/linux-mtd/cover/20181018143718.26298-1-richard@nod.at/

This appears to encrypt all the files on the partition using the
supplied key. When I look at fscrypt on ext4 it seems here that you
can't do this. The root directory cannot itself be encrypted, only
sub-directories. Reading here:
https://www.kernel.org/doc/html/v4.17/filesystems/fscrypt.html it
says:

"Note that the ext4 filesystem does not allow the root directory to be
encrypted, even if it is empty. Users who want to encrypt an entire
filesystem with one key should consider using dm-crypt instead."

So this is different. It also seems that with ubifs I can't apply
encryption to the subdirectories like I could in ext4. The README.md
here https://github.com/google/fscryptctl gives an example using ext4.
This encrypts a subdirectory called test. I don't see how to do the
same thing using ubifs. Could someone help me?

I've been using the NANDSIM kernel module for testing. At the end of
this post is a script for building an encrypted overlay ubifs
filesystem. As you can see the mkfs.ubifs is taking the key directory
and it appears to apply it to all the files on the partition. You
can't then apply policies to any sub-directories as they are already
encrypted.

I would like to use some of the other features that the userspace
fscrypt tool provides e.g. protectors (so I don't need to use the
master key directly). I can't however see any way to get this fscrypt
tool to setup encryption on a ubifs. The userspace fscrypt command
seems to want to create a .fscrypt directory in the root of the
partition to store information about policies and protectors. This
seems to fit more with the ext4 implementation.

When I try to set-up an unencrypted ubifs with "fscrypt setup" I run
into trouble as making a standard ubifs seems to create a v4 format
version rather than the required v5. This means the "fscrypt encrypt"
command fails. (Errors like this are seen in the dmesg output
[12022.576268] UBIFS error (ubi0:7 pid 6006): ubifs_enable_encryption
[ubifs]: on-flash format version 5 is needed for encryption).  Is
there some way to get mkfs.ubifs to create an unencrypted v5 formatted
filesystem?

Here is my script to create an encrypted ubifs using the fscryptctl tool:

#!/bin/bash

MTD_UTILS_ROOT=../../mtd-utils
FSCRYPTCTL=../../fscryptctl/fscryptctl
MOUNTPOINT=./mnt

dd if=/dev/urandom of=overlay.keyfile count=64 bs=1 # XTS needs a 512bit key
descriptor=`$FSCRYPTCTL get_descriptor < overlay.keyfile`
$MTD_UTILS_ROOT/mkfs.ubifs --cipher AES-256-XTS --key overlay.keyfile
-m 2048 -e 129024 -c 32 -r ./overlay -o overlay.enc.img

$MTD_UTILS_ROOT/ubiupdatevol /dev/ubi0_6 overlay.enc.img

# Try it out
$FSCRYPTCTL insert_key < overlay.keyfile
key=`keyctl show | grep $descriptor | awk '{print $1}'`
mount -t ubifs /dev/ubi0_6 $MOUNTPOINT
ls $MOUNTPOINT
umount $MOUNTPOINT
keyctl unlink $key

NB I've been working with mtd-utils v2.1.2 on a 5.4 kernel.