Re: [PATCH 5/7] ima: add securityfs interface to save a measurements list with kexec header

Roberto Sassu
2017-06-13T07:27:37Z

On 6/6/2017 3:23 PM, Mimi Zohar wrote:
> On Tue, 2017-06-06 at 14:45 +0200, Roberto Sassu wrote:
>> On 6/6/2017 12:56 PM, Mimi Zohar wrote:
>>> On Tue, 2017-06-06 at 10:49 +0200, Roberto Sassu wrote:
>>>> On 6/5/2017 8:04 AM, Mimi Zohar wrote:
>>>>> On Tue, 2017-05-16 at 14:53 +0200, Roberto Sassu wrote:
>>>>>> Through the new interface binary_kexec_runtime_measurements, it will be
>>>>>> possible to read the same content returned by binary_runtime_measurements,
>>>>>> with the kexec header prepended.
>>>>>>
>>>>>> The new interface has been added for testing ima_restore_measurement_list()
>>>>>> which, at the moment, works only on PPC systems. The interface for reading
>>>>>> the binary list with the kexec header will be provided in a separate patch.
>>>>>>
>>>>>> The patch reuses ima_measurements_start() and ima_measurements_next()
>>>>>> to send the measurements list to userspace. Their behavior changes
>>>>>> depending on the current dentry.
>>>>>>
>>>>>> To provide the correct information in the kexec header,
>>>>>> ima_measurements_start() has to iterate over the whole list and calculate
>>>>>> the number of entries and the total size. It is not possible to read
>>>>>> the value of the global variable binary_runtime_size and ima_htable.len
>>>>>> without taking ima_extend_list_mutex, because there might have been a list
>>>>>> add between the two read operations.
>>>>>
>>>>> Please correct me if I'm wrong. Your code walks the measurement list
>>>>> calculating the total number of measurements and the memory size
>>>>> needed to store in the kexec header. Can't there be additional
>>>>> measurements between the time these values - total number of
>>>>> measurements and memory needed - were calculated and actually saving
>>>>> the measurements? How would that be any different than the problem
>>>>> you're trying to solve? In both cases, the number of measurements
>>>>> might be less than the actual number of measurements.
>>>>>
>>>>> As long as you query the number of measurements before getting the
>>>>> memory needed, unless you're trying to verify a TPM quote, having
>>>>> fewer measurements shouldn't be a problem for testing.
>>>>
>>>> The problem is that the total number of entries and the required
>>>> memory size might be inconsistent without taking ima_extend_list_mutex.
>>>> ima_measurements_start() could read the entries counter before
>>>> it is incremented by ima_add_digest_entry() and the required memory
>>>> size after it is updated. If this happens, the parser returns an error
>>>> because ENFORCE_BUFEND is set for the last entry and there would be
>>>> still data to read (the new entry added to the list).
>>>
>>> I don't see this as being any different than what happens when the
>>> kernel saves the measurement list. Originally, the memory size was
>>> defined at kexec load, but only populated at kexec execute. There was
>>> plenty of time between the kexec load and execute for additional
>>> measurement records to be added.
>>>
>>> The upstreamed version defines the buffer size and populates it at
>>> kexec load. However kexec load itself generates additional
>>> measurements, so it has to reserve more memory than what is returned
>>> by ima_get_binary_runtime_size(). (Refer to ima_add_kexec_buffer.)
>>
>> ima_dump_measurement_list() determines the total number of entries and
>> the required memory size (which are written to the kexec header) after
>> iterating over the whole list. Are new entries added to the kexec buffer
>> after ima_dump_measurement_list() is called?
>
> The upstreamed version allocates the segment in kexec load and then
> fills the buffer. However, in between getting the current memory size
> needed and filling the buffer, additional measurements can be added.
> Thus the segment size needs to be larger than the current memory
> size.
>
> The header reflects the number of measurements and the actual buffer
> size, not the segment size. When restoring the measurement list,
> however, we rely on the number of measurements and use the buffer size
> as a reference to prevent accessing memory beyond the buffer. The
> buffer size does not need to be exact.

In this case, I have to modify patch 2/7 and remove ENFORCE_BUFEND
from the enforcing mask. Otherwise, ima_restore_measurement_list()
would return an error when parsing the last entry and buffer size
in the kexec header is greater than the exact size required to
store the measurements list.

Should I just send the modified patch with the [RESEND] tag
or should I send the whole patch set with an incremented version
number?

Also, since patches 4-6 are for testing, I would prefer to skip
them for now and push a new version of the patch set for the
Crypto Agile format first.

Thanks

Roberto

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