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

[Roberto Sassu]

On 6/3/2019 4:31 PM, James Bottomley wrote:
> On Mon, 2019-06-03 at 16:29 +0200, Roberto Sassu wrote:
>> 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?
> 
> We don't.  We only guarantee immutability for unprivileged containers,
> so root can't be inside.

Ok.

Regarding the new behavior, this must be explicitly enabled by adding
ima_appraise=enforce-evm or log-evm to the kernel command line.
Otherwise, the current behavior is preserved with this patch. Would this
be ok?

Roberto

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