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

From: Roberto Sassu <roberto.sassu@huawei.com>
To: Mimi Zohar <zohar@linux.ibm.com>, <dmitry.kasatkin@huawei.com>,
	<mjg59@google.com>
Cc: <linux-integrity@vger.kernel.org>,
	<linux-security-module@vger.kernel.org>,
	<linux-doc@vger.kernel.org>, <stable@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, <silviu.vlasceanu@huawei.com>
Subject: Re: [PATCH v3 2/2] ima: add enforce-evm and log-evm modes to strictly check EVM status
Date: Fri, 7 Jun 2019 17:14:07 +0200	[thread overview]
Message-ID: <773c3301-7861-f28b-813a-1f2ff657bae8@huawei.com> (raw)
In-Reply-To: <1559920112.4278.264.camel@linux.ibm.com>

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.

Roberto

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