Re: [RFC V2] IMA Log Snapshotting Design Proposal

[Tushar Sugandhi <tusharsu@linux.microsoft.com>]

On 11/16/23 14:28, Paul Moore wrote:
> On Tue, Oct 31, 2023 at 3:15 PM Mimi Zohar <zohar@linux.ibm.com> wrote:
>> On Thu, 2023-10-19 at 11:49 -0700, Tushar Sugandhi wrote:
>>
>> [...]
>>> -----------------------------------------------------------------------
>>> | C.1 Solution Summary                                                |
>>> -----------------------------------------------------------------------
>>> To achieve the goals described in the section above, we propose the
>>> following changes to the IMA subsystem.
>>>
>>>       a. The IMA log from Kernel memory will be offloaded to some
>>>          persistent storage disk to keep the system running reliably
>>>          without facing memory pressure.
>>>          More details, alternate approaches considered etc. are present
>>>          in section "D.3 Choices for Storing Snapshots" below.
>>>
>>>       b. The IMA log will be divided into multiple chunks (snapshots).
>>>          Each snapshot would be a delta between the two instances when
>>>          the log was offloaded from memory to the persistent storage
>>>          disk.
>>>
>>>       c. Some UM process (like a remote-attestation-client) will be
>>>          responsible for writing the IMA log snapshot to the disk.
>>>
>>>       d. The same UM process would be responsible for triggering the IMA
>>>          log snapshot.
>>>
>>>       e. There will be a well-known location for storing the IMA log
>>>          snapshots on the disk.  It will be non-trivial for UM processes
>>>          to change that location after booting into the Kernel.
>>>
>>>       f. A new event, "snapshot_aggregate", will be computed and measured
>>>          in the IMA log as part of this feature.  It should help the
>>>          remote-attestation client/service to benefit from the IMA log
>>>          snapshot feature.
>>>          The "snapshot_aggregate" event is described in more details in
>>>          section "D.1 Snapshot Aggregate Event" below.
>>>
>>>       g. If the existing remote-attestation client/services do not change
>>>          to benefit from this feature or do not trigger the snapshot,
>>>          the Kernel will continue to have it's current functionality of
>>>          maintaining an in-memory full IMA log.
>>>
>>> Additionally, the remote-attestation client/services need to be updated
>>> to benefit from the IMA log snapshot feature.  These proposed changes
>>>
>>> are described in section "D.4 Remote-Attestation Client/Service Side
>>> Changes" below, but their implementation is out of scope for this
>>> proposal.
>>
>> As previously said on v1,
>>     This design seems overly complex and requires synchronization between the
>>     "snapshot" record and exporting the records from the measurement list. [...]
>>
>>     Concerns:
>>     - Pausing extending the measurement list.
>>
>> Nothing has changed in terms of the complexity or in terms of pausing
>> the measurement list.   Pausing the measurement list is a non starter.
> 
> The measurement list would only need to be paused for the amount of
> time it would require to generate the snapshot_aggregate entry, which
> should be minimal and only occurs when a privileged userspace requests
> a snapshot operation.  The snapshot remains opt-in functionality, and
> even then there is the possibility that the kernel could reject the
> snapshot request if generating the snapshot_aggregate entry was deemed
> too costly (as determined by the kernel) at that point in time.
> 
Thanks Paul for responding and sharing your thoughts.


Hi Mimi,
To address your concern about pausing the measurements -
We are not proposing to pause the measurements for the entire duration
of UM <--> Kernel interaction while taking a snapshot.

We are simply proposing to pause the measurements when we get the TPM
PCR quotes to add them to "snapshot_aggregate". (which should be a very
small time window). IMA already has this mechanism when two separate
modules try to add entry to IMA log - by using
mutex_lock(&ima_extend_list_mutex); in ima_add_template_entry.


We plan to use this existing locking functionality.
Hope this addresses your concern about pausing extending the measurement
list.

~Tushar

>> Userspace can already export the IMA measurement list(s) via the
>> securityfs {ascii,binary}_runtime_measurements file(s) and do whatever
>> it wants with it.  All that is missing in the kernel is the ability to
>> trim the measurement list, which doesn't seem all that complicated.
> 
>>From my perspective what has been presented is basically just trimming
> the in-memory measurement log, the additional complexity (which really
> doesn't look that bad IMO) is there to ensure robustness in the face
> of an unreliable userspace (processes die, get killed, etc.) and to
> establish a new, transitive root of trust in the newly trimmed
> in-memory log.
> 
> I suppose one could simplify things greatly by having a design where
> userspace  captures the measurement log and then writes the number of
> measurement records to trim from the start of the measurement log to a
> sysfs file and the kernel acts on that.  You could do this with, or
> without, the snapshot_aggregate entry concept; in fact that could be
> something that was controlled by userspace, e.g. write the number of
> lines and a flag to indicate if a snapshot_aggregate was desired to
> the sysfs file.  I can't say I've thought it all the way through to
> make sure there are no gotchas, but I'm guessing that is about as
> simple as one can get.
> 
> If there is something else you had in mind, Mimi, please share the
> details.  This is a very real problem we are facing and we want to
> work to get a solution upstream.
>