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Subject: Re: [PATCH v2 2/3] ima: don't ignore INTEGRITY_UNKNOWN EVM status
To:     James Bottomley <James.Bottomley@HansenPartnership.com>,
        Mimi Zohar <zohar@linux.ibm.com>, <dmitry.kasatkin@huawei.com>,
        <mjg59@google.com>
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        <linux-security-module@vger.kernel.org>,
        <linux-doc@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <silviu.vlasceanu@huawei.com>, <stable@vger.kernel.org>
References: <20190529133035.28724-1-roberto.sassu@huawei.com>
 <20190529133035.28724-3-roberto.sassu@huawei.com>
 <1559217621.4008.7.camel@linux.ibm.com>
 <e6b31aa9-0319-1805-bdfc-3ddde5884494@huawei.com>
 <1559569401.5052.17.camel@HansenPartnership.com>
From:   Roberto Sassu <roberto.sassu@huawei.com>
Message-ID: <3667fbd4-b6ed-6a76-9ff4-84ec3c2dda12@huawei.com>
Date:   Mon, 3 Jun 2019 16:29:40 +0200
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On 6/3/2019 3:43 PM, James Bottomley wrote:
> On Mon, 2019-06-03 at 11:25 +0200, Roberto Sassu wrote:
>> On 5/30/2019 2:00 PM, Mimi Zohar wrote:
>>> On Wed, 2019-05-29 at 15:30 +0200, Roberto Sassu wrote:
>>>> Currently, ima_appraise_measurement() ignores the EVM status when
>>>> evm_verifyxattr() returns INTEGRITY_UNKNOWN. If a file has a
>>>> valid security.ima xattr with type IMA_XATTR_DIGEST or
>>>> IMA_XATTR_DIGEST_NG, ima_appraise_measurement() returns
>>>> INTEGRITY_PASS regardless of the EVM status. The problem is that
>>>> the EVM status is overwritten with the appraisal statu
>>>
>>> Roberto, your framing of this problem is harsh and misleading.  IMA
>>> and EVM are intentionally independent of each other and can be
>>> configured independently of each other.  The intersection of the
>>> two is the call to evm_verifyxattr().  INTEGRITY_UNKNOWN is
>>> returned for a number of reasons - when EVM is not configured, the
>>> EVM hmac key has not yet been loaded, the protected security
>>> attribute is unknown, or the file is not in policy.
>>>
>>> This patch does not differentiate between any of the above cases,
>>> requiring mutable files to always be protected by EVM, when
>>> specified as an "ima_appraise=" option on the boot command line.
>>>
>>> IMA could be extended to require EVM on a per IMA policy rule
>>> basis. Instead of framing allowing IMA file hashes without EVM as a
>>> bug that has existed from the very beginning, now that IMA/EVM have
>>> matured and is being used, you could frame it as extending IMA
>>> or hardening.
>>
>> I'm seeing it from the perspective of an administrator that manages
>> an already hardened system, and expects that the system only grants
>> access to files with a valid signature/HMAC. That system would not
>> enforce this behavior if EVM keys are removed and the digest in
>> security.ima is set to the actual file digest.
>>
>> Framing it as a bug rather than an extension would in my opinion help
>> to convince people about the necessity to switch to the safe mode, if
>> their system is already hardened.
> 
> I have a use case for IMA where I use it to enforce immutability of
> containers.  In this use case, the cluster admin places hashes on
> executables as the image is unpacked so that if an executable file is
> changed, IMA will cause an execution failure.  For this use case, I
> don't care about the EVM, in fact we don't use it, because the only
> object is to fail execution if a binary is mutated.

How would you prevent root in the container from updating security.ima?

Roberto

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