tree b5ab8397fc3b6c7e93e677670f76cbd046e6da28
parent 2e6ee6670a5d450bc880e77a892ea0227a2cc3b4
author Lukas Wunner <lukas@wunner.de> 1719051789 +0200
committer Lukas Wunner <lukas@wunner.de> 1719761817 +0200

spdm: Authenticate devices despite invalid certificate chain

The SPDM library has just been amended to keep a log of received
signatures from a device and expose it in sysfs.

Currently challenge-response authentication with a device is only
performed if one of its up to 8 certificate chains is considered valid
by the kernel.

Valid means several things:

* That the certificate chain adheres to requirements in the SPDM
  specification (e.g. each certificate in the chain is signed by the
  preceding certificate),
* that the certificate chain adheres to requirements in other
  specifications such as PCIe r6.1 sec 6.31.3,
* that the first certificate in the chain is signed by a trusted root
  certificate on the kernel's keyring
* or that none of the certificates in the chain is on the kernel's
  blacklist_keyring.

User space should be given the chance to make up its own mind on the
validity of a certificate chain and the signature generated with it.
So if none of the 8 certificate chains is considered valid by the
kernel, pick one of them and perform challenge-response authentication
with it for the sole purpose of exposing a signature to user space.

Do not verify that signature because if the kernel considers the
certificate chain invalid, the signature implicitly is as well.

Arbitrarily select the certificate chain in the first provisioned slot
(which is normally slot 0) for such "for user space only" authentication
attempts.

Signed-off-by: Lukas Wunner <lukas@wunner.de>