Prior to the introduction of the machine keyring, most distros simply
allowed all keys contained within the platform keyring to be used
for both kernel and module verification. This was done by an out of
tree patch. Some distros took it even further and loaded all these keys
into the secondary trusted keyring. This also allowed the system owner
to add their own key for IMA usage.
Each distro contains similar documentation on how to sign kernel modules
and enroll the key into the MOK. The process is fairly straightforward.
With the introduction of the machine keyring, the process remains
basically the same, without the need for any out of tree patches.
The machine keyring allowed distros to eliminate the out of tree patches
for kernel module signing. However, it falls short in allowing the end
user to add their own keys for IMA. Currently, 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. This would expand the current
integrity gap. The IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
Kconfig states that keys may be added to the ima keyrings if the key is
validly signed by a CA cert in the system built-in or secondary trusted
keyring. Currently, there is not code that enforces the contents of a
CA cert.
This series introduces a way to do CA enforement with the machine
keyring. It introduces three different ways to configure the machine
keyring. A new menu option is added to control the type of keys that may
be added to it. The options include none, min, and max restrictions. The
default is CONFIG_INTEGRITY_CA_MACHINE_KEYRING_NONE. This allows all MOK
keys into the machine keyring. When CONFIG_INTEGRITY_CA_MACHINE_KEYRING_MIN
is selected, the X.509 CA bit must be true. Also, the key usage must
contain keyCertSign, any other usage field may also be set. When
CONFIG_INTEGRITY_CA_MACHINE_KEYRING_MAX is selected, the X.509 CA bit
must be true. Also, the key usage must contain keyCertSign and the
digitialSignature usage may not be set. If a key doesn't pass the CA
restriction check, instead of going into the machine keyring, it is
added to the platform keyring. With the ability to configure the machine
keyring with CA restrictions, code that prevented the machine keyring
from being enabled with IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
has been removed.
Changelog:
v4:
- Removed all code that validated the certificate chain back to the root
CA. Now the only restriction is what is initially placed in the
machine keyring.
- Check and store if the X.509 usage contains digitalSignature
- New Kconfig menu item with none, min and max CA restriction on the
machine keyring
v3:
- Allow Intermediate CA certs to be enrolled through the MOK. The
Intermediate CA cert must contain keyCertSign key usage and have the
CA bit set to true. This was done by removing the self signed
requirement.
Eric Snowberg (6):
KEYS: Create static version of public_key_verify_signature
KEYS: Add missing function documentation
KEYS: X.509: Parse Basic Constraints for CA
KEYS: X.509: Parse Key Usage
KEYS: CA link restriction
integrity: machine keyring CA configuration
certs/system_keyring.c | 14 +++++--
crypto/asymmetric_keys/restrict.c | 40 ++++++++++++++++++
crypto/asymmetric_keys/x509_cert_parser.c | 50 +++++++++++++++++++++++
include/crypto/public_key.h | 28 +++++++++++++
security/integrity/Kconfig | 39 +++++++++++++++++-
security/integrity/digsig.c | 8 +++-
6 files changed, 173 insertions(+), 6 deletions(-)
base-commit: 4ec5183ec48656cec489c49f989c508b68b518e3
--
2.27.0
[CC'ing: Lee, Chun-Yi]
On Mon, 2023-02-06 at 21:59 -0500, Eric Snowberg wrote:
> Prior to the introduction of the machine keyring, most distros simply
> allowed all keys contained within the platform keyring to be used
> for both kernel and module verification. This was done by an out of
> tree patch. Some distros took it even further and loaded all these keys
> into the secondary trusted keyring. This also allowed the system owner
> to add their own key for IMA usage.
>
> Each distro contains similar documentation on how to sign kernel modules
> and enroll the key into the MOK. The process is fairly straightforward.
> With the introduction of the machine keyring, the process remains
> basically the same, without the need for any out of tree patches.
>
> The machine keyring allowed distros to eliminate the out of tree patches
> for kernel module signing. However, it falls short in allowing the end
> user to add their own keys for IMA. Currently, 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. This would expand the current
> integrity gap. The IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
> Kconfig states that keys may be added to the ima keyrings if the key is
> validly signed by a CA cert in the system built-in or secondary trusted
> keyring. Currently, there is not code that enforces the contents of a
> CA cert.
>
> This series introduces a way to do CA enforement with the machine
> keyring. It introduces three different ways to configure the machine
> keyring. A new menu option is added to control the type of keys that may
> be added to it. The options include none, min, and max restrictions. The
> default is CONFIG_INTEGRITY_CA_MACHINE_KEYRING_NONE. This allows all MOK
> keys into the machine keyring. When CONFIG_INTEGRITY_CA_MACHINE_KEYRING_MIN
> is selected, the X.509 CA bit must be true. Also, the key usage must
> contain keyCertSign, any other usage field may also be set. When
> CONFIG_INTEGRITY_CA_MACHINE_KEYRING_MAX is selected, the X.509 CA bit
> must be true. Also, the key usage must contain keyCertSign and the
> digitialSignature usage may not be set. If a key doesn't pass the CA
> restriction check, instead of going into the machine keyring, it is
> added to the platform keyring. With the ability to configure the machine
> keyring with CA restrictions, code that prevented the machine keyring
> from being enabled with IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
> has been removed.
>
> Changelog:
> v4:
> - Removed all code that validated the certificate chain back to the root
> CA. Now the only restriction is what is initially placed in the
> machine keyring.
> - Check and store if the X.509 usage contains digitalSignature
> - New Kconfig menu item with none, min and max CA restriction on the
> machine keyring
Thank you, Eric.
For complete separation of certificate usage, at least in the "max" CA
restriction case, the next step would be to limit certificates being
loaded onto the IMA keyring to those with key usage of
"digitalSignature".
Perhaps also require a "codeSigning" extendedKeyUsage, though that
might break existing usages. The "codeSigning" checking could
piggyback on Joey's proposed "Check codeSigning extended key usage
extension" patch set.
What do you think? Do you have any concerns with limiting the type of
certificate being loaded onto the IMA keyring to those with
"digitalSignature"?
--
thanks,
Mimi
> On Feb 8, 2023, at 5:38 AM, Mimi Zohar <[email protected]> wrote:
>
> [CC'ing: Lee, Chun-Yi]
>
> On Mon, 2023-02-06 at 21:59 -0500, Eric Snowberg wrote:
>> Prior to the introduction of the machine keyring, most distros simply
>> allowed all keys contained within the platform keyring to be used
>> for both kernel and module verification. This was done by an out of
>> tree patch. Some distros took it even further and loaded all these keys
>> into the secondary trusted keyring. This also allowed the system owner
>> to add their own key for IMA usage.
>>
>> Each distro contains similar documentation on how to sign kernel modules
>> and enroll the key into the MOK. The process is fairly straightforward.
>> With the introduction of the machine keyring, the process remains
>> basically the same, without the need for any out of tree patches.
>>
>> The machine keyring allowed distros to eliminate the out of tree patches
>> for kernel module signing. However, it falls short in allowing the end
>> user to add their own keys for IMA. Currently, 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. This would expand the current
>> integrity gap. The IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
>> Kconfig states that keys may be added to the ima keyrings if the key is
>> validly signed by a CA cert in the system built-in or secondary trusted
>> keyring. Currently, there is not code that enforces the contents of a
>> CA cert.
>>
>> This series introduces a way to do CA enforement with the machine
>> keyring. It introduces three different ways to configure the machine
>> keyring. A new menu option is added to control the type of keys that may
>> be added to it. The options include none, min, and max restrictions. The
>> default is CONFIG_INTEGRITY_CA_MACHINE_KEYRING_NONE. This allows all MOK
>> keys into the machine keyring. When CONFIG_INTEGRITY_CA_MACHINE_KEYRING_MIN
>> is selected, the X.509 CA bit must be true. Also, the key usage must
>> contain keyCertSign, any other usage field may also be set. When
>> CONFIG_INTEGRITY_CA_MACHINE_KEYRING_MAX is selected, the X.509 CA bit
>> must be true. Also, the key usage must contain keyCertSign and the
>> digitialSignature usage may not be set. If a key doesn't pass the CA
>> restriction check, instead of going into the machine keyring, it is
>> added to the platform keyring. With the ability to configure the machine
>> keyring with CA restrictions, code that prevented the machine keyring
>> from being enabled with IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
>> has been removed.
>>
>> Changelog:
>> v4:
>> - Removed all code that validated the certificate chain back to the root
>> CA. Now the only restriction is what is initially placed in the
>> machine keyring.
>> - Check and store if the X.509 usage contains digitalSignature
>> - New Kconfig menu item with none, min and max CA restriction on the
>> machine keyring
>
> Thank you, Eric.
>
> For complete separation of certificate usage, at least in the "max" CA
> restriction case, the next step would be to limit certificates being
> loaded onto the IMA keyring to those with key usage of
> "digitalSignature".
>
> Perhaps also require a "codeSigning" extendedKeyUsage, though that
> might break existing usages. The "codeSigning" checking could
> piggyback on Joey's proposed "Check codeSigning extended key usage
> extension" patch set.
>
> What do you think? Do you have any concerns with limiting the type of
> certificate being loaded onto the IMA keyring to those with
> "digitalSignature"?
In the MAX setting I would not have a concern. Instead of restrict_link_to_ima
being a macro, a new restriction similar to restrict_link_by_ca could be created.
The new restriction would simply verify digitialSignature is set and the key can be
vouched for by either the built-in or secondary keyrings. Joey’s work to parse
the extended key usage extension could also be included in this restriction.
I’m assuming this would be follow on work?
On Wed, 2023-02-08 at 23:26 +0000, Eric Snowberg wrote:
>
> > On Feb 8, 2023, at 5:38 AM, Mimi Zohar <[email protected]> wrote:
> >
> > [CC'ing: Lee, Chun-Yi]
> >
> > On Mon, 2023-02-06 at 21:59 -0500, Eric Snowberg wrote:
> >> Prior to the introduction of the machine keyring, most distros simply
> >> allowed all keys contained within the platform keyring to be used
> >> for both kernel and module verification. This was done by an out of
> >> tree patch. Some distros took it even further and loaded all these keys
> >> into the secondary trusted keyring. This also allowed the system owner
> >> to add their own key for IMA usage.
> >>
> >> Each distro contains similar documentation on how to sign kernel modules
> >> and enroll the key into the MOK. The process is fairly straightforward.
> >> With the introduction of the machine keyring, the process remains
> >> basically the same, without the need for any out of tree patches.
> >>
> >> The machine keyring allowed distros to eliminate the out of tree patches
> >> for kernel module signing. However, it falls short in allowing the end
> >> user to add their own keys for IMA. Currently, 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. This would expand the current
> >> integrity gap. The IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
> >> Kconfig states that keys may be added to the ima keyrings if the key is
> >> validly signed by a CA cert in the system built-in or secondary trusted
> >> keyring. Currently, there is not code that enforces the contents of a
> >> CA cert.
> >>
> >> This series introduces a way to do CA enforement with the machine
> >> keyring. It introduces three different ways to configure the machine
> >> keyring. A new menu option is added to control the type of keys that may
> >> be added to it. The options include none, min, and max restrictions. The
> >> default is CONFIG_INTEGRITY_CA_MACHINE_KEYRING_NONE. This allows all MOK
> >> keys into the machine keyring. When CONFIG_INTEGRITY_CA_MACHINE_KEYRING_MIN
> >> is selected, the X.509 CA bit must be true. Also, the key usage must
> >> contain keyCertSign, any other usage field may also be set. When
> >> CONFIG_INTEGRITY_CA_MACHINE_KEYRING_MAX is selected, the X.509 CA bit
> >> must be true. Also, the key usage must contain keyCertSign and the
> >> digitialSignature usage may not be set. If a key doesn't pass the CA
> >> restriction check, instead of going into the machine keyring, it is
> >> added to the platform keyring. With the ability to configure the machine
> >> keyring with CA restrictions, code that prevented the machine keyring
> >> from being enabled with IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
> >> has been removed.
> >>
> >> Changelog:
> >> v4:
> >> - Removed all code that validated the certificate chain back to the root
> >> CA. Now the only restriction is what is initially placed in the
> >> machine keyring.
> >> - Check and store if the X.509 usage contains digitalSignature
> >> - New Kconfig menu item with none, min and max CA restriction on the
> >> machine keyring
> >
> > Thank you, Eric.
> >
> > For complete separation of certificate usage, at least in the "max" CA
> > restriction case, the next step would be to limit certificates being
> > loaded onto the IMA keyring to those with key usage of
> > "digitalSignature".
> >
> > Perhaps also require a "codeSigning" extendedKeyUsage, though that
> > might break existing usages. The "codeSigning" checking could
> > piggyback on Joey's proposed "Check codeSigning extended key usage
> > extension" patch set.
> >
> > What do you think? Do you have any concerns with limiting the type of
> > certificate being loaded onto the IMA keyring to those with
> > "digitalSignature"?
>
> In the MAX setting I would not have a concern. Instead of restrict_link_to_ima
> being a macro, a new restriction similar to restrict_link_by_ca could be created.
> The new restriction would simply verify digitialSignature is set and the key can be
> vouched for by either the built-in or secondary keyrings. Joey’s work to parse
> the extended key usage extension could also be included in this restriction.
Sounds good.
> I’m assuming this would be follow on work?
Yes, that probably makes the most sense.
--
thanks,
Mimi