linux-kernel - Re: [PATCH 7/8] ima: use ima_hash_algo for collision detection in the measurement list

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Date:   Thu, 30 Jan 2020 17:26:09 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>,
        jarkko.sakkinen@...ux.intel.com,
        james.bottomley@...senpartnership.com,
        linux-integrity@...r.kernel.org
Cc:     linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, silviu.vlasceanu@...wei.com
Subject: Re: [PATCH 7/8] ima: use ima_hash_algo for collision detection in
 the measurement list

On Mon, 2020-01-27 at 18:04 +0100, Roberto Sassu wrote:
> Before calculating a digest for each PCR bank, collisions were detected
> with a SHA1 digest. This patch includes ima_hash_algo among the algorithms
> used to calculate the template digest and checks collisions on that digest.
> 
> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>

Definitely needed to protect against a sha1 collision attack.

<snip>

>  
> diff --git a/security/integrity/ima/ima_api.c b/security/integrity/ima/ima_api.c
> index ebaf0056735c..a9bb45de6db9 100644
> --- a/security/integrity/ima/ima_api.c
> +++ b/security/integrity/ima/ima_api.c
> @@ -51,7 +51,7 @@ int ima_alloc_init_template(struct ima_event_data *event_data,
>  	if (!*entry)
>  		return -ENOMEM;
>  
> -	(*entry)->digests = kcalloc(ima_tpm_chip->nr_allocated_banks + 1,
> +	(*entry)->digests = kcalloc(ima_tpm_chip->nr_allocated_banks + 2,
>  				    sizeof(*(*entry)->digests), GFP_NOFS);
>  	if (!(*entry)->digests) {
>  		result = -ENOMEM;

I would prefer not having to allocate and use "nr_allocated_banks + 1"
everywhere, but I understand the need for it.  I'm not sure this patch
warrants allocating +2.  Perhaps, if a TPM bank doesn't exist for the
IMA default hash algorithm, use a different algorithm or, worst case,
continue using the ima_sha1_idx.

Mimi