Re: [PATCH V4 0/2] Add support for arm64 to carry ima measurement

[James Morse]

Hi Mimi,

On 16/10/2019 02:44, Mimi Zohar wrote:
> On Tue, 2019-10-15 at 18:39 +0100, James Morse wrote:
>> If SecureBoot isn't relevant, I'm confused as to why kexec_file_load() is.
>>
>> I thought kexec_file_load() only existed because SecureBoot systems need to validate the
>> new OS images's signature before loading it, and we can't trust user-space calling Kexec
>> to do this.
>>
>> If there is no secure boot, why does this thing only work with kexec_file_load()?
>> (good news! With the UEFI memreseve table, it should work transparently with regular kexec
>> too)

> I'm so sorry for the confusion. ÂIMA was originally limited to
> extending trusted boot concepts to the OS. ÂAs of Linux 3.10, IMA
> added support for extending secure boot concepts and auditing file
> hashes (commit e7c568e0fd0cf).
> 
> True, kexec_file_load is required for verifying the kexec kernel
> image, but it is also required for measuring the kexec kernel image as
> well.
> 
> After reading the kernel image into memory (kernel_read_file_from_fd),
> the hash is calculated and then added to the IMA measurement list and
> used to extend the TPM. ÂAll of this is based on the IMA policy,
> including the TPM PCR.

Don't we get a set of segments with the regular kexec syscall? These could equally be
hashed and measured, and logged via IMA and/or extending the TPMs measurements.

(obviously this would include the command-line and maybe purgatory, which makes it less
predictable, but these are still the binary blobs that were given privileged access to the
system).


>>> I am not sure if i addressed all your concerns, please let me know
>>> if i missed anything. To me most concerns look to be towards the kexec case and dependency
>>> on hardware(ACPI/TPM) during boot and early boot services, where as carrying the logs is
>>> only during the kexec_file_load sys call and does not interfere with that code path.
>>> IMA documentation: https://sourceforge.net/p/linux-ima/wiki/Home/
>>
>> Supporting ACPI in the same way is something we need to do from day one. kexec_file_load()
>> already does this. I'm not sure "only kexec_file_load()" is a justifiable restriction...

> The TPM PCRs are not reset on a soft reboot. ÂAs a result, in order to
> validate the IMA measurement list against the TPM PCRs, the IMA
> measurement list is saved on kexec load, restored on boot, and then
> the memory allocated for carrying the measurement list across kexec is
> freed.

Hmm, this is why the reserved memory gets freed.

What happens to stuff that happens between kexec-load and boot?
There is a comment:
| /* segment size can't change between kexec load and execute */

But I can't see anywhere that enforces that. I guess those measurements will go missing,
and the TPM value will not match after kexec.



Thanks,

James