Re: [RFC][PATCH v3a 00/11] ima: support fs-verity digests and signatures (alternative)

[Stefan Berger <stefanb@linux.ibm.com>]

On 1/31/22 15:24, Eric Biggers wrote:
> On Mon, Jan 31, 2022 at 02:29:19PM -0500, Stefan Berger wrote:
>>>> don't think I realized there was a more direct, PKCS#7-less way to do it and
>>>> that IMA used that way.)  However, it would be better to use this as an
>>>> opportunity to move people off of the built-in signatures entirely, either by
>>>> switching to a full userspace solution or by switching to IMA.
>>> If what we sign remains the same, then we could support multiple
>>> methods and use a selector to let fsverity_verify_signature() know
>>> how it should verify the signature. I don't know what would be a
>>> proper place for the selector.
>>>
>>> PKCS#7 seems ok, as it is used for kernel modules. IMA would be
>>> also ok, as it can verify the signature more directly. I would also
>>> be interested in supporting PGP, to avoid the requirement for
>>> Linux distributions to manage a secondary key. I have a small
>>> extension for rpmsign, that I would like to test in the Fedora
>>> infrastructure.
>>>
>>> Both the PKCS#7 and the PGP methods don't require additional
>>> support from outside, the functions verify_pkcs7_signature()
>>> and verify_pgp_signature() (proposed, not yet in the upstream
>>> kernel) would be sufficient.
>> FYI: An empty file signed with pkcs7 and an ecc key for NIST p256 generates
>> a signature of size 817 bytes. If an RPM needs to carry such signatures on a
>> per-file basis we are back to the size increase of nearly an RSA signature.
>> I would say for packages this is probably too much size increase.. and this
>> is what drove the implementation of ECC support.
> I am getting 256 bytes for an ECC signature in PKCS#7 format:
>
> 	cd src/fsverity-utils
> 	make
> 	openssl ecparam -name prime256v1 -genkey -noout -out key.pem
> 	openssl req -new -x509 -key key.pem -out cert.pem -days 360
> 	touch file
> 	./fsverity sign file file.sig --key=key.pem --cert=cert.pem
> 	stat -c %s file.sig
>
> Probably you accidentally included the whole certificate in the PKCS#7 message.
> That's not required here.
>
> There are definitely problems with PKCS#7, and it does have space overhead.  But
> the space overhead is not as bad as you state.

You are right. I used openssl cms without -nocerts and -noattr 
(unintentionately). Though 256 bytes is RSA 2048 signature size again. 
ECDSA with NIST p256 key is around 70 bytes.


>
> - Eric