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

[Tyler Hicks <tyhicks@linux.microsoft.com>]

On 2020-12-10 17:21:19, Tushar Sugandhi wrote:
> 
> 
> 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);

I think that looks fine. You could also just pass event_data.buf to
ima_store_template().

Tyler

> 
> ~Tushar
>