[PATCH v2 2/3] ima: don't ignore INTEGRITY_UNKNOWN EVM status

[Roberto Sassu]

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

-- 
HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Bo PENG, Jian LI, Yanli SHI