linux-kernel - Re: [PATCH 4/7] KEYS: Introduce a builtin root of trust key flag

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Date:   Mon, 11 Apr 2022 11:30:48 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Eric Snowberg <eric.snowberg@...cle.com>
Cc:     David Howells <dhowells@...hat.com>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "dmitry.kasatkin@...il.com" <dmitry.kasatkin@...il.com>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "serge@...lyn.com" <serge@...lyn.com>,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        "pvorel@...e.cz" <pvorel@...e.cz>, "tiwai@...e.de" <tiwai@...e.de>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 4/7] KEYS: Introduce a builtin root of trust key flag

On Fri, 2022-04-08 at 21:59 +0000, Eric Snowberg wrote:
> > On Apr 8, 2022, at 12:49 PM, Mimi Zohar <zohar@...ux.ibm.com> wrote:
> > 
> > On Fri, 2022-04-08 at 17:34 +0000, Eric Snowberg wrote:
> >> 
> >>> On Apr 8, 2022, at 10:55 AM, Mimi Zohar <zohar@...ux.ibm.com> wrote:
> >>> 
> >>> On Fri, 2022-04-08 at 15:27 +0000, Eric Snowberg wrote:
> >>>> 
> >>>>> On Apr 8, 2022, at 8:40 AM, Mimi Zohar <zohar@...ux.ibm.com> wrote:
> >>>>> 
> >>>>> On Tue, 2022-04-05 at 21:53 -0400, Eric Snowberg wrote:
> >>>>>> 
> >>>>>> The first type of key to use this is X.509.  When a X.509 certificate
> >>>>>> is self signed, has the kernCertSign Key Usage set and contains the
> >>>>>> CA bit set this new flag is set.
> >>>>>> 
> >>>>>> Signed-off-by: Eric Snowberg <eric.snowberg@...cle.com>
> >>>>>> 
> >>>>>> diff --git a/include/linux/key.h b/include/linux/key.h
> >>>>>> index 7febc4881363..97f6a1f86a27 100644
> >>>>>> --- a/include/linux/key.h
> >>>>>> +++ b/include/linux/key.h
> >>>>>> @@ -230,6 +230,7 @@ struct key {
> >>>>>> #define KEY_FLAG_ROOT_CAN_INVAL  7       /* set if key can be invalidated by root without permission */
> >>>>>> #define KEY_FLAG_KEEP            8       /* set if key should not be removed */
> >>>>>> #define KEY_FLAG_UID_KEYRING     9       /* set if key is a user or user session keyring */
> >>>>>> +#define KEY_FLAG_BUILTIN_ROT    10      /* set if key is a builtin Root of Trust key */
> >>>>>> 
> >>>>>>  /* the key type and key description string
> >>>>>>   * - the desc is used to match a key against search criteria
> >>>>>> @@ -290,6 +291,7 @@ extern struct key *key_alloc(struct key_type *type,
> >>>>>> #define KEY_ALLOC_BYPASS_RESTRICTION     0x0008  /* Override the check on restricted keyrings */
> >>>>>> #define KEY_ALLOC_UID_KEYRING            0x0010  /* allocating a user or user session keyring */
> >>>>>> #define KEY_ALLOC_SET_KEEP               0x0020  /* Set the KEEP flag on the key/keyring */
> >>>>>> +#define KEY_ALLOC_BUILT_IN_ROT          0x0040  /* Add builtin root of trust key */
> >>>>> 
> >>>>> Since the concept of root of trust is not generic, but limited to
> >>>>> specific keyrings, the root CA certificate signing keys on the
> >>>>> "machine" keyring need to be identified.  Similar to the
> >>>>> KEY_ALLOC_BUILT_IN/KEY_FLAG_BUILTIN, new flags
> >>>>> KEY_ALLOC_MACHINE/KEY_FLAG_MACHINE should be defined instead.
> >>>> 
> >>>> I’m open to renaming these, however this name change seems confusing to me.  
> >>>> This flag gets set when the X.509 certificate contains the three CA requirements 
> >>>> identified above.  The remaining keys in the machine keyring can be used for 
> >>>> anything else.
> >>> 
> >>> Renaming the flag to KEY_ALLOC_MACHINE/KEY_FLAG_MACHINE differentiates
> >>> between the "builtin" keys from the "machine" keys.  The trust models
> >>> are very different.
> >> 
> >> Isn’t the trust model the same for machine and secondary keys?  Both are supplied by 
> >> the end-user. That is why I’m confused by naming something _MACHINE when it applies 
> >> to more than one keyring.
> > 
> > True both are supplied by the end-user, but the trust models are
> > different.
> 
> I think I need more information here, I’m not seeing how they are different trust 
> models.

In order to discuss trust models, we need to understand the different
use-cases that are being discussed here without ever having been
explicitly stated.  Here are a few:
- Allow users to sign their own kernel modules.
- Allow users to selectively authorize 3rd party certificates to verify
kernel modules.
- From an IMA perspective, allow users to sign files within their own
software packages.

Each of the above use-cases needs to be independently configurable,
thoroughly explained, and enforced.

thanks,

Mimi


> 
> >  In one case the certificates are coming indirectly from
> > firmware,