IMA appraisal master plan?

[Roberto Sassu]

On 11/16/2017 2:06 PM, Mimi Zohar wrote:
> On Thu, 2017-11-16 at 10:23 +0100, Roberto Sassu wrote:
>> On 11/16/2017 3:13 AM, Mimi Zohar wrote:
>>> On Wed, 2017-11-15 at 16:05 -0800, Matthew Garrett wrote:
>>>> On Wed, Nov 15, 2017 at 4:02 PM, James Morris <james.l.morris at oracle.com> wrote:
>>>>> On Wed, 15 Nov 2017, Patrick Ohly wrote:
>>>>>
>>>>>> I have some experience with SMACK, but not with Apparmor. At least with
>>>>>> SMACK the problem is that the LSM depends on integrity protection of
>>>>>> the xattrs, but the integrity protection itself depends on the LSM, so
>>>>>> there's a cycle. An attacker can much too easily make offline changes
>>>>>> which then defeat whatever IMA policy the system might be using.
>>>>>
>>>>> Isn't this what EVM is supposed to mitigate?
>>
>> With the default appraisal policy, it can't. IMA determines if a file
>> must be appraised depending on metadata whose integrity has not been
>> verified yet. A root process is able to load appraised files with
>> i_uid = 0 and files with missing/invalid HMAC and i_uid != 0, at the
>> same time.
> 
> The LSMs are responsible for protecting their own labels.  Theyhave
> the opportunity to verify and deny access to files based on LSM
> labels, BEFORE IMA-appraisal is called to verify the file's integrity.

Adding in CC the linux-security-module mailing list.

LSMs are responsible to enforce a security policy at run-time, while
IMA/EVM protect data and metadata against offline attacks. However, if
IMA/EVM protect only part of the system, the security policy might not
be enforced as expected. I give an example.

Suppose that a security policy preserves the integrity of a database by
allowing only one application to modify it. Suppose also, that the
security policy allows that application to modify files which are not
appraised by IMA. Only the database is appraised.

Then, the integrity of the database cannot be guaranteed anymore. When
the system is offline, the database label is swapped with one that is
not included in the IMA policy. When the system is online again, LSMs
would allow the application to access the database, but its integrity
is no longer verified. From the users perspective, the application is
working correctly, while unauthorized modifications could have be done
on the database.

In my opinion, protecting the integrity of a TCB against offline and
online attacks with LSMs and IMA/EVM, can be achieved in two ways:

- all objects accessed by LSM TCB subjects are a subset of IMA TCB
   objects, and LSM prevents accesses to LSM TCB objects by processes
   outside LSM TCB

- all objects accessed by IMA TCB subjects are protected by IMA, IMA
   prevents accesses to IMA TCB objects by processes outside IMA TCB, and
   LSM TCB subjects are a subset of IMA TCB subjects

As you can see, in both cases there is a dependency between the LSM
policy and the IMA policy. In the first case, the dependency is on
objects and LSM is enforcing integrity. In the second case, the
dependency is on subjects, IMA is enforcing integrity and LSM could
enforce a more strict integrity policy or a policy with different goals.

I prefer the second option because:

1) it easier to write a policy in term of subjects rather than objects

2) LSM does not necessarily enforce an integrity policy; LSM could
    enforce a policy for isolation and containment, while IMA could
    enforce an integrity policy

3) an integrity policy can be enforced without LSM, and both LSM and IMA
    can enforce their own integrity policy

4) the effort necessary to enforce an integrity policy with IMA is very
    low: if files with valid signature/HMAC are in the IMA TCB and the
    IMA policy identifies TCB subjects, the required modification would
    be to simply deny access to appraised files if the subject does not
    match policy criteria

The first version of the patch set which adds support for the
enforcement of the Biba strict policy can be found at the URL:

https://www.spinics.net/lists/linux-integrity/msg00392.html

Roberto


> Look at security/security.c and see that IMA is called AFTER the
> LSMs.  The same is true for the other IMA hooks, that are not co-
> located with LSM hooks.  For example, the security_file_open hook is
> called before the ima_file_check() hook.
> 
> Mimi
> 

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