Re: [PATCH 0/7] Add CA enforcement keyring restrictions

[Eric Snowberg]

> On Apr 6, 2022, at 2:45 PM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> 
> Hi Eric,
> 
> On Tue, 2022-04-05 at 21:53 -0400, Eric Snowberg wrote:
>> A key added to the ima keyring must be signed by a key contained within 
>> either the builtin trusted or secondary trusted keyrings. Currently, there are 
>> CA restrictions described in IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY,
>> but these restrictions are not enforced within code. Therefore, keys within 
>> either the builtin or secondary may not be a CA and could be used to
>> vouch for an ima key.
>> 
>> The machine keyring can not be used as another trust anchor for adding keys 
>> to the ima keyring, since CA enforcement does not currently exist [1]. This 
>> would expand the current integrity gap.
>> 
>> Introduce a new root of trust key flag to close this integrity gap for
>> all keyrings.  The first key type to use this is X.509.  When a X.509 
>> certificate is self signed, contains kernCertSign Key Usage and contains 
>> the CA bit, the new flag is set.  Introduce new keyring restrictions 
>> that not only validates a key is signed by a key contained within the 
>> keyring, but also validates the key has the new root of trust key flag 
>> set.  Use this new restriction for keys added to the ima keyring.  Now 
>> that we have CA enforcement, allow the machine keyring to be used as another 
>> trust anchor for the ima keyring.
>> 
>> To recap, all keys that previously loaded into the builtin, secondary or
>> machine keyring will still load after applying this series.  Keys
>> contained within these keyrings may carry the root of trust flag. The
>> ima keyring will use the new root of trust restriction to validate
>> CA enforcement. Other keyrings that require a root of trust could also 
>> use this in the future.
> 
> Your initial patch set indicated that you were addressing Linus'
> request to allow end-users the ability "to add their own keys and sign
> modules they trust".  However, from the design of the previous patch
> set and now this one, everything indicates a lot more is going on than
> just allowing end-users to add their own keys.  There would be no
> reason for loading all the MOK keys, rather than just the CA keys, onto
> the "machine" keyring.  Please provide the motivation for this design.

The motivation is to satisfy both Linus and your requests. Linus requested 
the ability to allow users to add their own keys and sign modules they trust.  
A code signing CA certificate does not require kernCertSign in the usage. Adding 
this as a requirement for kernel modules would be a regression (or a bug).

This series addresses your request to only trust validly signed CA certs. 
As you pointed out in the Kconfig help for 
IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY:

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.

requires keys to be “validly” signed by a CA cert. Later the definition of a 
validly signed CA cert was defined as: self signed, contains kernCertSign 
key usage and contains the CA bit. While this help file states the CA restriction, 
nothing in code enforces it.  One can place any type of self signed cert in either 
keyring and ima will use it.  The motivation is for all keys added to the ima 
keyring to abide by the restriction defined in the Kconfig help.  With this series 
this can be accomplished without introducing a regression on keys placed in 
any of the system keyrings.

> Please note that Patch 6/7 permits intermediary CA keys, without any
> mention of it in the cover letter.  Please include this in the
> motivation for this design.

Ok, I’ll add that in the next round.