[PATCH v3 3/4] tools: binman: fit: add support for OpenSSL engines
Quentin Schulz
quentin.schulz at cherry.de
Wed Nov 26 12:28:55 CET 2025
Hi Simon,
On 11/25/25 11:15 PM, Simon Glass wrote:
> Hi Quentin,
>
> On Fri, 21 Nov 2025 at 10:15, Quentin Schulz <foss+uboot at 0leil.net> wrote:
>>
>> From: Quentin Schulz <quentin.schulz at cherry.de>
>>
>> This adds support for using an OpenSSL engine for signing a FIT image.
>> To use it, one should set the fit,engine property at the FIT node level
>> with the engine to use. This will in turn call mkimage with the -N
>> option.
>>
>> The -k argument to mkimage can be specified via fit,engine-keydir. If
>> not specified, -k is not passed to mkimage. This property is especially
>> useful for pkcs11 engine to specify slots, token label, etc...
>>
>> As far as I could tell, mkimage encrypts and signs a FIT in one go, thus
>> the -k argument applies to both signing and encrypting. Considering we
>> reuse the -k argument for two different meanings (info to pass to the
>> engine when using an engine otherwise the directory where keys are
>> stored), we cannot reasonably encrypt using local keys and signing with
>> an engine, hence the enforced check. I believe it should be possible to
>> support encrypting and signing with the same engine (using different
>> key pairs of course, via different key-name-hint likely), but this is
>> left for the next person to implement.
>> This is why the property is named fit,engine and not fit,sign-engine.
>> Ditto for fit,engine-keydir.
>>
>> The public key (with .crt extension) is still required if it needs to be
>> embedded in the SPL DTB for example. We could probably support
>> retrieving the public key from an engine, but this is a change to make
>> to fdt_add_pubkey.c.
>>
>> Signed-off-by: Quentin Schulz <quentin.schulz at cherry.de>
>> ---
>> tools/binman/entries.rst | 54 +++++++++++++++++++++++++--
>> tools/binman/etype/fit.py | 93 +++++++++++++++++++++++++++++++++++++++++++++--
>> 2 files changed, 140 insertions(+), 7 deletions(-)
>>
>> diff --git a/tools/binman/entries.rst b/tools/binman/entries.rst
>> index 8922d6cd070..a81fcbd3891 100644
>> --- a/tools/binman/entries.rst
>> +++ b/tools/binman/entries.rst
>> @@ -885,9 +885,10 @@ The top-level 'fit' node supports the following special properties:
>>
>> fit,sign
>> Enable signing FIT images via mkimage as described in
>> - verified-boot.rst. If the property is found, the private keys path
>> - is detected among binman include directories and passed to mkimage
>> - via -k flag. All the keys required for signing FIT must be
>> + verified-boot.rst.
>> + If the property is found and fit,engine is not set, the private
>> + keys path is detected among binman include directories and passed to
>> + mkimage via -k flag. All the keys required for signing FIT must be
>> available at time of signing and must be located in single include
>> directory.
>>
>> @@ -898,6 +899,53 @@ The top-level 'fit' node supports the following special properties:
>> required for encrypting the FIT must be available at the time of
>> encrypting and must be located in a single include directory.
>>
>> + Incompatible with fit,engine.
>> +
>> + fit,engine
>> + Indicates the OpenSSL engine to use for signing the FIT image. This
>> + is passed to mkimage via the `-N` flag. Example::
>> +
>> + fit,engine = "my-engine";
>> +
>> + A `-k` argument for mkimage may be passed via `fit,engine-keydir`.
>> +
>> + When `fit,engine` is set to `pkcs11`, the following applies:
>> +
>> + - If `fit,engine-keydir` is absent, the value of `key-name-hint` is
>> + prefixed with `pkcs11:object=` before being passed to the OpenSSL
>> + engine API::
>> +
>> + pkcs11:object=<key-name-hint>
>> +
>> + - If `fit,engine-keydir` contains either `object=` or `id=`, its
>> + value is passed verbatim to the OpenSSL engine API,
>> +
>> + - Otherwise, the value of `fit,engine-keydir` is followed by
>> + `;object=` and the value of `key-name-hint` before being passed
>> + to the OpenSSL engine API::
>> +
>> + <fit,engine-keydir>;object=<key-name-hint>
>> +
>> + If `fit,engine` is set to something different than `pkcs11`, the
>> + value of `key-name-hint` (prefixed with the value of
>> + `fit,engine-keydir` if present) and passed verbatim to the OpenSSL
>> + engine API.
>> +
>> + Depends on fit,sign.
>> +
>> + Incompatible with fit,encrypt.
>> +
>> + fit,engine-keydir
>> + Indicates the `-k` argument to pass to mkimage if an OpenSSL engine
>> + is to be used for signing the FIT image. Example::
>> +
>> + fit,engine-keydir = "pkcs11:model=xxx;manufacturer=xxx";
>> +
>> + Read `fit,engine` documentation for more info on special cases when
>> + using `pkcs11` as engine.
>> +
>> + Depends on fit,engine.
>> +
>> Substitutions
>> ~~~~~~~~~~~~~
>>
>> diff --git a/tools/binman/etype/fit.py b/tools/binman/etype/fit.py
>> index db40479d30e..f28b1e6b4cb 100644
>> --- a/tools/binman/etype/fit.py
>> +++ b/tools/binman/etype/fit.py
>> @@ -104,9 +104,10 @@ class Entry_fit(Entry_section):
>>
>> fit,sign
>> Enable signing FIT images via mkimage as described in
>> - verified-boot.rst. If the property is found, the private keys path
>> - is detected among binman include directories and passed to mkimage
>> - via -k flag. All the keys required for signing FIT must be
>> + verified-boot.rst.
>> + If the property is found and fit,engine is not set, the private
>> + keys path is detected among binman include directories and passed to
>> + mkimage via -k flag. All the keys required for signing FIT must be
>> available at time of signing and must be located in single include
>> directory.
>>
>> @@ -117,6 +118,53 @@ class Entry_fit(Entry_section):
>> required for encrypting the FIT must be available at the time of
>> encrypting and must be located in a single include directory.
>>
>> + Incompatible with fit,engine.
>> +
>> + fit,engine
>> + Indicates the OpenSSL engine to use for signing the FIT image. This
>> + is passed to mkimage via the `-N` flag. Example::
>> +
>> + fit,engine = "my-engine";
>> +
>> + A `-k` argument for mkimage may be passed via `fit,engine-keydir`.
>> +
>> + When `fit,engine` is set to `pkcs11`, the following applies:
>> +
>> + - If `fit,engine-keydir` is absent, the value of `key-name-hint` is
>> + prefixed with `pkcs11:object=` before being passed to the OpenSSL
>> + engine API::
>> +
>> + pkcs11:object=<key-name-hint>
>> +
>> + - If `fit,engine-keydir` contains either `object=` or `id=`, its
>> + value is passed verbatim to the OpenSSL engine API,
>> +
>> + - Otherwise, the value of `fit,engine-keydir` is followed by
>> + `;object=` and the value of `key-name-hint` before being passed to
>> + the OpenSSL engine API::
>> +
>> + <fit,engine-keydir>;object=<key-name-hint>
>> +
>> + If `fit,engine` is set to something different than `pkcs11`, the
>> + value of `key-name-hint` (prefixed with the value of
>> + `fit,engine-keydir` if present) and passed verbatim to the OpenSSL
>> + engine API.
>> +
>> + Depends on fit,sign.
>> +
>> + Incompatible with fit,encrypt.
>> +
>> + fit,engine-keydir
>> + Indicates the `-k` argument to pass to mkimage if an OpenSSL engine
>> + is to be used for signing the FIT image. Example::
>> +
>> + fit,engine-keydir = "pkcs11:model=xxx;manufacturer=xxx";
>> +
>> + Read `fit,engine` documentation for more info on special cases when
>> + using `pkcs11` as engine.
>> +
>> + Depends on fit,engine.
>> +
>> Substitutions
>> ~~~~~~~~~~~~~
>>
>> @@ -588,6 +636,29 @@ class Entry_fit(Entry_section):
>>
>> return paths[0] if len(paths) else None
>>
>> + def _get_fit_engine(self):
>> + """Detect whether an OpenSSL engine is to be used for the FIT
>> +
>> + Returns:
>> + Tuple(str, str): Name of the engine to use, as first element of the
>> + Tuple. None if no engine to use.
>> + keydir arguments to pass with the engine to the
>> + OpenSSL API, as second element of the Tuple. None
>> + if no keydir to pass.
>> + """
>> + engine = None
>> + engine_keydir = None
>> +
>> + prop = self._fit_props.get('fit,engine')
>> + if prop is not None:
>> + engine = prop.value
>> +
>> + prop = self._fit_props.get('fit,engine-keydir')
>> + if prop is not None:
>> + engine_keydir = prop.value
>> +
>> + return engine, engine_keydir
>> +
>> def BuildSectionData(self, required):
>> """Build FIT entry contents
>>
>> @@ -620,7 +691,21 @@ class Entry_fit(Entry_section):
>> args.update({'align': fdt_util.fdt32_to_cpu(align.value)})
>> if (self._fit_props.get('fit,sign') is not None or
>> self._fit_props.get('fit,encrypt') is not None):
>> - args.update({'keys_dir': self._get_keys_dir(data)})
>> + engine = None
>> + keydir = None
>> +
>> + # Engine only supported for signing for now
>> + if self._fit_props.get('fit,sign') is not None:
>> + engine, keydir = self._get_fit_engine()
>> +
>> + args.update({'engine': engine})
>> + # If no engine, keys must exist locally, find them
>> + if engine is None:
>> + keydir = self._get_keys_dir(data)
>> + elif self._fit_props.get('fit,encrypt') is not None:
>> + self.Raise('fit,engine currently does not support encryption')
>> +
>> + args.update({'keys_dir': keydir})
>> if self.mkimage.run(reset_timestamp=True, output_fname=output_fname,
>> **args) is None:
>> if not self.GetAllowMissing():
>>
>> --
>> 2.51.1
>>
>
> Were there any changes on this one? It looks OK to me.
>
Yes. You can see the git-range-diff between both versions with:
$ b4 diff -v 2 3 --
https://lore.kernel.org/u-boot/20251121-binman-engine-v3-0-b80180aaa783@cherry.de/T/\#t
Also see cover letter where I list the changes made to the v3 compared
to v2 and why I decided to not add your and Wolfgang's trailers.
> You should be able to do: if 'fit,sign' not in self._fit_props
>
$ git grep "in self._fit_props" tools/binman/etype/fit.py
returns nothing. I would rather have the same logic to get properties
within a given file.
Cheers,
Quentin
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