linux-kernel - Re: [PATCH v7 3/5] ima: permit fsverity's file digests in the IMA measurement list

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Date:   Tue, 5 Apr 2022 19:28:19 +0000
From:   Eric Biggers <ebiggers@...nel.org>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     linux-integrity@...r.kernel.org,
        Stefan Berger <stefanb@...ux.ibm.com>,
        linux-fscrypt@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 3/5] ima: permit fsverity's file digests in the IMA
 measurement list

On Fri, Mar 25, 2022 at 06:38:22PM -0400, Mimi Zohar wrote:
> Permit fsverity's file digest (a hash of struct fsverity_digest) to be
> included in the IMA measurement list, based on the new measurement
> policy rule 'digest_type=verity' option.

"fsverity's file digest" *is* 'struct fsverity_digest', not a hash of it.
Did you mean to write 'struct fsverity_descriptor'?

> diff --git a/Documentation/security/IMA-templates.rst b/Documentation/security/IMA-templates.rst
> index 1a91d92950a7..2d4789dc7750 100644
> --- a/Documentation/security/IMA-templates.rst
> +++ b/Documentation/security/IMA-templates.rst
> @@ -68,6 +68,9 @@ descriptors by adding their identifier to the format string
>   - 'd-ng': the digest of the event, calculated with an arbitrary hash
>     algorithm (field format: [<hash algo>:]digest, where the digest
>     prefix is shown only if the hash algorithm is not SHA1 or MD5);
> + - 'd-ngv2': same as d-ng, but prefixed with the digest type.
> +    field format: [<digest type>:<hash algo>:]digest,
> +        where the digest type is either "ima" or "verity".

As in patch 2, it is not clear what the square brackets mean here.  Maybe they
mean that "<digest type>:<hash algo>:" is optional, but it is not explained when
they will be present and when they will not be present.

>   - 'd-modsig': the digest of the event without the appended modsig;
>   - 'n-ng': the name of the event, without size limitations;
>   - 'sig': the file signature, or the EVM portable signature if the file
> @@ -106,3 +109,8 @@ currently the following methods are supported:
>     the ``ima_template=`` parameter;
>   - register a new template descriptor with custom format through the kernel
>     command line parameter ``ima_template_fmt=``.
> +
> +
> +References
> +==========
> +[1] Documentation/filesystems/fsverity.rst

Is this meant to be a footnote?  There are no references to it above.

> @@ -242,14 +267,29 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,
>  	 */
>  	i_version = inode_query_iversion(inode);
>  	hash.hdr.algo = algo;
> +	hash.hdr.length = hash_digest_size[algo];
>  
>  	/* Initialize hash digest to 0's in case of failure */
>  	memset(&hash.digest, 0, sizeof(hash.digest));
>  
> -	if (buf)
> +	if (buf) {
>  		result = ima_calc_buffer_hash(buf, size, &hash.hdr);
> -	else
> +	} else if (iint->flags & IMA_VERITY_REQUIRED) {
> +		result = ima_get_verity_digest(iint, &hash);
> +		switch (result) {
> +		case 0:
> +			break;
> +		case -ENODATA:
> +			audit_cause = "no-verity-digest";
> +			result = -EINVAL;
> +			break;
> +		default:
> +			audit_cause = "invalid-verity-digest";
> +			break;
> +		}
> +	} else {
>  		result = ima_calc_file_hash(file, &hash.hdr);
> +	}
>  
>  	if (result && result != -EBADF && result != -EINVAL)
>  		goto out;

The above code only calls ima_get_verity_digest() if 'buf' is non-NULL,
otherwise it calls ima_calc_buffer_hash().  Under what circumstances is 'buf'
non-NULL?  Does this imply that 'digest_type=verity' does not always use verity
digests, and if not, when are they used and when are they not used?

> +/*
> + * Make sure the policy rule and template format are in sync.
> + */
> +static void check_template_field(const struct ima_template_desc *template,
> +				 const char *field, const char *msg)
> +{
> +	int i;
> +
> +	for (i = 0; i < template->num_fields; i++)
> +		if (!strcmp(template->fields[i]->field_id, field))
> +			return;
> +
> +	pr_notice_once("%s", msg);
> +}

A better description for this function would be something like "Warn if the
template does not contain the given field."

> index daf49894fd7d..d42a01903f08 100644
> --- a/security/integrity/integrity.h
> +++ b/security/integrity/integrity.h
> @@ -32,7 +32,7 @@
>  #define IMA_HASHED		0x00000200
>  
>  /* iint policy rule cache flags */
> -#define IMA_NONACTION_FLAGS	0xff000000
> +#define IMA_NONACTION_FLAGS	0xff800000
>  #define IMA_DIGSIG_REQUIRED	0x01000000
>  #define IMA_PERMIT_DIRECTIO	0x02000000
>  #define IMA_NEW_FILE		0x04000000
> @@ -40,6 +40,7 @@
>  #define IMA_FAIL_UNVERIFIABLE_SIGS	0x10000000
>  #define IMA_MODSIG_ALLOWED	0x20000000
>  #define IMA_CHECK_BLACKLIST	0x40000000
> +#define IMA_VERITY_REQUIRED	0x80000000

It is intentional that the new bit added to IMA_NONACTION_FLAGS is not the same
as IMA_VERITY_REQUIRED?

- Eric