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

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Date:   Fri, 07 Jun 2019 11:08:32 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>,
        dmitry.kasatkin@...wei.com, mjg59@...gle.com
Cc:     linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-doc@...r.kernel.org,
        stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        silviu.vlasceanu@...wei.com
Subject: Re: [PATCH v3 2/2] ima: add enforce-evm and log-evm modes to
 strictly check EVM status

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.  With CONFIG_IMA_LOAD_X509 enabled, the public key
can be loaded onto the IMA keyring with IMA-appraisal enabled, but
without verifying the file signature.

Mimi