[v7,01/12] Documentation/x86: Document Key Locker

Commit Message

Chang S. Bae May 24, 2023, 4:57 p.m. UTC

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 <[e-mail]>
Reviewed-by: Dan Williams <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
---
Changes from v6:
* Rebase on the upstream -- commit ff61f0791ce9 ("docs: move x86
  documentation into Documentation/arch/"). (Nathan Huckleberry)
* Remove a duplicated sentence -- 'But there is no AES-KL instruction
  to process a 192-bit key.'
* Update the text for clarity and readability:
  - Clarify the error code and exemplify the backup failure
  - Use 'wrapping key' instead of less readable 'IWKey'

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/arch/x86/keylocker.html
---
 Documentation/arch/x86/index.rst     |  1 +
 Documentation/arch/x86/keylocker.rst | 97 ++++++++++++++++++++++++++++
 2 files changed, 98 insertions(+)
 create mode 100644 Documentation/arch/x86/keylocker.rst

Patch

diff --git a/Documentation/arch/x86/index.rst b/Documentation/arch/x86/index.rst
index c73d133fd37c..256359c24669 100644
--- a/Documentation/arch/x86/index.rst
+++ b/Documentation/arch/x86/index.rst
@@ -42,3 +42,4 @@  x86-specific Documentation
    features
    elf_auxvec
    xstate
+   keylocker
diff --git a/Documentation/arch/x86/keylocker.rst b/Documentation/arch/x86/keylocker.rst
new file mode 100644
index 000000000000..5557b8d0659a
--- /dev/null
+++ b/Documentation/arch/x86/keylocker.rst
@@ -0,0 +1,97 @@ 
+.. 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.
+
+CPU-internal Wrapping Key
+=========================
+
+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:
+
+Wrapping Key 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 a wrapping key 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 a wrapping key 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 wrapping key 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 wrapping key state, but the
+architecture does not provide that. An emulation of that capability, by
+caching per-VM wrapping keys 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 -EINVAL.
+  Note that this flag is also set if the wrapping key is changed, e.g.,
+  because of the backup error.
+
+* AES-KL implements support for 128-bit and 256-bit keys, but there is no
+  AES-KL instruction to process an 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.
+