Re: [PATCH v7 2/8] IMA: add support to measure buffer data hash

[Tushar Sugandhi <tusharsu@linux.microsoft.com>]

On 2020-12-10 2:38 p.m., Tyler Hicks wrote:
> On 2020-12-09 11:42:06, Tushar Sugandhi wrote:
>> The original IMA buffer data measurement sizes were small (e.g. boot
>> command line), but the new buffer data measurement use cases have data
>> sizes that are a lot larger.  Just as IMA measures the file data hash,
>> not the file data, IMA should similarly support the option for measuring
>> the hash of the buffer data.
>>
>> Measuring in-memory buffer-data/buffer-data-hash is different than
>> measuring file-data/file-data-hash. For the file, IMA stores the
>> measurements in both measurement log and the file's extended attribute -
>> which can later be used for appraisal as well. For buffer, the
>> measurements are only stored in the IMA log, since the buffer has no
>> extended attributes associated with it.
>>
>> Introduce a boolean parameter measure_buf_hash to support measuring
>> hash of a buffer, which would be much smaller, instead of the buffer
>> itself.
>>
>> Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
>> ---
>>   security/integrity/ima/ima.h                 |  3 +-
>>   security/integrity/ima/ima_appraise.c        |  2 +-
>>   security/integrity/ima/ima_asymmetric_keys.c |  2 +-
>>   security/integrity/ima/ima_main.c            | 36 +++++++++++++++++---
>>   security/integrity/ima/ima_queue_keys.c      |  3 +-
>>   5 files changed, 38 insertions(+), 8 deletions(-)
>>
>> diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
>> index e5622ce8cbb1..fa3044a7539f 100644
>> --- a/security/integrity/ima/ima.h
>> +++ b/security/integrity/ima/ima.h
>> @@ -268,7 +268,8 @@ void ima_store_measurement(struct integrity_iint_cache *iint, struct file *file,
>>   			   struct ima_template_desc *template_desc);
>>   void process_buffer_measurement(struct inode *inode, const void *buf, int size,
>>   				const char *eventname, enum ima_hooks func,
>> -				int pcr, const char *func_data);
>> +				int pcr, const char *func_data,
>> +				bool measure_buf_hash);
>>   void ima_audit_measurement(struct integrity_iint_cache *iint,
>>   			   const unsigned char *filename);
>>   int ima_alloc_init_template(struct ima_event_data *event_data,
>> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
>> index 8361941ee0a1..46ffa38bab12 100644
>> --- a/security/integrity/ima/ima_appraise.c
>> +++ b/security/integrity/ima/ima_appraise.c
>> @@ -352,7 +352,7 @@ int ima_check_blacklist(struct integrity_iint_cache *iint,
>>   		if ((rc == -EPERM) && (iint->flags & IMA_MEASURE))
>>   			process_buffer_measurement(NULL, digest, digestsize,
>>   						   "blacklisted-hash", NONE,
>> -						   pcr, NULL);
>> +						   pcr, NULL, false);
>>   	}
>>   
>>   	return rc;
>> diff --git a/security/integrity/ima/ima_asymmetric_keys.c b/security/integrity/ima/ima_asymmetric_keys.c
>> index 1c68c500c26f..a74095793936 100644
>> --- a/security/integrity/ima/ima_asymmetric_keys.c
>> +++ b/security/integrity/ima/ima_asymmetric_keys.c
>> @@ -60,5 +60,5 @@ void ima_post_key_create_or_update(struct key *keyring, struct key *key,
>>   	 */
>>   	process_buffer_measurement(NULL, payload, payload_len,
>>   				   keyring->description, KEY_CHECK, 0,
>> -				   keyring->description);
>> +				   keyring->description, false);
>>   }
>> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
>> index e76ef4bfd0f4..03aad13e9e70 100644
>> --- a/security/integrity/ima/ima_main.c
>> +++ b/security/integrity/ima/ima_main.c
>> @@ -779,7 +779,7 @@ int ima_post_load_data(char *buf, loff_t size,
>>   }
>>   
>>   /*
>> - * process_buffer_measurement - Measure the buffer to ima log.
>> + * process_buffer_measurement - Measure the buffer or the buffer data hash
>>    * @inode: inode associated with the object being measured (NULL for KEY_CHECK)
>>    * @buf: pointer to the buffer that needs to be added to the log.
>>    * @size: size of buffer(in bytes).
>> @@ -787,12 +787,23 @@ int ima_post_load_data(char *buf, loff_t size,
>>    * @func: IMA hook
>>    * @pcr: pcr to extend the measurement
>>    * @func_data: private data specific to @func, can be NULL.
>> + * @measure_buf_hash: measure buffer hash
>>    *
>> - * Based on policy, the buffer is measured into the ima log.
>> + * Measure the buffer into the IMA log, and extend the @pcr.
>> + *
>> + * Determine what buffers are allowed to be measured, based on the policy rules
>> + * and the IMA hook passed using @func.
>> + *
>> + * Use @func_data, if provided, to match against the measurement policy rule
>> + * data for @func.
>> + *
>> + * If @measure_buf_hash is set to true - measure hash of the buffer data,
>> + * else measure the buffer data itself.
>>    */
>>   void process_buffer_measurement(struct inode *inode, const void *buf, int size,
>>   				const char *eventname, enum ima_hooks func,
>> -				int pcr, const char *func_data)
>> +				int pcr, const char *func_data,
>> +				bool measure_buf_hash)
>>   {
>>   	int ret = 0;
>>   	const char *audit_cause = "ENOMEM";
>> @@ -807,6 +818,8 @@ void process_buffer_measurement(struct inode *inode, const void *buf, int size,
>>   		struct ima_digest_data hdr;
>>   		char digest[IMA_MAX_DIGEST_SIZE];
>>   	} hash = {};
>> +	char buf_hash[IMA_MAX_DIGEST_SIZE];
>> +	int buf_hash_len = hash_digest_size[ima_hash_algo];
>>   	int violation = 0;
>>   	int action = 0;
>>   	u32 secid;
>> @@ -849,6 +862,20 @@ void process_buffer_measurement(struct inode *inode, const void *buf, int size,
>>   		goto out;
>>   	}
>>   
>> +	if (measure_buf_hash) {
>> +		memcpy(buf_hash, hash.hdr.digest, buf_hash_len);
>> +
>> +		ret = ima_calc_buffer_hash(buf_hash, buf_hash_len,
>> +					   iint.ima_hash);
>> +		if (ret < 0) {
>> +			audit_cause = "measure_buf_hash_error";
>> +			goto out;
>> +		}
>> +
>> +		event_data.buf = buf_hash;
>> +		event_data.buf_len = buf_hash_len;
>> +	}
>> +
>>   	ret = ima_alloc_init_template(&event_data, &entry, template);
>>   	if (ret < 0) {
>>   		audit_cause = "alloc_entry";
> 
> A few more lines below, not present in this context, is a call to
> ima_store_template() with buf as the fourth parameter passed in. That
> parameter eventually makes its way to integrity_audit_message() and ends
> up as part of an audit message as the value of the "name=" field. This
> is usually a filename, the name of a key, or a kexec cmdline. In the
> case of measuring SELinux policy, do we want the entire buf to be
> included in the audit message?
> 
> Tyler
> 
Great catch.
We obviously don't want to include the entire buf in the audit message,
especially when the measure_buf_hash is set to true. (the buffer being
measured is expected to be large in that case)

How about the following? Does it look ok to you? Mimi?

if (measure_buf_hash)
     ret = ima_store_template(entry, violation, NULL, buf_hash, pcr);
else
     ret = ima_store_template(entry, violation, NULL, buf, pcr);

~Tushar