Re: [PATCH v9 2/8] integrity: Introduce a Linux keyring called machine

From: Eric Snowberg
Date: Wed Jan 12 2022 - 18:01:44 EST




> On Jan 12, 2022, at 12:41 PM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
>
> On Tue, 2022-01-11 at 20:14 -0500, Mimi Zohar wrote:
>> On Tue, 2022-01-11 at 21:26 +0000, Eric Snowberg wrote:
>>>
>>>> On Jan 11, 2022, at 11:16 AM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
>>>>
>>>> On Mon, 2022-01-10 at 23:25 +0000, Eric Snowberg wrote:
>>>>>> Jarkko, my concern is that once this version of the patch set is
>>>>>> upstreamed, would limiting which keys may be loaded onto the .machine
>>>>>> keyring be considered a regression?
>>>>>
>>>>>
>>>>> Currently certificates built into the kernel do not have a CA restriction on them.
>>>>> IMA will trust anything in this keyring even if the CA bit is not set. While it would
>>>>> be advisable for a kernel to be built with a CA, nothing currently enforces it.
>>>>>
>>>>> My thinking for the dropped CA restriction patches was to introduce a new Kconfig.
>>>>> This Kconfig would do the CA enforcement on the machine keyring. However if the
>>>>> Kconfig option was not set for enforcement, it would work as it does in this series,
>>>>> plus it would allow IMA to work with non-CA keys. This would be done by removing
>>>>> the restriction placed in this patch. Let me know your thoughts on whether this would
>>>>> be an appropriate solution. I believe this would get around what you are identifying as
>>>>> a possible regression.
>>>>
>>>> True the problem currently exists with the builtin keys, but there's a
>>>> major difference between trusting the builtin keys and those being
>>>> loading via MOK. This is an integrity gap that needs to be closed and
>>>> shouldn't be expanded to keys on the .machine keyring.
>>>>
>>>> "plus it would allow IMA to work with non-CA keys" is unacceptable.
>>>
>>> Ok, I’ll leave that part out. Could you clarify the wording I should include in the future
>>> cover letter, which adds IMA support, on why it is unacceptable for the end-user to
>>> make this decision?
>>
>> The Kconfig IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
>> "help" is very clear:
>
> [Reposting the text due to email formatting issues.]
>
> help
> Keys may be added to the IMA or IMA blacklist keyrings, if the
> key is validly signed by a CA cert in the system built-in or
> secondary trusted keyrings.
>
> Intermediate keys between those the kernel has compiled in and the
> IMA keys to be added may be added to the system secondary keyring,
> provided they are validly signed by a key already resident in the
> built-in or secondary trusted keyrings.
>
>
> The first paragraph requires "validly signed by a CA cert in the system
> built-in or secondary trusted keyrings" for keys to be loaded onto the
> IMA keyring. This Kconfig is limited to just the builtin and secondary
> keyrings. Changing this silently to include the ".machine" keyring
> introduces integrity risks that previously did not exist. A new IMA
> Kconfig needs to be defined to allow all three keyrings - builtin,
> machine, and secondary.
>
> The second paragraph implies that only CA and intermediate CA keys are
> on secondary keyring, or as in our case the ".machine" keyring linked
> to the secondary keyring.

Got it, thanks. I’ll use this in the cover letter that introduces the CA restrictions
to enable IMA.