linux-kernel - Re: [PATCH] ima: extend boot_aggregate with kernel measurements

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Message-ID: <1592920990.5437.15.camel@linux.ibm.com>
Date:   Tue, 23 Jun 2020 10:03:10 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Maurizio Drocco <maurizio.drocco@....com>
Cc:     Silviu.Vlasceanu@...wei.com, dmitry.kasatkin@...il.com,
        jejb@...ux.ibm.com, jmorris@...ei.org,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, mdrocco@...ux.vnet.ibm.com,
        roberto.sassu@...wei.com, serge@...lyn.com
Subject: Re: [PATCH] ima: extend boot_aggregate with kernel measurements

Hi Maurizio,

When re-posting patches, please include the version number (e.g.
[PATCH v4] ima: ... ).

On Mon, 2020-06-22 at 00:50 -0400, Maurizio Drocco wrote:
> IMA is not considering TPM registers 8-9 when calculating the boot
> aggregate.

This line is unnecessary with the following change.

> When registers 8-9 are used to store measurements of the
> kernel and its command line (e.g., grub2 bootloader with tpm module
> enabled), IMA should include them in the boot aggregate.

The "When" clause makes this sound like PCRs 8 & 9 are not always
included.  I would split this into two sentences.

>  Registers
> 8-9 are only included in non-SHA1 boot_aggregate digests to avoid
> ambiguity.
> 
> Signed-off-by: Maurizio Drocco <maurizio.drocco@....com>
> ---

Missing "Changelog:".

Changelog:
v2: 
- Limit including PCRs 8 & 9 to non-sha1 hashes
v1:
- Include non zero PCRs 8 & 9 in the boot_aggregate

>  security/integrity/ima/ima.h        |  2 +-
>  security/integrity/ima/ima_crypto.c | 15 ++++++++++++++-
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
> index df93ac258e01..9d94080bdad8 100644
> --- a/security/integrity/ima/ima.h
> +++ b/security/integrity/ima/ima.h
> @@ -30,7 +30,7 @@
>  
>  enum ima_show_type { IMA_SHOW_BINARY, IMA_SHOW_BINARY_NO_FIELD_LEN,
>  		     IMA_SHOW_BINARY_OLD_STRING_FMT, IMA_SHOW_ASCII };
> -enum tpm_pcrs { TPM_PCR0 = 0, TPM_PCR8 = 8 };
> +enum tpm_pcrs { TPM_PCR0 = 0, TPM_PCR8 = 8, TPM_PCR10 = 10 };
>  
>  /* digest size for IMA, fits SHA1 or MD5 */
>  #define IMA_DIGEST_SIZE		SHA1_DIGEST_SIZE
> diff --git a/security/integrity/ima/ima_crypto.c b/security/integrity/ima/ima_crypto.c
> index 220b14920c37..d02917d85033 100644
> --- a/security/integrity/ima/ima_crypto.c
> +++ b/security/integrity/ima/ima_crypto.c
> @@ -823,13 +823,26 @@ static int ima_calc_boot_aggregate_tfm(char *digest, u16 alg_id,
>  	if (rc != 0)
>  		return rc;
>  
> -	/* cumulative sha1 over tpm registers 0-7 */
> +	/* cumulative digest over tpm registers 0-7 */

Please uppercase "tpm" here and below.

>  	for (i = TPM_PCR0; i < TPM_PCR8; i++) {
>  		ima_pcrread(i, &d);
>  		/* now accumulate with current aggregate */
>  		rc = crypto_shash_update(shash, d.digest,
>  					 crypto_shash_digestsize(tfm));
>  	}
> +	/*
> +	 * extend cumulative digest over tpm registers 8-9, which contain
> +	 * measurement for the kernel command line (reg. 8) and image (reg. 9)
> +	 * in a typical PCR allocation. Registers 8-9 are only included in
> +	 * non-SHA1 boot_aggregate digests to avoid ambiguity.
> +	 */

Comments that are full sentences should start with an uppercase letter
and end with a period (e.g. Extend).

thanks,

Mimi

> +	if (alg_id != TPM_ALG_SHA1) {
> +		for (i = TPM_PCR8; i < TPM_PCR10; i++) {
> +			ima_pcrread(i, &d);
> +			rc = crypto_shash_update(shash, d.digest,
> +						crypto_shash_digestsize(tfm));
> +		}
> +	}
>  	if (!rc)
>  		crypto_shash_final(shash, digest);
>  	return rc;