Re: [PATCH 0/2] public key: IMA signer logging: Log public key of IMA Signature signer in IMA log

Ken Goldman
2019-05-22T18:57:38Z

On 5/20/2019 7:15 PM, Lakshmi wrote:
> On 5/17/19 7:41 AM, Ken Goldman wrote:
>
> Hi Ken,
>
> Apologize for the delay in responding.
>
>> Since a platform typically uses only a few signing keys, 4 bytes makes
>> the chance of a collision quite small.  The collision would have to be
>> within the same log, not global.
>>
>> In that worst case, the verifier would have to try two keys.  It's a
>> slight performance penalty, but does anything break?
>
> Problem Statement:
> - If the attestation service has to re-validate the signature reported
> in the IMA log, the service has to maintain the hash\signature of all
> the binaries deployed on all the client nodes. This approach will not
> scale for large cloud deployments.

1 - How is your solution - including a public key with each event -
related to this issue?

2 - I don't understand how a large cloud affects scale. Wouldn't the
verifier would typically be checking known machines - those of their
enterprise - not every machine on the cloud?

Can't we assume a typical attestation use case has a fairly locked down
OS with a limited number of applications.

> - Possibility of collision of "Key Ids" is non-zero
> - In the service if the "Key Id" alone is used to verify using a map of
> "Key Id" to "Signing Key(s)", the service cannot determine if the
> trusted signing key was indeed used by the client for signature
> validation (Due to "Key Id" collision issue or malicious signature).

Like I said, it should be rare. In the worst case, can't the service
tell by trying both keys?

>
> Proposed Solution:
> - The service receives known\trusted signing key(s) from a trusted
> source (that is different from the client machines)
> - The clients measure the keys in key rings such as IMA, Platform,
> BuiltIn Trusted, etc. as early as possible in the boot sequence.
> - Leave all IMA measurements the same - i.e., we don't log public keys
> in the IMA log for each file, but just use what is currently available
> in IMA.

I thought your solution was to change the IMA measurements, adding the
public key to each entry with a new template? Did I misunderstand, or
do you have a new proposal?

>
> Impact:
> - The service can verify that the keyrings have only known\trusted keys.

If the service already has trusted keys from a trusted source, why do
they have to come from the client at all?

> - The service can cross check the "key id" with the key rings measured.
> - The look up of keys using the key id would be simpler and faster on
> the service side.
> - It can also handle collision of Key Ids.

How does this solve the collision issue? If there are two keys with the
same key ID, isn't there still a collision?

>
> Note that the following is a key assumption:
>
> - Only keys signed by a key in the "BuiltIn Trusted Keyring" can be
> added to IMA\Platform keyrings.

I understand how the client keyring is used in IMA to check file
signatures, but how is that related to the attestation service?

>
>
> Thanks,
>  -lakshmi
>