[PATCH v5 3/7] IMA: add hook to measure critical data

[Tushar Sugandhi]

On 2020-11-06 5:24 a.m., Mimi Zohar wrote:
> Hi Tushar,
> 
> On Sun, 2020-11-01 at 14:26 -0800, Tushar Sugandhi wrote:
>> Currently, IMA does not provide a generic function for kernel subsystems
>> to measure their critical data. Examples of critical data in this context
>> could be kernel in-memory r/o structures, hash of the memory structures,
>> or data that represents a linux kernel subsystem state change. The
>> critical data, if accidentally or maliciously altered, can compromise
>> the integrity of the system.
> 
> Start out with what IMA does do (e.g. measures files and more recently
> buffer data).  Afterwards continue with kernel integrity critical data
> should also be measured.  Please include a definition of kernel
> integrity critical data here, as well as in the cover letter.
> 
Yes, will do. I will also describe what kernel integrity critical data
is – by providing a definition, and not by example -  as you suggested.
(here and in the cover letter)

>>
>> A generic function provided by IMA to measure critical data would enable
>> various subsystems with easier and faster on-boarding to use IMA
>> infrastructure and would also avoid code duplication.
> 
> By definition LSM and IMA hooks are generic with callers in appropriate
> places in the kernel.   This paragraph is redundant.
> 
Sounds good. I will remove this paragraph.
>>
>> Add a new IMA func CRITICAL_DATA and a corresponding IMA hook
>> ima_measure_critical_data() to support measuring critical data for
>> various kernel subsystems.
> 
> Instead of using the word "add", it would be more appropriate to use
> the word "define".   Define a new IMA hook named
> ima_measure_critical_data to measure kernel integrity critical data.
> Please also update the Subject line as well.  "ima: define an IMA hook
> to measure kernel integrity critical data".
> 
Sounds good. Will do.
>>
>> Signed-off-by: Tushar Sugandhi <tusharsu at linux.microsoft.com>
>> ---
>>
>> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
>> index 4485d87c0aa5..6e1b11dcba53 100644
>> --- a/security/integrity/ima/ima_main.c
>> +++ b/security/integrity/ima/ima_main.c
>> @@ -921,6 +921,44 @@ void ima_kexec_cmdline(int kernel_fd, const void *buf, int size)
>>   	fdput(f);
>>   }
>>   
>> +/**
>> + * ima_measure_critical_data - measure kernel subsystem data
>> + * critical to integrity of the kernel
> 
> Please change this to "measure kernel integrity critical data".
> 
*Question*
Thanks Mimi. Do you want us just to update the description, or do you
want us to update the function name too?

I believe you meant just description, but still want to clarify.

ima_measure_kernel_integrity_critical_data() would be too long.
Maybe ima_measure_integrity_critical_data()?

Or do you want us to keep the existing ima_measure_critical_data()?
Could you please let us know?

>> + * @event_data_source: name of the data source being measured;
>> + * typically it should be the name of the kernel subsystem that is sending
>> + * the data for measurement
> 
> Including "data_source" here isn't quite right.  "data source" should
> only be added in the first patch which uses it, not here.   When adding
> it please shorten the field description to "kernel data source".   The
> longer explanation can be included in the longer function description.
> 
*Question*
Do you mean the parameter @event_data_source should be removed from this
patch? And then later added in patch 7/7 – where SeLinux uses it?

But ima_measure_critical_data() calls process_buffer_measurement(), and
p_b_m() accepts it as part of the param @func_data.

What should we pass to p_b_m() @func_data in this patch, if we remove
@event_data_source from this patch?

>> + * @event_name: name of an event from the kernel subsystem that is sending
>> + * the data for measurement
> 
> As this is being passed to process_buffer_measurement(), this should be
> the same or similar to the existing definition.
> 
Ok. I will change this to @eventname to be consistemt with p_b_m().

>> + * @buf: pointer to buffer containing data to measure
>> + * @buf_len: length of buffer(in bytes)
>> + * @measure_buf_hash: if set to true - will measure hash of the buf,
>> + *                    instead of buf
> 
>   kernel doc requires a single line.  In this case, please shorten the
> argument definition to "measure buffer data or buffer data hash".   The
> details can be included in the longer function description.
> 
Sounds good. Will do.
>> + *
>> + * A given kernel subsystem (event_data_source) may send
>> + * data (buf) to be measured when the data or the subsystem state changes.
>> + * The state/data change can be described by event_name.
>> + * Examples of critical data (buf) could be kernel in-memory r/o structures,
>> + * hash of the memory structures, or data that represents subsystem
>> + * state change.
>> + * measure_buf_hash can be used to save space, if the data being measured
>> + * is too large.
>> + * The data (buf) can only be measured, not appraised.
>> + */
> 
> Please remove this longer function description, replacing it something
> more appropriate.  The subsequent patch that introduces the "data
> source" parameter would expand the description.
> 
> thanks,
> 
> Mimi
> 
*Question*
Hi Mimi, will do.
Do you want the data_source to be part of SeLinux patch? (patch 7/7 of
this series).
Or a separate patch before it?
~Tushar

>> +void ima_measure_critical_data(const char *event_data_source,
>> +			       const char *event_name,
>> +			       const void *buf, int buf_len,
>> +			       bool measure_buf_hash)
>> +{
>> +	if (!event_name || !event_data_source || !buf || !buf_len) {
>> +		pr_err("Invalid arguments passed to %s().\n", __func__);
>> +		return;
>> +	}
>> +
>> +	process_buffer_measurement(NULL, buf, buf_len, event_name,
>> +				   CRITICAL_DATA, 0, event_data_source,
>> +				   measure_buf_hash);
>> +}
>> +
>>   static int __init init_ima(void)
>>   {
>>   	int error;