Re: [PATCH 0/7] ima: carry the measurement list across kexec

[Balbir Singh <bsingharora@gmail.com>]

On 09/08/16 22:36, Mimi Zohar wrote:
> On Tue, 2016-08-09 at 15:19 +1000, Balbir Singh wrote:
>>
>> On 04/08/16 22:24, Mimi Zohar wrote:
>>> The TPM PCRs are only reset on a hard reboot.  In order to validate a
>>> TPM's quote after a soft reboot (eg. kexec -e), the IMA measurement list
>>> of the running kernel must be saved and then restored on the subsequent
>>> boot.
>>>
>>> The existing securityfs binary_runtime_measurements file conveniently
>>> provides a serialized format of the IMA measurement list. This patch
>>> set serializes the measurement list in this format and restores it.
>>>
>>> This patch set pre-req's Thiago Bauermann's "kexec_file: Add buffer
>>> hand-over for the next kernel" patch set* for actually carrying the
>>> serialized measurement list across the kexec.
>>>
>>> Mimi
>>>
>>
>> Hi, Mimi
>>
>> I am trying to convince myself of the security of the solution. I asked
>> Thiago as well, but may be I am be lagging behind in understanding.
>>
>> We trust the kernel to hand over PCR values of the old kernel (which
>> cannot be validated) to the IMA subsystem in the new kernel for storage.
>> I guess the idea is for ima_add_boot_aggregate to do the right thing?
>> How do we validate what the old kernel is giving us? Why do we care for
>> the old measurement list? Is it still of significance in the new kernel?
>>
> 
> Hi Balbir,
> 
> To validate the hardware TPM PCR values requires walking the measurement
> list simulating the TPM extend operation.  The resulting values should
> match the hardware TPM PCRs.
> 
> In the case of a soft reboot, the TPM PCRs are not reset to 0, so all
> the measurements of the running system, including those from previous
> soft reboots, need to be included in the measurement list.   Without
> these measurements, the simulated PCR values will not match the hardware
> TPM PCR values.  Thus the need for this patch set.
> 
> Measurements can not be added/removed/changed in the measurement list
> without it being detectable.
> 

Thanks Mimi

I think that makes sense

So effectively we do

first kernel boot -> <measurements match PCR and measurements are saved>
second kernel boot -> <new PCR = first save measurements + new measurements>

and so on

Balbir Singh