[PATCH v3 00/15] x86: Support Key Locker

[Chang S. Bae]

Recall that AES-NI [1] is a set of CPU instructions to perform encryption
operations. Like all other software encryption implementations it relies on
OS memory protections to keep clear-text key material from being
exfiltrated across security domains. However, there are demonstrated
methods for exfiltrating data across security domains.

Key Locker [2] 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' [3]. 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.

As mentioned, Key Locker introduces a CPU-internal wrapping key. This key
is loaded in a CPU state that software can not access, and then it is used
to convert the AES keys. At every boot, a new randomized key is
generated and loaded. Thus, the key handle is revoked upon system reboot
(including kexec-reboot).

== Threat Model and Mitigation Description ==

A targeted attack is with brief physical access [4] to the victim device in
unlock state or with the screen locked, but with keys in memory. For
example, the attacker might use a malicious USB peripheral to exploit a
kernel bug or perform a cold boot attack [4], and then extract the disk
encryption master key. Using this master key, the attacker will be able to
extract the contents of the disk. This includes data yet-to-be-written when
acquiring the device from the victim user at a later point in time and
performing forensic analysis.

Key Locker reduces opportunities for long-lived keys to be exfiltrated from
system RAM. Once the key is converted to a key handle, the clear text key
is no longer available to a class of data exfiltration techniques.

== Disk Encryption Use Case ==

Disk encryption uses Key Locker to mitigate key exfiltration as follows:

1. Configuration for Key Locker: AES-KL shows up in /proc/crypto as a
   distinct cipher option. From there, tools like cryptsetup [5] can select
   AES-KL vs AES-NI. For example,

   $ cryptsetup luksFormat --cipher="capi:xts-aes-aeskl-plain" <device>

Note: AES-KL has a performance tradeoff. See details in 'Performance'
below.

2. Disk encryption flow with key protection:

* The cryptsetup utility is responsible for loading the volume key into the
  kernel's keyring and passing a reference of the key. Once dm-crypt [6]
  has set up the volume, user space is responsible for invalidating the key
  material so that only the key handle remains in memory. Cryptsetup does
  this, e.g. via crypt_free_volume_key() and crypt_safe_free().

* The AES-KL code in the kernel's crypto library uses the key handle
  instead of the actual clear text key.

== Non Use Cases ==

Bare metal disk encryption is the only use case intended by these patches.
Userspace usage is not supported because there is no ABI provided to
communicate and coordinate wrapping-key restore failures to userspace.
For now, the key restore failures are only coordinated with kernel users.
For this reason a "keylocker" indicator is not published in /proc/cpuinfo.
At the same time, the kernel can not prevent userspace from using the
AES-KL instructions when Key Locker support has been enabled, so the lack
of userspace support is only documented, not actively enforced. Key Locker
support is not enumerated to VM guests.

== Performance ==

This feature comes with some performance penalty vs AESNI. The cryptsetup
utility [5] has been used to measure the Key Locker performance. Below
results have been measured [8] on an Intel 11th-gen Core Processor, code
named Tigerlake mobile part [9].

Below is the average encryption and decryption rates with key size of 256b.

Commands:
cryptsetup version – 2.3.4
$ cryptsetup benchmark-c aes-cbc -s 256
$ cryptsetup benchmark-c aes-xts -s 256

Tests are approximate using memory only (no storage IO).

+-----------+---------------+---------------+
| Cipher    |   Encryption  | Decryption    |
| (AES-NI)  |    (MiB/s)    | (MiB/s)       |
+-----------+---------------+---------------+
| AES-CBC   |     1242.6    |   4446.5      |
| AES-XTS   |     4233.3    |   4359.7      |
+-----------+-------------------------------+

+-----------+---------------+---------------+
| Cipher    |   Encryption  | Decryption    |
| (AES-KL)  |    (MiB/s)    | (MiB/s)       |
+-----------+---------------+---------------+
| AES-CBC   |     505.3     |   2097.8      |
| AES-XTS   |     1130      |   696.4       |
+-----------+-------------------------------+

The cryptsetup benchmark indicates Key Locker raw throughput can be  ~5x
slower than AES-NI. For disk encryption, storage bandwidth may be the
bottleneck before encryption bandwidth, but the potential performance
difference is why AES-KL is advertised as a distinct cipher in /proc/crypto
rather than the kernel transparently replacing AES-NI usage with AES-KL.

== Patch Series ==

The series touches two areas -- the x86 core and the x86 crypto library:

* PATCH01-09: Implement Key Locker support in the x86 core.  A new internal
  wrapping key is loaded at boot time and then it is restored from deep
  sleep states. The implication is that, e.g., a dm-crypt user needs to
  re-enter the private key at every power-on, per typical expectations, but
  it does not expect the user to re-enter the key over suspend events.
  The AES-KL code in the kernel's crypto library depends on this key
  support. Build up this support via helpers in the feature-dedicated .c
  file. Include documentation.

* PATCH10-15: For the x86 crypto library, it first prepares the AES-NI code
  to accommodate the new AES implementation. Then incrementally add base
  functions and various modes support -- ECB, CBC, CTR, and XTS. The code
  was found to pass the crypto test.

Changes from RFC v2 [7]:
* Clarify the usage case -- bare metal disk encryption. (Andy Lutomirski)
* Change the backup key failure handling. (Dan Williams) (PATCH8)
* Do not publish 'keylocker' indicator to userspace via /proc/cpuinfo.
  (PATCH2)
* Make CONFIG_X86_KEYLOCKER selected by CONFIG_CRYPTO_AES_KL.
  (Dan Williams) (PATCH9)
* Clarify AES-KL limitation and tradeoff. (Andy Lutomirski) (PATCH11)
* Polish the changelog, and refactor patches.
* Drop the self-test as recommending no userspace use. (Dan Williams)
* Drop the hardware randomization option.
* Limit to support bare metal only. (PATCH7)
* Rename MSRs. (Dan Williams) (PATCH5)
* Clean up code. (Dan Williams) (PATCH7)
* Add documentation. (PATCH1)

Thanks to Dan Williams, Charishma Gairuboyina, and Kumar Dwarakanath for
help with the cover letter.

== Reference ==

[1] Intel Advanced Encryption Standard Instructions (AES-NI):
    https://www.intel.com/content/www/us/en/developer/articles/technical/advanced-encryption-standard-instructions-aes-ni.html
[2] Intel Key Locker Specification:
    https://software.intel.com/content/dam/develop/external/us/en/documents/343965-intel-key-locker-specification.pdf
[3] This encoded form contains ciphertext of AES key, Additional
    Authentication Data, and integrity tag information. Section 1.4 Handle
    Format [2] describes the format.
[4] Key Locker cannot protect the user data in the event of a full system
    compromise, or against the scenarios where the attack can observe the
    creation of the key handle from the original key.
[5] cryptsetup: https://gitlab.com/cryptsetup/cryptsetup
[6] DM-crypt:
    https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/dm-crypt.html
[7] RFC V2: https://lore.kernel.org/lkml/20210514201508.27967-1-chang.seok.bae@xxxxxxxxx/
[8] Intel publishes information about product performance at
    www.Intel.com/PerformanceIndex.
[9] Tigerlake:
    https://www.intel.com/content/www/us/en/products/docs/processors/embedded/11th-gen-product-brief.html

Chang S. Bae (15):
  Documentation/x86: Document Key Locker
  x86/cpufeature: Enumerate Key Locker feature
  x86/insn: Add Key Locker instructions to the opcode map
  x86/asm: Add a wrapper function for the LOADIWKEY instruction
  x86/msr-index: Add MSRs for Key Locker internal wrapping key
  x86/keylocker: Define Key Locker CPUID leaf
  x86/cpu/keylocker: Load an internal wrapping key at boot-time
  x86/power/keylocker: Restore internal wrapping key from the ACPI S3/4
    sleep states
  x86/cpu: Add a configuration and command line option for Key Locker
  crypto: x86/aes - Prepare for a new AES implementation
  crypto: x86/aes-kl - Support AES algorithm using Key Locker
    instructions
  crypto: x86/aes-kl - Support ECB mode
  crypto: x86/aes-kl - Support CBC mode
  crypto: x86/aes-kl - Support CTR mode
  crypto: x86/aes-kl - Support XTS mode

 .../admin-guide/kernel-parameters.txt         |    2 +
 Documentation/x86/index.rst                   |    1 +
 Documentation/x86/keylocker.rst               |   98 ++
 arch/x86/Kconfig                              |    3 +
 arch/x86/crypto/Makefile                      |    5 +-
 arch/x86/crypto/aes-intel_asm.S               |   26 +
 arch/x86/crypto/aes-intel_glue.c              |  219 +++
 arch/x86/crypto/aes-intel_glue.h              |   61 +
 arch/x86/crypto/aeskl-intel_asm.S             | 1186 +++++++++++++++++
 arch/x86/crypto/aeskl-intel_glue.c            |  401 ++++++
 arch/x86/crypto/aesni-intel_asm.S             |   90 +-
 arch/x86/crypto/aesni-intel_glue.c            |  313 +----
 arch/x86/crypto/aesni-intel_glue.h            |   88 ++
 arch/x86/include/asm/cpufeatures.h            |    1 +
 arch/x86/include/asm/disabled-features.h      |    8 +-
 arch/x86/include/asm/keylocker.h              |   45 +
 arch/x86/include/asm/msr-index.h              |    6 +
 arch/x86/include/asm/special_insns.h          |   33 +
 arch/x86/include/uapi/asm/processor-flags.h   |    2 +
 arch/x86/kernel/Makefile                      |    1 +
 arch/x86/kernel/cpu/common.c                  |   21 +-
 arch/x86/kernel/cpu/cpuid-deps.c              |    1 +
 arch/x86/kernel/keylocker.c                   |  199 +++
 arch/x86/kernel/smpboot.c                     |    2 +
 arch/x86/lib/x86-opcode-map.txt               |   11 +-
 arch/x86/power/cpu.c                          |    2 +
 crypto/Kconfig                                |   44 +
 tools/arch/x86/lib/x86-opcode-map.txt         |   11 +-
 28 files changed, 2534 insertions(+), 346 deletions(-)
 create mode 100644 Documentation/x86/keylocker.rst
 create mode 100644 arch/x86/crypto/aes-intel_asm.S
 create mode 100644 arch/x86/crypto/aes-intel_glue.c
 create mode 100644 arch/x86/crypto/aes-intel_glue.h
 create mode 100644 arch/x86/crypto/aeskl-intel_asm.S
 create mode 100644 arch/x86/crypto/aeskl-intel_glue.c
 create mode 100644 arch/x86/crypto/aesni-intel_glue.h
 create mode 100644 arch/x86/include/asm/keylocker.h
 create mode 100644 arch/x86/kernel/keylocker.c


base-commit: 7284bd9822f33a3be80ac6d92b4540d6dcfb5219
--
2.17.1