[PATCH v6 01/12] Documentation/x86: Document Key Locker

Chang S. Bae
2023-04-10T23:11:44Z

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: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: x86@kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes from v5:
* Fix a typo: 'feature feature' -> 'feature'

Changes from RFC v2:
* Add as a new patch.

The preview is available here:
https://htmlpreview.github.io/?https://github.com/intel-staging/keylocker/kdoc/x86/keylocker.html
---
Documentation/x86/index.rst | 1 +
Documentation/x86/keylocker.rst | 98 +++++++++++++++++++++++++++++++++
2 files changed, 99 insertions(+)
create mode 100644 Documentation/x86/keylocker.rst

diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
index 8ac64d7de4dc..669c239c009f 100644
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -43,3 +43,4 @@ x86-specific Documentation
features
elf_auxvec
xstate
+ keylocker
diff --git a/Documentation/x86/keylocker.rst b/Documentation/x86/keylocker.rst
new file mode 100644
index 000000000000..3b405fade7d8
--- /dev/null
+++ b/Documentation/x86/keylocker.rst
@@ -0,0 +1,98 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============
+x86 Key Locker
+==============
+
+Introduction
+============
+
+Key Locker is a CPU 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
+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.
+
--
2.17.1