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

[Eric Biggers <ebiggers@kernel.org>]

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.

- Eric