Re: [PATCH v3 ima-evm-utils] extend ima_measurement --pcrs option to support per-bank pcr files

[Mimi Zohar <zohar@linux.ibm.com>]

On Fri, 2020-07-24 at 10:52 -0400, Stephen Smalley wrote:
> Extend the ima_measurement --pcrs option to support per-bank pcr files.
> The extended syntax is "--pcrs algorithm,pathname".  If no algorithm
> is specified, it defaults to sha1 as before.  Multiple --pcrs options
> are now supported, one per bank of PCRs. The file format remains
> unchanged.  If --pcrs is specified, only try to read PCRs from the
> specified file(s); do not fall back to trying to read from sysfs
> or the TPM itself in this case since the user requested use of
> the files.
> 
> Create per-bank pcr files, depends on "tpm: add sysfs exports for all
> banks of PCR registers" kernel patch:
> $ cat tpm2pcrread.sh
> #!/bin/sh
> for alg in sha1 sha256
> do
>   rm -f pcr-$alg
>   pcr=0;
>   while [ $pcr -lt 24 ];
>   do
>     printf "PCR-%02d: " $pcr >> pcr-$alg;
>     cat /sys/class/tpm/tpm0/pcr-$alg/$pcr >> pcr-$alg;
>     pcr=$[$pcr+1];
>   done
> done
> $ sh ./tpm2pcrread.sh
> 
> Pass only the sha1 PCRs to evmctl defaulting to sha1:
> $ sudo evmctl ima_measurement --pcrs pcr-sha1 /sys/kernel/security/integrity/ima/binary_runtime_measurements
> 
> Pass only the sha1 PCRs to evmctl with explicit selection of sha1:
> $ sudo evmctl ima_measurement --pcrs sha1,pcr-sha1 /sys/kernel/security/integrity/ima/binary_runtime_measurements
> 
> Pass both sha1 and sha256 PCRs to evmctl:
> $ sudo evmctl ima_measurement --pcrs sha1,pcr-sha1 --pcrs sha256,pcr-sha256 /sys/kernel/security/integrity/ima/binary_runtime_measurements
> 
> Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>

I reviewed the patch and everything looks good, other than a few
comments below.  Weirdly the order in which the TPM bank files
containing the PCRs are supplied on the command line is affecting
being able to verify the measurement list - sha1, sha256 versus
sha256, sha1.  Perhaps during the time it takes to read the different
banks, the PCRs have changed?  Not quite sure what is happening.
 
> ---
> v3 splits reading the pcr files and the sysfs files into two separate functions
> each called from read_tpm_banks() with different error handling. Thus, if
> --pcrs is specified, then evmctl will only try to read from the specified
> file(s) and any errors will cause it to fail rather than trying to read
> directly from the TPM.  If --pcrs is not specified, then evmctl will try
> to read from the sysfs files and then fall back to reading directly
> from the TPM if that fails. v3 also reduces the limit on the number of
> PCRs read from the files to NUM_PCRS since otherwise it will overflow
> the pcrs[] array.  Further improvements to the file parser are deferred
> to future patches.
> 
> @@ -1829,20 +1783,108 @@ static void extend_tpm_banks(struct template_entry *entry, int num_banks,
>  #endif
>  }
>  
> -/* Read TPM 1.2 PCRs */
> -static int read_tpm_pcrs(int num_banks, struct tpm_bank_info *tpm_banks)
> +static int read_one_bank(struct tpm_bank_info *tpm_bank, FILE *fp)
>  {
> -	int i;
> +	char *p, pcr_str[8], buf[80];

As log as we're changing this from 70 to 80, might as well define it
as MAX_DITEST_SIZE * 2 + X.  I'm pretty sure support for additional,
larger digests will be added.  This can be deferred to the subsequent
patch. 

> +	int i = 0;
> +	int result = -1;
> +	for (;;) {
> +		p = fgets(buf, sizeof(buf), fp);
> +		if (!p || i >= NUM_PCRS)
> +			break;
> +		sprintf(pcr_str, "PCR-%2.2d", i);
> +		if (!strncmp(p, pcr_str, 6))
> +			hex2bin(tpm_bank->pcr[i++], p + 7, tpm_bank->digest_size);
> +		result = 0;
> +	}
> +	return result;
> +}
> +
> +static char *pcrs = "/sys/class/tpm/tpm0/device/pcrs";  /* Kernels >= 4.0 */
> +static char *misc_pcrs = "/sys/class/misc/tpm0/device/pcrs";
> +
> +/* Read one of the TPM 1.2 sysfs files if present */
> +static int read_sysfs_pcrs(int num_banks, struct tpm_bank_info *tpm_banks)
> +{
> +	FILE *fp;
> +	int i, result;
>  
> -	if (tpm_pcr_read(tpm_banks, SHA_DIGEST_LENGTH)) {
> -		log_debug("Failed to read TPM 1.2 PCRs.\n");
> +	fp = fopen(pcrs, "r");
> +	if (!fp)
> +		fp = fopen(misc_pcrs, "r");
> +	if (!fp)
>  		return -1;
> -	}
>  
> +	result = read_one_bank(&tpm_banks[0], fp);
> +	fclose(fp);
> +	if (result < 0)
> +		return result;
>  	tpm_banks[0].supported = 1;
>  	for (i = 1; i < num_banks; i++)
>  		tpm_banks[i].supported = 0;
>  	return 0;
> +
> +}
> +
> +/* Read PCRs from per-bank file(s) specified via --pcrs */
> +static int read_file_pcrs(int num_banks, struct tpm_bank_info *tpm_banks)
> +{
> +	struct stat s;
> +	FILE *fp;
> +	char *p;
> +	const char *alg, *path;
> +	int i, j, bank, result;
> +
> +	for (i = 0; i < num_banks; i++)
> +		tpm_banks[i].supported = 0;
> +
> +	for (i = 0; i < npcrfile; i++) {

Being able to provide more PCR files than banks is a bit strange.  I
guess the TPM could export additional banks, than what is currently
supported by ima-evm-utils, and programmatically supply them to ima-
evm-utils.

Mimi

> +		p = strchr(pcrfile[i], ',');
> +		if (p) {
> +			*p = 0;
> +			alg = pcrfile[i];
> +			path = ++p;
> +		} else {
>