Hi James, On Tue, Jun 20, 2023 at 08:43:00AM -0400, James Bottomley wrote: > On Tue, 2023-06-20 at 01:42 +0100, Luca Boccassi wrote: > > On Tue, 20 Jun 2023 at 00:49, Eric Biggers <ebiggers@xxxxxxxxxx> > > wrote: > > > On Tue, Jun 20, 2023 at 12:04:39AM +0100, Luca Boccassi wrote: > [...] > > > > > +- fs-verity builtin signatures are in PKCS#7 format, and the > > > > > public > > > > > + keys are in X.509 format. These data formats are > > > > > unnecessarily > > > > > + complex and prone to vulnerabilities. (fs-verity builtin > > > > > signatures > > > > > + were made to use these formats because other kernel > > > > > subsystems, such > > > > > + as the module loader, unfortunately used these formats > > > > > already. > > > > > + Note, these formats should still be used when they are the > > > > > only > > > > > + option to have signatures at all.) Userspace signature > > > > > verification > > > > > + avoids having to enable CONFIG_FS_VERITY_BUILTIN_SIGNATURES > > > > > and the > > > > > + associated kernel attack surface. Userspace also has the > > > > > + flexibility to choose simpler formats. For example, > > > > > consider using > > > > > + straightforward Ed25519 keys and signatures with `libsodium > > > > > + > > > > > <https://libsodium.gitbook.io/doc/public-key_cryptography/publi > > > > > c-key_signatures>`_. > > > > > + > > > > > + IMA appraisal, which supports fs-verity, does not use > > > > > PKCS#7, so it > > > > > + partially avoids this issue as well (though it does use > > > > > X.509). > > > > > + > > > > > + If you are considering making use of "advanced" features of > > > > > X.509 > > > > > + and/or PKCS#7, please also keep in mind that these > > > > > "advanced" > > > > > + features do not always work as intended with the kernel. > > > > > For > > > > > + example, the kernel does not check X.509 certificate > > > > > validity times. > > > > > > > > Sorry but this still reads as way too opinionated and generic, > > > > rather than being fsverity-specific. > > > > > > > > Please simplify to convey the same > > > > message in more concise way, perhaps something along these lines: > > > > > > > > - fs-verity builtin signatures are in PKCS#7 format, and the > > > > public keys are in X.509 format. IMA appraisal, which supports > > > > fs-verity, uses a custom signature format rather than PKCS#7 and > > > > X.509 for public keys. Alternative formats for signatures and > > > > public keys are not supported for builtin signatures or IMA > > > > appraisal. For fully flexible and customized signature and public > > > > keys formats, while also avoiding to expose the kernel to > > > > untrusted input, signature verification can be implemented by a > > > > trusted userspace component as described at <pointer to > > > > appropriate section> > > > > > > That is not the same message at all, as it omits the mention of the > > > disadvantages of PKCS#7 and X.509 compared to raw signatures, which > > > was the main point. So no, I don't think your version would be > > > better. > > > > The 'disadvantages' are your personal opinions. It's fine to have > > opinions, it's less fine to present them as if they were industry > > consensus in public project-wide documentation. > > > > > It seems that what is going on here is that you are invested > > > heavily into existing X.509 and PKCS#7 based systems, and as a > > > result you do not want the problems with these formats to be > > > described anywhere in the kernel documentation. That is > > > understandable, but that is a special interest that should not be > > > catered to here. This documentation is trying to help users make > > > a decision of what type of signature to use in new systems. And > > > yes, it is fsverity specific documentation, but there is no way for > > > it to make the needed point without dicussing the underlying data > > > formats. > > > > Industry standards are by very definition the opposite of 'special > > interests'. Look, I tried my best to provide constructive and > > actionable feedback in previous replies, but given you seem only > > interested in casting aspersions and hijacking kernel documentation > > to promote the latest proprietary google-toy-of-the-month: > > OK, could I try to share the perspective of of someone who's worked in > security with users for a long time. I get that every security expert > thinks standard key formats are insecure, usually for two reasons > > 1. In security simpler is better > 2. They can always come up with a simple, totally secure, key > representation > > The problem with the above is that it means every tool invented by a > different security expert has a different key format and we don't have > any interoperability between them. This is great from a security point > of view but terrible for usability: users want both interoperability > and pluggability (particularly the ability to use tokens or other > security devices in place of keys). Our security systems are not coded > for absolute security (otherwise they would be completely balkanized); > they're a tradeoff between usability and security. There's actually > even a security reason for this: a security tool no-one can use isn't > contributing to the security of the ecosystem, however internally > secure and up to date the tool is. It's not just Google who has a habit > of special coding security systems to work on Android (with Titan-M), I > can't use wireguard on Linux because I long ago mandated that critical > things like VPN keys should be in secure keystores and I use the TPM on > Linux for openvpn. Wireguard, because it has a simple and secure key > format, won't plug into the kernel TPM system without a whole load of > speciallised glue, so I'm stuck with openvpn. > > The debate you two are having now is about the most important thing we > can ever decide: where to draw the line between usability and security. > If you conduct it in a more constructive manner and try to see each > other's point of view (Eric: how would I use a TPM with fsverity on > Linux? and Luca: how much interoperability does fsverity really need > for debian?) because usability without security is as equally useless > as security without usability. By using a TPM with fsverity, I assume you mean doing the signing with a TPM? (Note that signing usually occurs on a system different from the one that verifies the signature.) Even that implies the use of X.509, which I don't think it does, I don't think it implies the use of PKCS#7. And it certainly doesn't imply the use of in-kernel signature verification. So don't immediately see the relevance of this. In any case, I think my latest patch (v3) is very reasonable. I've worked hard to address Luca's concerns, and the patch makes it clear that there are multiple options for verification of signatures of fsverity files, both with and without PKCS#7, and with and without in-kernel verification. Each has its advantages and disadvantages. I would encourage you to read the latest patch fully; I think Luca's comments are making it sound like it says things that it doesn't actually say (and I also made some further revisions v2 => v3). - Eric
