classes: provide a class for generating dm-verity meta-data images

This adds a class that allows to generate conversions of ext[234] and
btrfs partitions images with dm-verity hash data appended at the end as
well as a corresponding .env file containing the root hash and data
offset that can be stored in a secure location (e.g. signed fitImage)
or signed and verified at run-time on its own.

The class depends on two variables:
  DM_VERITY_IMAGE:      defines the name of the main image (normally the
                        one that is used with the bitbake command to
                        build the main image)
  DM_VERITY_IMAGE_TYPE: defines exactly one type for which to generate
                        the protected image.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>

classes/dm-verity-img.bbclass

@@ -0,0 +1,88 @@
+# SPDX-License-Identifier: MIT
+#
+# Copyright (C) 2020 BayLibre SAS
+# Author: Bartosz Golaszewski <bgolaszewski@baylibre.com>
+#
+# This bbclass allows creating of dm-verity protected partition images. It
+# generates a device image file with dm-verity hash data appended at the end
+# plus the corresponding .env file containing additional information needed
+# to mount the image such as the root hash in the form of ell variables. To
+# assure data integrity, the root hash must be stored in a trusted location
+# or cryptographically signed and verified.
+#
+# Usage:
+#     DM_VERITY_IMAGE = "core-image-full-cmdline" # or other image
+#     DM_VERITY_IMAGE_TYPE = "ext4" # or ext2, ext3 & btrfs
+#     IMAGE_CLASSES += "dm-verity-img"
+#
+# The resulting image can then be used to implement the device mapper block
+# integrity checking on the target device.
+
+# Process the output from veritysetup and generate the corresponding .env
+# file. The output from veritysetup is not very machine-friendly so we need to
+# convert it to some better format. Let's drop the first line (doesn't contain
+# any useful info) and feed the rest to a script.
+process_verity() {
+    local ENV="$OUTPUT.env"
+
+    # Each line contains a key and a value string delimited by ':'. Read the
+    # two parts into separate variables and process them separately. For the
+    # key part: convert the names to upper case and replace spaces with
+    # underscores to create correct shell variable names. For the value part:
+    # just trim all white-spaces.
+    IFS=":"
+    while read KEY VAL; do
+        echo -ne "$KEY" | tr '[:lower:]' '[:upper:]' | sed 's/ /_/g' >> $ENV
+        echo -ne "=" >> $ENV
+        echo "$VAL" | tr -d " \t" >> $ENV
+    done
+
+    # Add partition size
+    echo "DATA_SIZE=$SIZE" >> $ENV
+
+    ln -sf $ENV ${IMAGE_BASENAME}-${MACHINE}.$TYPE.verity.env
+}
+
+verity_setup() {
+    local TYPE=$1
+    local INPUT=${IMAGE_NAME}${IMAGE_NAME_SUFFIX}.$TYPE
+    local SIZE=$(stat --printf="%s" $INPUT)
+    local OUTPUT=$INPUT.verity
+
+    cp -a $INPUT $OUTPUT
+
+    # Let's drop the first line of output (doesn't contain any useful info)
+    # and feed the rest to another function.
+    veritysetup --data-block-size=1024 --hash-offset=$SIZE format $OUTPUT $OUTPUT | tail -n +2 | process_verity
+}
+
+VERITY_TYPES = "ext2.verity ext3.verity ext4.verity btrfs.verity"
+IMAGE_TYPES += "${VERITY_TYPES}"
+CONVERSIONTYPES += "verity"
+CONVERSION_CMD_verity = "verity_setup ${type}"
+CONVERSION_DEPENDS_verity = "cryptsetup-native"
+
+python __anonymous() {
+    verity_image = d.getVar('DM_VERITY_IMAGE')
+    verity_type = d.getVar('DM_VERITY_IMAGE_TYPE')
+    image_fstypes = d.getVar('IMAGE_FSTYPES')
+    pn = d.getVar('PN')
+
+    if verity_image != pn:
+        return # This doesn't concern this image
+
+    if not verity_image or not verity_type:
+        bb.warn('dm-verity-img class inherited but not used')
+        return
+
+    if len(verity_type.split()) is not 1:
+        bb.fatal('DM_VERITY_IMAGE_TYPE must contain exactly one type')
+
+    d.appendVar('IMAGE_FSTYPES', ' %s.verity' % verity_type)
+
+    # If we're using wic: we'll have to use partition images and not the rootfs
+    # source plugin so add the appropriate dependency.
+    if 'wic' in image_fstypes:
+        dep = ' %s:do_image_%s' % (pn, verity_type)
+        d.appendVarFlag('do_image_wic', 'depends', dep)
+}