Re: [RFC] IMA Log Snapshotting Design Proposal

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

Thanks a lot James for looking at this proposal,
and sharing your thoughts. Really appreciate it.

On 8/1/23 14:21, James Bottomley wrote:
> On Tue, 2023-08-01 at 12:12 -0700, Sush Shringarputale wrote:
> [...]
>> 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).
Here we meant just truncating IMA log is not a complete
solution in itself. As you said, we have to take additional steps
like logging TPM2_Quotes etc. Logging AK is an interesting proposal
which we didn’t consider earlier. I am not sure if embedding AK to IMA
log/snapshot is needed. If the client sends them separately with "signed
PCR quotes" + "IMA log" + snapshots, it should still serve the purpose,
right?

>
> 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
> should have a beginning and end quote and a record of the AK used.
A new IMA log snapshot file (or shard as you call it) will have
the TPM2_Quote record (plus some additional metadata) at the beginning.
I don't believe it needs to be logged at the end of the snapshot (since 
it can
be computed by replaying the remaining entries in the snapshot).

See the snapshot_aggregate field in section B.5 in the original RFC mail 
[1].
> 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.
Could you please point to the Keylime source and/or documentation
which explains the use of beginning and end quotes? We would like to
understand how the verifiers are addressing this problem currently.


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

~Tushar
>
> Regards,
>
> James