[v5,01/12] Documentation/x86: Document Key Locker

Document the overview of the feature along with relevant consideration when
provisioning dm-crypt volumes with AES-KL instead of AES-NI.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes from RFC v2:
* Add as a new patch.
---
 Documentation/x86/index.rst     |  1 +
 Documentation/x86/keylocker.rst | 98 +++++++++++++++++++++++++++++++++
 2 files changed, 99 insertions(+)
 create mode 100644 Documentation/x86/keylocker.rst

On 1/13/22 04:12, Chang S. Bae wrote:
> +==============
> +x86 Key Locker
> +==============
> +
> +Introduction
> +============
> +
> +Key Locker is a CPU feature feature to reduce key exfiltration
> +opportunities while maintaining a programming interface similar to AES-NI.
> +It converts the AES key into an encoded form, called the 'key handle'. The
> +key handle is a wrapped version of the clear-text key where the wrapping
> +key has limited exposure. Once converted, all subsequent data encryption
> +using new AES instructions (AES-KL) uses this key handle, reducing the
> +exposure of private key material in memory.
> +
> +Internal Wrapping Key (IWKey)
> +=============================
> +
> +The CPU-internal wrapping key is an entity in a software-invisible CPU
> +state. On every system boot, a new key is loaded. So the key handle that
> +was encoded by the old wrapping key is no longer usable on system shutdown
> +or reboot.
> +
> +And the key may be lost on the following exceptional situation upon wakeup:
> +
> +IWKey Restore Failure
> +---------------------
> +
> +The CPU state is volatile with the ACPI S3/4 sleep states. When the system
> +supports those states, the key has to be backed up so that it is restored
> +on wake up. The kernel saves the key in non-volatile media.
> +
> +The event of an IWKey restore failure upon resume from suspend, all
> +established key handles become invalid. In flight dm-crypt operations
> +receive error results from pending operations. In the likely scenario that
> +dm-crypt is hosting the root filesystem the recovery is identical to if a
> +storage controller failed to resume from suspend, reboot. If the volume
                                            "suspend and reboot"?
> +impacted by an IWKey restore failure is a data-volume then it is possible
> +that I/O errors on that volume do not bring down the rest of the system.
> +However, a reboot is still required because the kernel will have
> +soft-disabled Key Locker. Upon the failure, the crypto library code will
> +return -ENODEV on every AES-KL function call. The Key Locker implementation
> +only loads a new IWKey at initial boot, not any time after like resume from
> +suspend.
> +
> +Use Case and Non-use Cases
> +==========================
> +
> +Bare metal disk encryption is the only intended use case.
> +
> +Userspace usage is not supported because there is no ABI provided to
> +communicate and coordinate wrapping-key restore failure to userspace. For
> +now, key restore failures are only coordinated with kernel users. But the
> +kernel can not prevent userspace from using the feature's AES instructions
> +('AES-KL') when the feature has been enabled. So, the lack of userspace
> +support is only documented, not actively enforced.
> +
> +Key Locker is not expected to be advertised to guest VMs and the kernel
> +implementation ignores it even if the VMM enumerates the capability. The
> +expectation is that a guest VM wants private IWKey state, but the
> +architecture does not provide that. An emulation of that capability, by
> +caching per VM IWKeys in memory, defeats the purpose of Key Locker. The
> +backup / restore facility is also not performant enough to be suitable for
> +guest VM context switches.
> +
> +AES Instruction Set
> +===================
> +
> +The feature accompanies a new AES instruction set. This instruction set is
> +analogous to AES-NI. A set of AES-NI instructions can be mapped to an
> +AES-KL instruction. For example, AESENC128KL is responsible for ten rounds
> +of transformation, which is equivalent to nine times AESENC and one
> +AESENCLAST in AES-NI.
> +
> +But they have some notable differences:
> +
> +* AES-KL provides a secure data transformation using an encrypted key.
> +
> +* If an invalid key handle is provided, e.g. a corrupted one or a handle
> +  restriction failure, the instruction fails with setting RFLAGS.ZF. The
> +  crypto library implementation includes the flag check to return an error
> +  code. Note that the flag is also set when the internal wrapping key is
> +  changed because of missing backup.
> +
> +* AES-KL implements support for 128-bit and 256-bit keys, but there is no
> +  AES-KL instruction to process an 192-bit key. But there is no AES-KL
> +  instruction to process a 192-bit key. The AES-KL cipher implementation
> +  logs a warning message with a 192-bit key and then falls back to AES-NI.
> +  So, this 192-bit key-size limitation is only documented, not enforced. It
> +  means the key will remain in clear-text in memory. This is to meet Linux
> +  crypto-cipher expectation that each implementation must support all the
> +  AES-compliant key sizes.
> +
> +* Some AES-KL hardware implementation may have noticeable performance
> +  overhead when compared with AES-NI instructions.
> +

The rest is LGTM.