Re: [RFC PATCH v2 03/13] ima: invalidate unsupported PCR banks

[Mimi Zohar]

> diff --git a/security/integrity/ima/ima_crypto.c b/security/integrity/ima/ima_crypto.c
> index 6f5696d999d0..a43080fb8edc 100644
> --- a/security/integrity/ima/ima_crypto.c
> +++ b/security/integrity/ima/ima_crypto.c
> @@ -625,26 +625,43 @@ int ima_calc_field_array_hash(struct ima_field_data *field_data,
>  	u16 alg_id;
>  	int rc, i;
>  
> +#if IS_ENABLED(CONFIG_IMA_COMPAT_FALLBACK_TPM_EXTEND)
>  	rc = ima_calc_field_array_hash_tfm(field_data, entry, ima_sha1_idx);
>  	if (rc)
>  		return rc;
>  
>  	entry->digests[ima_sha1_idx].alg_id = TPM_ALG_SHA1;
> +#endif
>  
>  	for (i = 0; i < NR_BANKS(ima_tpm_chip) + ima_extra_slots; i++) {
> +#if IS_ENABLED(CONFIG_IMA_COMPAT_FALLBACK_TPM_EXTEND)
>  		if (i == ima_sha1_idx)
>  			continue;
> +#endif
>  
>  		if (i < NR_BANKS(ima_tpm_chip)) {
>  			alg_id = ima_tpm_chip->allocated_banks[i].alg_id;
>  			entry->digests[i].alg_id = alg_id;
>  		}
>  
> -		/* for unmapped TPM algorithms digest is still a padded SHA1 */
> +		/*
> +		 * For unmapped TPM algorithms, the digest is still a
> +		 * padded SHA1 if backwards-compatibility fallback PCR
> +		 * extension is enabled. Otherwise fill with
> +		 * 0xfes. This is the value to invalidate unsupported
> +		 * PCR banks with. Also, a non-all-zeroes value serves
> +		 * as an indicator to kexec measurement restoration
> +		 * that the entry is not a violation and all its
> +		 * template digests need to get recomputed.
> +		 */
>  		if (!ima_algo_array[i].tfm) {
> +#if IS_ENABLED(CONFIG_IMA_COMPAT_FALLBACK_TPM_EXTEND)
>  			memcpy(entry->digests[i].digest,
>  			       entry->digests[ima_sha1_idx].digest,
>  			       TPM_DIGEST_SIZE);
> +#else
> +			memset(entry->digests[i].digest, 0xfe, TPM_DIGEST_SIZE);
> +#endif

Using TPM_DIGEST_SIZE will result in a padded 0xfe value.

>  			continue;
>  		}
>