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

[Eric Snowberg]

> 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.