On 1/31/22 10:12, Roberto Sassu wrote:
From: Eric Biggers [mailto:ebiggers@xxxxxxxxxx] Sent: Friday, January 28, 2022 9:26 PM On Fri, Jan 28, 2022 at 09:05:01AM +0000, Roberto Sassu wrote:From: Eric Biggers [mailto:ebiggers@xxxxxxxxxx] Sent: Thursday, January 27, 2022 8:40 PM On Thu, Jan 27, 2022 at 11:35:12AM -0800, Eric Biggers wrote:On Thu, Jan 27, 2022 at 07:46:09PM +0100, Roberto Sassu wrote:I wanted to propose a different approach for handling fsverity digestsandsignatures, compared to: https://lore.kernel.org/linux-integrity/20220126000658.138345-1-zohar@xxxxxxxxxxxxx/In the original proposal, a new signature version has been introduced (v3) to allow the possibility of signing the digest of a more flexible data structure, ima_file_id, which could also include the fsverity file digest. While the new signature type would be sufficient to handle fsverity file digests, the problem is that its format would not be compatible with the signature format supported by the built-in verification module in fsverity. The rpm package manager already has an extension to include fsverity signatures, with the existing format, in the RPM header. Given that the fsverity signature is in the PKCS#7 format, IMA has already the capability of handling it with the existing code, more specifically the modsig code. It would be sufficient to provide to modsig the correct data to avoid introducing a new signature format.I think it would be best to get people moved off of the fs-verity built-in signatures, rather than further extend the use of it. PKCS#7 is a pretty terrible signature format. The IMA one is better, though it's unfortunatethatIMA still relies on X.509 for keys.Note, the only reason that support for fs-verity built-in signatures was added to RPM is that people didn't want to use IMA: https://lore.kernel.org/linux-fscrypt/b49b4367-51e7-f62a-6209- b46a6880824b@xxxxxxxxx If people are going to use IMA anyway, then there would be no point.Hi Eric I thought that the solution I came with could satisfy multiple needs. For people that don't want to use IMA, they could still continue to use the existing signature format, and wait for an LSM that satisfy their needs. They also have the option to migrate to the new signature format you are defining. But will those people be willing to switch to something IMA-specific? For people that use IMA, they could benefit from the effort of people creating packages with the original fsverity signature. For people that are skeptical about IMA, they could be interested in trying the full solution, which would probably be more easily available if the efforts from both sides converge. If, as you say, you have concerns about the existing signature format, wouldn't it be better that you address them from the fsverity side, so that all users of fsverity can benefit from it? Currently, fsverity hashes the formatted digest whose format is FSVerity<digest algo><digest size><digest>. Couldn't IMA hash the same data as well? An idea could be to always sign the formatted digest, and have a selector for the signature format: IMA, PKCS#7 or PGP.Adding support for the new IMA signature format to fsverity_verify_signature() *might* make sense. (When I added this code, my understanding was that it was just verifying signatures the way the kernel usually verifies signatures. IOk. Do we need something more to sign other than the fsverity formatted digest? If not, this could be the same for any method we support.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.
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