Re: [PATCH 1/3] binman: Add support for externally encrypted blobs

Hello Simon,

thank you for the initial review. Replies are below.

Am 29.06.2023 um 21:09 schrieb Simon Glass:

Hi Christian,

On Tue, 27 Jun 2023 at 08:39, christian.taedcke-oss@weidmueller.com wrote:

...

From: Christian Taedcke christian.taedcke@weidmueller.com

This adds a new etype encrypted that is derived from collection.

It creates a new cipher node in the related image similar to the cipher node used by u-boot, see boot/image-cipher.c. Optionally it creates a global /cipher node containing a key and iv using the same nameing convention as used in boot/image-cipher.c.

Signed-off-by: Christian Taedcke christian.taedcke@weidmueller.com

tools/binman/etype/encrypted.py | 98 +++++++++++++++++++++++++++++++++ 1 file changed, 98 insertions(+) create mode 100644 tools/binman/etype/encrypted.py

diff --git a/tools/binman/etype/encrypted.py b/tools/binman/etype/encrypted.py new file mode 100644 index 0000000000..005ae56acf --- /dev/null +++ b/tools/binman/etype/encrypted.py @@ -0,0 +1,98 @@ +# SPDX-License-Identifier: GPL-2.0+ +# Copyright 2023 Weidmüller Interface GmbH & Co. KG +# Written by Christian Taedcke christian.taedcke@weidmueller.com +# +# Entry-type module for cipher information of encrypted blobs/images +#

+from binman.etype.collection import Entry_collection +from dtoc import fdt_util +from u_boot_pylib import tools

+# This is imported if needed +state = None

+class Entry_encrypted(Entry_collection):

• """Externally built encrypted binary blob
• If the file providing this blob is missing, binman can optionally ignore it
• and produce a broken image with a warning.
• In addition to the inherited 'collection' for Properties / Entry arguments:

•    - algo: The encryption algorithm
  

What possible values are available? Please list them

Currently the evaluation of the generated cipher nodes is not implemented in c code in upstream U-Boot. I use aes256-gcm and decrypt the relevant blobs/images in board-specific code. We plan to also upstream the c code for decryption later.

I expect we will support at least aes[128/192/256]-cbc in the future, since these are already implemented in software in U-Boot and in addition aes256-gcm via a crypto driver.

Since decryption is not implemented yet, i didn't want to restrict the possible algos for now, since board-specific code could implement decryption for any algorithm here that uses a key and iv (initialization vector).

Should i list aes[128/192/256]-cbc and aes256-gcm here or should i state that the supported algorithms (for now) are board specific?

...

•    - iv-name-hint: The name hint for the iv
  

what is the iv?

Initialization Vector. Should i use the full name here?

...

•    - key-name-hint: The name hint for the key
  

•    - iv-filename: The name of the file containing the iv
  

•    - key-filename: The name of the file containing the key
  

• This entry creates a cipher node in the entries' parent node (i.e. the

entry's

...

• image). Optionally it also creates a cipher node in the root of the device
• tree containg key and iv information.

containing

Overall I think this documentation needs to be expanded.

Ok. I tried to explain the motivation in the cover letter, see https://lists.denx.de/pipermail/u-boot/2023-June/521160.html

Is the cover letter the wrong place for this (should i move the motivation into the first commit message)?

I will also try to improve the code documentation here.

I wonder why this needs to be an entry type? Could the node be added to the description by the user, instead of the entry adding it? It feels a little strange to me, but perhaps I just need more info.

This new entry type basically reads the files containing the initialization vector and the key and puts it into the device tree. The initialization vector normally changes whenever the encrypted blob changes.

Having this as an entry type simplifies the build process of the resulting image. Otherwise some external script would have to run during the build process to patch the iv and key into the generated device tree.

...

• """
• def __init__(self, section, etype, node):

•    # Put this here to allow entry-docs and help to work without libfdt
  

•    global state
  

•    from binman import state
  

•    super().__init__(section, etype, node)
  

•    # The property key-filename is not required, because some implementations use keys that
  

•    # are not embedded in the device tree, but e.g. in the device itself
  

•    self.required_props = ['algo', 'key-name-hint', 'iv-filename']
  

•    self._algo = fdt_util.GetString(self._node, 'algo')
  

•    self._iv_name_hint = fdt_util.GetString(self._node, 'iv-name-hint')
  

•    self._key_name_hint = fdt_util.GetString(self._node, 'key-name-hint')
  

•    self._filename_iv = fdt_util.GetString(self._node, 'iv-filename')
  

•    self._filename_key = fdt_util.GetString(self._node, 'key-filename')
  

Here you should set these variables to None. Move the reading to ReadNode()

Sorry if there are counter-examples in the source code. But this is the correct way.

...

• def ReadNode(self):

•    super().ReadNode()
  

•    iv_filename = tools.get_input_filename(self._filename_iv)
  

•    self._iv = tools.read_file(iv_filename, binary=True)
  

Please only read the node in this method. Move file reading until where it is needed.

...

•    self._key = None
  

•    if self._filename_key:
  

•        key_filename = tools.get_input_filename(self._filename_key)
  

•        self._key = tools.read_file(key_filename, binary=True)
  

• def gen_entries(self):

•    super().gen_entries()
  

•    cipher_node = state.AddSubnode(self._node.parent, "cipher")
  

•    cipher_node.AddString("algo", self._algo)
  

•    cipher_node.AddString("key-name-hint", self._key_name_hint)
  

•    if self._iv_name_hint:
  

•        cipher_node.AddString("iv-name-hint", self._iv_name_hint)
  

•    else:
  

•        cipher_node.AddData("iv", self._iv)
  

•    if self._key or self._iv_name_hint:
  

•        # add cipher node in root
  

•        root_node = self._node.parent.parent.parent
  

The root node is self.GetImage()._node

But why are you adding something to the root node? This seems quite strange.

This is shown in the example in the cover letter. The generated device tree looks like this:

\ { cipher { key-aes256-gcm-keyname { key = <0x...>; iv = <0x...>; }; };

images { ... some-bitstream { ... data = [...] cipher { algo = "aes256-gcm"; key-name-hint = "keyname"; }; }; ...

The cipher node right below the root node (may) contain the actually used key and iv. The cipher node below the images node just points to the node inside the global cipher node. For this the values of the algo, key-name-hint and optionally the iv-name-hint are used. The actual key is found in /cipher/key-<algo>-<key-name-hint> or /cipher/key-<algo>-<key-name-hint>-<iv-name-hint>. This is implemented this way, because it is also used in boot/image-cipher.c.

Side note: If the hardware/board already contains a key in some secure storage, it is not necessary to put the key into the device tree. In that case the property key-name-hint can be used to identify which key should be used for decryption.

...

•        name = "cipher"
  

•        cipher_node = root_node.FindNode(name)
  

•        if not cipher_node:
  

•            cipher_node = state.AddSubnode(root_node, name)
  

•        key_node_name = (
  

•            f"key-{self._algo}-{self._key_name_hint}-{self._iv_name_hint}"
  

•            if self._iv_name_hint
  

•            else f"key-{self._algo}-{self._key_name_hint}"
  

•        )
  

•        key_node = cipher_node.FindNode(key_node_name)
  

•        if not key_node:
  

This behaviour is not clearly documented above.

Should i document this in the class doc of this entry or in the method doc of gen_entries()?

...

•            key_node = state.AddSubnode(cipher_node, key_node_name)
  

•            if self._key:
  

•                key_node.AddData("key", self._key)
  

•            if self._iv_name_hint:
  

•                key_node.AddData("iv", self._iv)
  

• def ObtainContents(self):

•    # ensure that linked content is not added to the device tree again from this entry
  

•    self.SetContents(b'')
  

•    return True
  

• def ProcessContents(self):

•    # ensure that linked content is not added to the device tree again from this entry
  

•    return self.ProcessContentsUpdate(b'')
  

-- 2.34.1

Regards, Simon

Regards, Christian