Thanks a lot Stefan for looking into this proposal,
and providing your feedback. We really appreciate it.

On 8/7/23 15:49, Stefan Berger wrote:

On 8/1/23 17:21, James Bottomley wrote:

quoted

On Tue, 2023-08-01 at 12:12 -0700, Sush Shringarputale wrote:
[...]

quoted

Truncating IMA log to reclaim memory is not feasible, since it makes
the log go out of sync with the TPM PCR quote making remote
attestation fail.

This assumption isn't entirely true.  It's perfectly possible to shard
an IMA log using two TPM2_Quote's for the beginning and end PCR values
to validate the shard.  The IMA log could be truncated in the same way
(replace the removed part of the log with a TPM2_Quote and AK, so the
log still validates from the beginning quote to the end).

If you use a TPM2_Quote mechanism to save the log, all you need to do
is have the kernel generate the quote with an internal AK.  You can
keep a record of the quote and the AK at the beginning of the truncated
kernel log.  If the truncated entries are saved in a file shard it

The truncation seems dangerous to me. Maybe not all the scenarios with 
an attestation
client (client = reading logs and quoting) are possible then anymore, 
such as starting
an attestation client only after truncation but a verifier must have 
witnessed the
system's PCRs and log state before the truncation occurred.

You are correct that truncation on it’s own is dangerous. It needs to be
accompanied by (a) saving the IMA log data to disk as snapshots, (b) 
adding the
necessary TPM PCR quotes to the current IMA log (as James mentioned above),
(c) attestation clients having an ability to send the past snapshots to the
remote-attestation-service (verifiers), (d) and verifiers having an ability
to use the snapshots along with current IMA logs for the purpose of 
attestation.
All these points are explained in the original RFC email in sections B.1 
through B.5 [1].

I think an ima-buf (or similar) log entry in IMA log would have to 
appear at the beginning of the
truncated log stating the value of all PCRs that IMA touched 
(typically only PCR 10
but it can be others). The needs to be done since the quote itself 
doesn't
provide the state of the individual PCRs. This would at least allow an 
attestation
client to re-read the log from the beginning (when it is re-start or 
started for the
first time after the truncation). 

  Agreed. See the description of snapshot_aggregate in Section B.5 in the
original RFC email [1].

However, this alone (without the
internal AK quoting the old state) could lead to abuse where I could 
create totally
fake IMA logs stating the state of the PCRs at the beginning (so the 
verifier
syncs its internal PCR state to this state). 

Yes, the PCR quotes sent to the verifier must be signed by the AK that
is trusted by the verifier. That assumption is true regardless of IMA log
snapshotting feature.

Further, even with the AK-quote that
you propose I may be able to create fake logs and trick a verifier into
trusting the machine IFF it doesn't know what kernel this system was 
booted with
that I may have hacked to provide a fake AK-quote that just happens to 
match the
PCR state presented at the beginning of the log.

If the Kernel is compromised, then all-bets are off.
(Regardless of IMA log snapshotting feature.)

=> Can a truncated log be made safe for attestation when the 
attestation starts
only after the truncation occurred?

Yes. If the “PCR quotes in the snapshot_aggregate event in IMA log”
+ "replay of rest of the events in IMA log" results in the “final PCR 
quotes”
that matches with the “AK signed PCR quotes” sent by the client, then 
the truncated
IMA log can be trusted. The verifier can either ‘trust’ the “PCR quotes 
in the
snapshot_aggregate event in IMA log” or it can ask for the (n-1)th 
snapshot shard
to check the past events.

=> Even if attestation was occurring 'what' state does an attestation 
server
need to carry around for an attested-to system so that the truncation 
is 'safe'
and I cannot create fake AK-quotes and fake IMA logs with initial PCR 
states?

Assuming most of the client devices take a snapshot at specific checkpoints,
the “PCR quotes in the snapshot_aggregate event in IMA log” will be the 
same for them.
The remote attestation server will have to remember these golden PCR quotes.
It doesn't have to remember the state of each client device.

Can I ever restart the client and have it read the truncated log from the
beginning and what type of verification needs to happen on the server 
then?

Yes, restarting the client should be possible.

It seems like the server would have to remember the state of the IMA 
PCRs upon
last truncation to detect a possible attack. This would make staring 
to monitor
a system after truncation impossible -- would be good to know these 
details.

The server is not forced to remember the state of IMA PCRs. It can
always ask for the last n snapshot files (shards) and replay the events. 
Even
though the data is truncated from the IMA log, it is not totally lost. It is
simply being transferred to the disk. It is saved by UM as snapshot 
files/shards.
The goal of IMA snapshotting is to reduce the Kernel memory pressure on the
client devices - to save them from out-of-memory errors which are harder 
to manage
on long running clients. It comes with a cost of additional work on the 
server
side to attest those clients.


Being said that, in the current proposal, taking a snapshots is totally 
optional
and controlled by UM attestation clients. If the 
attestation-clients/services are
not-ready/don’t-want to take advantage of IMA log snapshotting, they 
don’t have to.

No snapshot will be taken, and the client-service can process the 
monolithic IMA
log just like they do today.

[1] 
https://lore.kernel.org/all/c5737141-7827-1c83-ab38-0119dcfea485@linux.microsoft.com/#t (local) 

quoted

should have a beginning and end quote and a record of the AK used.
Since verifiers like Keylime are already using this beginning and end
quote for sharded logs, it's the most natural format to feed to
something externally for verification and it means you don't have to
invent a new format to do the same thing.

Regards,

James


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