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

[Simon Glass]

Hi Christian,

On Fri, 30 Jun 2023 at 09:20, Taedcke, Christian
<christian.taedcke-oss at weidmueller.com> wrote:
>
> 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 at weidmueller.com> wrote:
> >>
> >> From: Christian Taedcke <christian.taedcke at 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 at 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 at 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?

Perhaps the correct answer is to say that nothing is supported yet,
but future patches will add certain algorithms TBD?

>
> >
> >> +        - iv-name-hint: The name hint for the iv
> >
> > what is the iv?
>
> Initialization Vector. Should i use the full name here?

Yes, plus explain what it is or where it is documented.

>
> >
> >> +        - 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 mean in the docs for the entry itself (which ends up in
entries.rst), since this is what people read.

Yes it is good to comment the code as well, but it seems OK to me.

>
> >
> > 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.

OK, so the 'cipher' node ends up providing information on how to
decode the image. But why put it in the root node? Would it not be
better to put it in the node next to the one with the encrypted data?
People might encrypt several images separately.
>
> >
> >> +    """
> >> +
> >> +    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.

Oh so how about putting the 'cipher' node in some-bitstream instead,
or even just add the encryption info to the 'cipher' node you have
shown? Why does it need to be in the root node?

>
> 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.

OK. In that case, perhaps we should have a property indicating that
the key is external, if that isn't obvious.

>
> >
> >> +            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()?

The class doc has the 'user' documentation, so it should go there. You
can copy in the info from your cover letter and also explain the
naming of the key node.

I might have a few more comments once this is updated.

> >
> >> +                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