Re: [PATCH v6 4/5] ima: support fs-verity file digest based version 3 signatures

[Stefan Berger <stefanb@linux.ibm.com>]

On 3/25/22 08:31, Mimi Zohar wrote:
> On Mon, 2022-03-21 at 09:10 -0400, Stefan Berger wrote:
>>
>> On 3/18/22 14:21, Mimi Zohar wrote:
>>> IMA may verify a file's integrity against a "good" value stored in the
>>> 'security.ima' xattr or as an appended signature, based on policy.  When
>>> the "good value" is stored in the xattr, the xattr may contain a file
>>> hash or signature.  In either case, the "good" value is preceded by a
>>> header.  The first byte of the xattr header indicates the type of data
>>> - hash, signature - stored in the xattr.  To support storing fs-verity
>>> signatures in the 'security.ima' xattr requires further differentiating
>>> the fs-verity signature from the existing IMA signature.
>>>
>>> In addition the signatures stored in 'security.ima' xattr, need to be
>>> disambiguated.  Instead of directly signing the fs-verity digest, a new
>>> signature version 3 is defined as the hash of the ima_file_id structure,
>>> which identifies the type of signature and the digest.
>>
>> Would it not be enough to just differentiat by the type of signature
>> rather than also bumping the version? It's still signature_v2_hdr but a
>> new type IMA_VERITY_DIGSIG is introduced there that shoud be sufficient
>> to indicate that a different method for calculating the hash is to be
>> used than for anything that existed before? sigv3 would then become the
>> more obvious veriftysig... ?
> 
> One of Eric's concerns was that, "an attacker (who controls the file's
> contents and IMA xattr) [could] replace the file with one with a

Reference: 
https://lore.kernel.org/linux-integrity/20220126000658.138345-1-zohar@linux.ibm.com/T/#m8929fa29fbdfc875dbf5f384a4c082d303d2040e

This seem to describe the root user. A restrictions of root's power 
maybe that root may not have access to the file signing key use on the 
local system... ?

> differrent content and still be able to pass the IMA check."  His

Is this a scenario of concern? : /usr/bin/foobar is signed by verity and 
there's a rule in the IMA policy that would appraise this file. Can root 
now remove /usr/bin/foobar and copy the regularly signed /usr/bin/bash 
to /usr/bin/foobar along with bash's security.ima and have it execute 
either since there's no appraise rule covering non-fsverity signatures 
or due to a rule that covers non-fsverity signatures?

Since the signature header of security.ima is not signed root could also 
just rewrite the header and modify the signature type (and also version) 
and circumvent appraisal rules specific to fsverity.

> solution was to only allow one signature version on a running system.
> For the complete description of the attack, refer to Eric's comments on
> v3.


I am trying to figure out a concrete scenario that one has to defend 
against what seems to be the power of the root user. A more concrete 
example may be helpful.

> 
> Instead of only allowing one signature version on a running system,
> subsequent versions of this patch set addressed his concern, by
> limiting the signature version based on policy.
>