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

[Mimi Zohar]

Hi Lakshmi, Eric,

On Mon, 2021-11-29 at 21:56 -0800, Lakshmi Ramasubramanian wrote:
> Hi Mimi,
> 
> On 11/29/2021 9:00 AM, Mimi Zohar wrote:
> > Instead of calculating a file hash and verifying the signature stored
> > in the security.ima xattr against the calculated file hash, verify the
> > signature of the fs-verity's file digest.  The fs-verity file digest is
> > a hash that includes the Merkle tree root hash.

> This patch is reading the fs-verity signature for the given file using 
> the new function fsverity_measure() that was defined in [Patch 1/4]. Is 
> it also verifying the fs-verity signature here?

Yes, the signature stored in the security.ima xattr may be a file hash,
a regular file signature, or a signature of the fs-verity file digest. 
The signature is verified like any other signature stored as an xattr.

>  
> > +static int ima_collect_verity_measurement(struct integrity_iint_cache *iint,
> > +					  struct ima_digest_data *hash)
> > +{
> > +	u8 verity_digest[FS_VERITY_MAX_DIGEST_SIZE];
> > +	enum hash_algo verity_alg;
> > +	int rc;
> > +
> > +	rc = fsverity_measure(iint->inode, verity_digest, &verity_alg);
> nit: fsverity_collect_measurement() may be more appropriate for this 
> function (defined in [PATCH 1/4]).

>From an IMA perspective it certainly would be a better function name,
but this function may be used by other kernel subsystems.  Eric
suggested renaming the function as fsverity_get_digest(), as opposed to
fsverity_read_digest().   get/put are normally used to bump a reference
count or to get/release a lock.   Perhaps a combination like
fsverity_collect_digest() would be acceptable.

thanks,

Mimi