Re: [PATCH v9 6/7] ima: support fs-verity file digest based version 3 signatures

[Stefan Berger]

On 5/5/22 08:31, 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 format version 3 is defined as the hash of the ima_file_id
> structure, which identifies the type of signature and the digest.
>
> The IMA policy defines "which" files are to be measured, verified, and/or
> audited.  For those files being verified, the policy rules indicate "how"
> the file should be verified.  For example to require a file be signed,
> the appraise policy rule must include the 'appraise_type' option.
>
> 	appraise_type:= [imasig] | [imasig|modsig] | [sigv3]
>             where 'imasig' is the original or signature format v2 (default),
>             where 'modsig' is an appended signature,
>             where 'sigv3' is the signature format v3.
>
> The policy rule must also indicate the type of digest, if not the IMA
> default, by first specifying the digest type:
>
> 	digest_type:= [verity]
>
> The following policy rule requires fsverity signatures.  The rule may be
> constrained, for example based on a fsuuid or LSM label.
>
>        appraise func=BPRM_CHECK digest_type=verity appraise_type=sigv3
>
> Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>

Acked-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>