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

From: Ken Goldman <kgold@linux.ibm.com>
To: Lakshmi <nramas@linux.microsoft.com>,
	Linux Integrity <linux-integrity@vger.kernel.org>,
	Mimi Zohar <zohar@linux.ibm.com>,
	David Howells <dhowells@redhat.com>,
	James Morris <jamorris@linux.microsoft.com>,
	Linux Kernel <linux-kernel@vger.kernel.org>
Cc: Balaji Balasubramanyan <balajib@linux.microsoft.com>,
	Prakhar Srivastava <prsriva@linux.microsoft.com>,
	jorhand@linux.microsoft.com
Subject: Re: [PATCH 0/2] public key: IMA signer logging: Log public key of IMA Signature signer in IMA log
Date: Wed, 22 May 2019 14:57:28 -0400	[thread overview]
Message-ID: <b1a2edc1-45c7-7a9f-7a77-e252b2f85a64@linux.ibm.com> (raw)
In-Reply-To: <54663a75-a601-ae6c-8068-bc2c3923a948@linux.microsoft.com>

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
> 