Re: [PATCH v3 1/3] ima: use ahash API for file hash calculation

[Dmitry Kasatkin]

On 07/07/14 14:56, Mimi Zohar wrote:
> On Fri, 2014-07-04 at 15:05 +0300, Dmitry Kasatkin wrote: 
>> Async hash API allows to use HW acceleration for hash calculation.
>> It may give significant performance gain or/and reduce power consumption,
>> which might be very beneficial for battery powered devices.
>>
>> This patch introduces hash calculation using ahash API.
>>
>> ahash performance depends on data size and particular HW. Under certain
>> limit, depending on the system, shash performance may be better.
>>
>> This patch defines 'ahash_minsize' module parameter which can be used to
>> define minimal file size to use with ahash. When file size is not set
>> or smaller than defined by the parameter, shash will be used.
>>
>> Changes in v3:
>> - kernel parameter replaced with module parameter
>> - pr_crit replaced with pr_crit_ratelimited
>>
>> Changes in v2:
>> - ima_ahash_size became as ima_ahash
>> - ahash pre-allocation moved out from __init code to be able to use
>>   ahash crypto modules. Ahash allocated once on the first use.
>> - hash calculation falls back to shash if ahash allocation/calculation fails
>> - complex initialization separated from variable declaration
>> - improved comments
>>
>> Signed-off-by: Dmitry Kasatkin <d.kasatkin@xxxxxxxxxxx>
>> ---
>>  Documentation/kernel-parameters.txt |   9 ++
>>  security/integrity/ima/ima_crypto.c | 185 +++++++++++++++++++++++++++++++++++-
>>  2 files changed, 190 insertions(+), 4 deletions(-)
>>
>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>> index 5a214a3..03c8452 100644
>> --- a/Documentation/kernel-parameters.txt
>> +++ b/Documentation/kernel-parameters.txt
>> @@ -1318,6 +1318,15 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>  			Formats: { "ima" | "ima-ng" }
>>  			Default: "ima-ng"
>>
>> +	ima.ahash_minsize= [IMA] Minimum file size for asynchronous hash usage
>> +			Format: <min_file_size>
>> +			Set the minimal file size for using asynchronous hash.
>> +			If left unspecified, ahash usage is disabled.
>> +
>> +			ahash performance varies for different data sizes on
>> +			different crypto accelerators. This option can be used
>> +			to achieve best performance for particular HW.
>> +
>>  	init=		[KNL]
>>  			Format: <full_path>
>>  			Run specified binary instead of /sbin/init as init
>> diff --git a/security/integrity/ima/ima_crypto.c b/security/integrity/ima/ima_crypto.c
>> index f82d1d7..bc38160 100644
>> --- a/security/integrity/ima/ima_crypto.c
>> +++ b/security/integrity/ima/ima_crypto.c
>> @@ -16,6 +16,8 @@
>>  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
>>
>>  #include <linux/kernel.h>
>> +#include <linux/moduleparam.h>
>> +#include <linux/ratelimit.h>
>>  #include <linux/file.h>
>>  #include <linux/crypto.h>
>>  #include <linux/scatterlist.h>
>> @@ -25,7 +27,18 @@
>>  #include <crypto/hash_info.h>
>>  #include "ima.h"
>>
>> +struct ahash_completion {
>> +	struct completion completion;
>> +	int err;
>> +};
>> +
>> +/* minimum file size for ahash use */
>> +static unsigned long ima_ahash_minsize;
>> +module_param_named(ahash_minsize, ima_ahash_minsize, ulong, 0644);
>> +MODULE_PARM_DESC(ahash_minsize, "Minimum file size for ahash use");
>> +
>>  static struct crypto_shash *ima_shash_tfm;
>> +static struct crypto_ahash *ima_ahash_tfm;
>>
>>  /**
>>   * ima_kernel_read - read file content
>> @@ -93,9 +106,146 @@ static void ima_free_tfm(struct crypto_shash *tfm)
>>  		crypto_free_shash(tfm);
>>  }
>>
>> -/*
>> - * Calculate the MD5/SHA1 file digest
>> - */
>> +static struct crypto_ahash *ima_alloc_atfm(enum hash_algo algo)
>> +{
>> +	struct crypto_ahash *tfm = ima_ahash_tfm;
>> +	int rc;
>> +
>> +	if ((algo != ima_hash_algo && algo < HASH_ALGO__LAST) || !tfm) {
>> +		tfm = crypto_alloc_ahash(hash_algo_name[algo], 0, 0);
>> +		if (!IS_ERR(tfm)) {
>> +			if (algo == ima_hash_algo)
>> +				ima_ahash_tfm = tfm;
>> +		} else {
>> +			rc = PTR_ERR(tfm);
>> +			pr_err("Can not allocate %s (reason: %d)\n",
>> +			       hash_algo_name[algo], rc);
>> +		}
>> +	}
>> +	return tfm;
>> +}
>> +
>> +static void ima_free_atfm(struct crypto_ahash *tfm)
>> +{
>> +	if (tfm != ima_ahash_tfm)
>> +		crypto_free_ahash(tfm);
>> +}
>> +
>> +static void ahash_complete(struct crypto_async_request *req, int err)
>> +{
>> +	struct ahash_completion *res = req->data;
>> +
>> +	if (err == -EINPROGRESS)
>> +		return;
>> +	res->err = err;
>> +	complete(&res->completion);
>> +}
>> +
>> +static int ahash_wait(int err, struct ahash_completion *res)
>> +{
>> +	switch (err) {
>> +	case 0:
>> +		break;
>> +	case -EINPROGRESS:
>> +	case -EBUSY:
>> +		wait_for_completion(&res->completion);
>> +		reinit_completion(&res->completion);
>> +		err = res->err;
>> +		/* fall through */
>> +	default:
>> +		pr_crit_ratelimited("ahash calculation failed: err: %d\n", err);
>> +	}
>> +
>> +	return err;
>> +}
>> +
>> +static int ima_calc_file_hash_atfm(struct file *file,
>> +				   struct ima_digest_data *hash,
>> +				   struct crypto_ahash *tfm)
>> +{
>> +	loff_t i_size, offset;
>> +	char *rbuf;
>> +	int rc, read = 0, rbuf_len;
>> +	struct ahash_request *req;
>> +	struct scatterlist sg[1];
>> +	struct ahash_completion res;
>> +
>> +	hash->length = crypto_ahash_digestsize(tfm);
>> +
>> +	req = ahash_request_alloc(tfm, GFP_KERNEL);
>> +	if (!req)
>> +		return -ENOMEM;
>> +
>> +	init_completion(&res.completion);
>> +	ahash_request_set_callback(req, CRYPTO_TFM_REQ_MAY_BACKLOG |
>> +				   CRYPTO_TFM_REQ_MAY_SLEEP,
>> +				   ahash_complete, &res);
>> +
>> +	rc = ahash_wait(crypto_ahash_init(req), &res);
>> +	if (rc)
>> +		goto out1;
>> +
>> +	i_size = i_size_read(file_inode(file));
>> +
>> +	if (i_size == 0)
>> +		goto out2;
>> +
>> +	rbuf = kzalloc(PAGE_SIZE, GFP_KERNEL);
>> +	if (!rbuf) {
>> +		rc = -ENOMEM;
>> +		goto out1;
>> +	}
>> +
>> +	if (!(file->f_mode & FMODE_READ)) {
>> +		file->f_mode |= FMODE_READ;
>> +		read = 1;
>> +	}
>> +
>> +	for (offset = 0; offset < i_size; offset += rbuf_len) {
>> +		rbuf_len = ima_kernel_read(file, offset, rbuf, PAGE_SIZE);
>> +		if (rbuf_len < 0) {
>> +			rc = rbuf_len;
>> +			break;
>> +		}
>> +		if (rbuf_len == 0)
>> +			break;
>> +
>> +		sg_init_one(&sg[0], rbuf, rbuf_len);
>> +		ahash_request_set_crypt(req, sg, NULL, rbuf_len);
>> +
>> +		rc = ahash_wait(crypto_ahash_update(req), &res);
>> +		if (rc)
>> +			break;
>> +	}
>> +	if (read)
>> +		file->f_mode &= ~FMODE_READ;
>> +	kfree(rbuf);
>> +out2:
>> +	if (!rc) {
>> +		ahash_request_set_crypt(req, NULL, hash->digest, 0);
>> +		rc = ahash_wait(crypto_ahash_final(req), &res);
>> +	}
>> +out1:
>> +	ahash_request_free(req);
>> +	return rc;
>> +}
>> +
>> +static int ima_calc_file_ahash(struct file *file, struct ima_digest_data *hash)
>> +{
>> +	struct crypto_ahash *tfm;
>> +	int rc;
>> +
>> +	tfm = ima_alloc_atfm(hash->algo);
>> +	if (IS_ERR(tfm))
>> +		return PTR_ERR(tfm);
>> +
>> +	rc = ima_calc_file_hash_atfm(file, hash, tfm);
>> +
>> +	ima_free_atfm(tfm);
>> +
>> +	return rc;
>> +}
>> +
>>  static int ima_calc_file_hash_tfm(struct file *file,
>>  				  struct ima_digest_data *hash,
>>  				  struct crypto_shash *tfm)
>> @@ -156,7 +306,7 @@ out:
>>  	return rc;
>>  }
>>
>> -int ima_calc_file_hash(struct file *file, struct ima_digest_data *hash)
>> +static int ima_calc_file_shash(struct file *file, struct ima_digest_data *hash)
>>  {
>>  	struct crypto_shash *tfm;
>>  	int rc;
>> @@ -172,6 +322,33 @@ int ima_calc_file_hash(struct file *file, struct ima_digest_data *hash)
>>  	return rc;
>>  }
>>
>> +/**
> This is the kernel-doc delimiter.
>
>> + * ima_calc_file_hash - calculae file hash
>> + *
> Missing kernel-doc argument descriptions.  Refer to
> Documentation/kernel-doc-nano-HOWTO.txt.
>

There is no need to explain arguments as they self-evident.

>> + * if ima_ahash_minsize parameter is non-zero, this function uses
>> + * ahash for hash caclulation. ahash performance varies for different
>> + * data sizes on different crypto accelerators. shash performance might
>> + * be better for small file. 'ima.ahash_minsize' module parameter allows
>> + * to specify the best value for the system.
>> + *
>> + * If ahash fails, it fallbacks to shash.
>> + */
>> +int ima_calc_file_hash(struct file *file, struct ima_digest_data *hash)
>> +{
>> +	loff_t i_size;
>> +	int rc;
>> +
>> +	i_size = i_size_read(file_inode(file));
>> +
>> +	if (ima_ahash_minsize && i_size >= ima_ahash_minsize) {
>> +		rc = ima_calc_file_ahash(file, hash);
>> +		if (!rc)
>> +			return 0;
>> +	}
>> +
>> +	return ima_calc_file_shash(file, hash);
>> +}
> If the crypto accelerator fails, it falls back to using shash.  Is their
> any indication that the HW error is intermittent or persistent?  Should
> ima_ahash_minsize be reset?

If hw constantly does not work then it is simply broken.

You want to be protected from "random" failures.

For me it is not the case either... If it works then it works...

> If the crypto accelerator, built as a kernel module, is removed,
> ima_ahash_minsize would still be set.  It would continue to use ahash.
> Is this the correct behavior? Or should ima_ahash_minsize be reset?
>
> thanks,
>
> Mimi

It cannot be removed, because it is used and module usage counter
protects from removing...


What is your worry?

>> +
>>  /*
>>   * Calculate the hash of template data
>>   */
>
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