Re: [PATCH v3 2/2] ima: add enforce-evm and log-evm modes to strictly check EVM status

From: Mimi Zohar
Date: Fri Jun 07 2019 - 11:13:08 EST

On Fri, 2019-06-07 at 16:40 +0200, Roberto Sassu wrote:
> > On Thu, 2019-06-06 at 13:26 +0200, Roberto Sassu wrote:

> >> Although this choice appears legitimate, it might not be suitable for
> >> hardened systems, where the administrator expects that access is denied if
> >> there is any error. An attacker could intentionally delete the EVM keys
> >> from the system and set the file digest in security.ima to the actual file
> >> digest so that the final appraisal status is INTEGRITY_PASS.
> >
> > Assuming that the EVM HMAC key is stored in the initramfs, not on some
> > other file system, and the initramfs is signed, INTEGRITY_UNKNOWN
> > would be limited to the rootfs filesystem.
> There is another issue. The HMAC key, like the public keys, should be
> loaded when appraisal is disabled. This means that we have to create a
> trusted key at early boot and defer the unsealing.

There is no need for IMA to appraise the public key file signature,
since the certificate is signed by a key on the builtin/secondary
trusted keyring. ÂWithÂCONFIG_IMA_LOAD_X509 enabled, the public key
can be loaded onto the IMA keyring with IMA-appraisal enabled, but
without verifying the file signature.