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 11:25:56 -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,
        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 17:14 +0200, Roberto Sassu wrote:
> On 6/7/2019 5:08 PM, Mimi Zohar wrote:
> > 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.
> 
> Yes, but access to the files containing the master key and the EVM key
> is denied if appraisal is enabled.

This is a key loading ordering issue.  Assuming you load the IMA key
first, you should be able to verify the master and EVM keys.

Mimi