Many UEFI Linux distributions boot using shim. The UEFI shim provides what is called Machine Owner Keys (MOK). Shim uses both the UEFI Secure Boot DB and MOK keys to validate the next step in the boot chain. The MOK facility can be used to import user generated keys. These keys can be used to sign an end-user development kernel build. When Linux boots, pre-boot keys (both UEFI Secure Boot DB and MOK keys) get loaded in the Linux .platform keyring. Currently, pre-boot keys are not trusted within the Linux trust boundary [1]. These platform keys can only be used for kexec. If an end-user wants to use their own key within the Linux trust boundary, they must either compile it into the kernel themselves or use the insert-sys-cert script. Both options present a problem. Many end-users do not want to compile their own kernels. With the insert-sys-cert option, there are missing upstream changes [2]. Also, with the insert-sys-cert option, the end-user must re-sign their kernel again with their own key, and then insert that key into the MOK db. Another problem with insert-sys-cert is that only a single key can be inserted into a compressed kernel. Having the ability to insert a key into the Linux trust boundary opens up various possibilities. The end-user can use a pre-built kernel and sign their own kernel modules. It also opens up the ability for an end-user to more easily use digital signature based IMA-appraisal. To get a key into the ima keyring, it must be signed by a key within the Linux trust boundary. Downstream Linux distros try to have a single signed kernel for each architecture. Each end-user may use this kernel in entirely different ways. Some downstream kernels have chosen to always trust platform keys within the Linux trust boundary for kernel module signing. These kernels have no way of using digital signature base IMA appraisal. This series introduces a new Linux kernel keyring containing the Machine Owner Keys (MOK) called .mok. It also adds a new MOK variable to shim. This variable allows the end-user to decide if they want to trust keys enrolled in the MOK within the Linux trust boundary. By default, nothing changes; MOK keys are not trusted within the Linux kernel. They are only trusted after the end-user makes the decision themselves. The end-user would set this through mokutil using a new --trust-mok option [3]. This would work similar to how the kernel uses MOK variables to enable/disable signature validation as well as use/ignore the db. When shim boots, it mirrors the new MokTML Boot Services variable to a new MokListTrustedRT Runtime Services variable and extends PCR14. MokListTrustedRT is written without EFI_VARIABLE_NON_VOLATILE set, preventing an end-user from setting it after booting and doing a kexec. When the kernel boots, if MokListTrustedRT is set and EFI_VARIABLE_NON_VOLATILE is not set, the MokListRT is loaded into the mok keyring instead of the platform keyring. Mimi has suggested that only CA keys be loaded into this keyring. All other certs will load into the platform keyring instead. The .mok keyring contains a new keyring permission that only allows CA keys to be loaded. If the permission fails, the key is later loaded into the platform keyring. After all keys are added into the .mok keyring, they are linked to the secondary trusted keyring. After the link is created, keys contained in the .mok keyring will automatically be searched when searching the secondary trusted keys. Secure Boot keys will never be trusted. They will always be loaded into the platform keyring. If an end-user wanted to trust one, they would need to enroll it into the MOK. I have included links to both the mokutil [3] and shim [4] changes I have made to support this new functionality. V2 changes: - The .mok keyring persists past boot - Removed the unrestricted move into the secondary keyring - Removed the keyring move bypass patch - Added restrictions to allow the .mok to be linked to either the builtin or secondary keyrings - Secondary keyring dependency has been removed V3 changes: - Only CA keys contained in the MOKList are loaded, nothing else - Support for kernels built without the secondary trusted keyring has been dropped. [1] https://lore.kernel.org/lkml/1556221605.24945.3.camel@xxxxxxxxxxxxxxxxxxxxx/ [2] https://lore.kernel.org/patchwork/cover/902768/ [3] https://github.com/esnowberg/mokutil/tree/0.3.0-mokvars-v2 [4] https://github.com/esnowberg/shim/tree/mokvars-v2 Eric Snowberg (14): integrity: Introduce a Linux keyring for the Machine Owner Key (MOK) KEYS: CA link restriction integrity: Trust MOK keys if MokListTrustedRT found integrity: add add_to_mok_keyring integrity: restrict INTEGRITY_KEYRING_MOK to restrict_link_by_ca integrity: accessor function to get trust_moklist integrity: add new keyring handler for mok keys KEYS: add a reference to mok keyring KEYS: Introduce link restriction to include builtin, secondary and mok keys KEYS: change link restriction for secondary to also trust mok KEYS: link secondary_trusted_keys to mok trusted keys integrity: Do not allow mok keyring updates following init integrity: store reference to mok keyring integrity: change ima link restriction to include mok keys certs/system_keyring.c | 33 ++++++- crypto/asymmetric_keys/restrict.c | 40 +++++++++ include/crypto/public_key.h | 5 ++ include/keys/system_keyring.h | 10 +++ security/integrity/Makefile | 3 +- security/integrity/digsig.c | 15 +++- security/integrity/integrity.h | 12 ++- .../platform_certs/keyring_handler.c | 17 +++- .../platform_certs/keyring_handler.h | 5 ++ security/integrity/platform_certs/load_uefi.c | 4 +- .../integrity/platform_certs/mok_keyring.c | 85 +++++++++++++++++++ 11 files changed, 220 insertions(+), 9 deletions(-) create mode 100644 security/integrity/platform_certs/mok_keyring.c base-commit: 36a21d51725af2ce0700c6ebcb6b9594aac658a6 -- 2.18.4
