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Subject: Re: [PATCH v3 2/2] ima: add enforce-evm and log-evm modes to strictly
 check EVM status
To:     Mimi Zohar <zohar@linux.ibm.com>, <dmitry.kasatkin@huawei.com>,
        <mjg59@google.com>
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        <linux-security-module@vger.kernel.org>,
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        <linux-kernel@vger.kernel.org>, <silviu.vlasceanu@huawei.com>
References: <20190606112620.26488-1-roberto.sassu@huawei.com>
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 <1559917462.4278.253.camel@linux.ibm.com>
 <93459fe8-f9b6-fe45-1ca7-2efb8854dc8b@huawei.com>
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From:   Roberto Sassu <roberto.sassu@huawei.com>
Message-ID: <773c3301-7861-f28b-813a-1f2ff657bae8@huawei.com>
Date:   Fri, 7 Jun 2019 17:14:07 +0200
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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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