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

Roberto Sassu
2017-06-13T12:37:37Z

On 6/13/2017 2:09 PM, Mimi Zohar wrote:
> On Tue, 2017-06-13 at 09:27 +0200, Roberto Sassu wrote:
>> 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.
>
> Or the testing patches could relax the ENFORCE_BUFEND requirement.
> Without the ENFORCE_BUFEND requirement, I'm not sure this patch (5/7)
> will be needed.

The ENFORCE_BUFEND bit is set in patch 2/7:
---
+ enforce_mask |= (count == khdr->count) ? ENFORCE_BUFEND : 0;
---

If this requirement is too strict for restoring measurements,
then patch 2/7 should be modified.

Regarding this patch, I agree that it is not strictly necessary.
With an userspace tool, it would be possible to prepend the
kexec header to the binary list after parsing the entries.

>> Should I just send the modified patch with the [RESEND] tag
>> or should I send the whole patch set with an incremented version
>> number?
>
> The testing patches could be upstreamed separately, if you prefer.

Ok.

Thanks

Roberto

>> 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?
>
> That's fine. I've pushed patches 1 - 3, and 7 to the next-4.12-
> ima_parse_buf branch for a day or so, before staging them in the next
> branch to be upstreamed.
>
> The patches can be found here
> https://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-
> integrity.git/next-4.12-ima_parse_buf.
>
> Mimi
>

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