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From: "Chang S. Bae" <chang.seok.bae@intel.com>
To: tglx@linutronix.de, bp@suse.de, dave.hansen@linux.intel.com,
	mingo@kernel.org, luto@kernel.org, x86@kernel.org,
	herbert@gondor.apana.org.au
Cc: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
	dan.j.williams@intel.com, charishma1.gairuboyina@intel.com,
	kumar.n.dwarakanath@intel.com,
	lalithambika.krishnakumar@intel.com, ravi.v.shankar@intel.com,
	chang.seok.bae@intel.com, linux-doc@vger.kernel.org
Subject: [PATCH v3 01/15] Documentation/x86: Document Key Locker
Date: Wed, 24 Nov 2021 12:06:46 -0800	[thread overview]
Message-ID: <20211124200700.15888-2-chang.seok.bae@intel.com> (raw)
In-Reply-To: <20211124200700.15888-1-chang.seok.bae@intel.com>

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

diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
index f498f1d36cd3..bbea47ea10f6 100644
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -38,3 +38,4 @@ 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..e65d936ef199
--- /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 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


  reply	other threads:[~2021-11-24 20:14 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-24 20:06 [PATCH v3 00/15] x86: Support " Chang S. Bae
2021-11-24 20:06 ` Chang S. Bae [this message]
2021-11-24 20:06 ` [PATCH v3 02/15] x86/cpufeature: Enumerate Key Locker feature Chang S. Bae
2021-11-24 20:06 ` [PATCH v3 03/15] x86/insn: Add Key Locker instructions to the opcode map Chang S. Bae
2021-11-24 20:06 ` [PATCH v3 04/15] x86/asm: Add a wrapper function for the LOADIWKEY instruction Chang S. Bae
2021-11-24 20:06 ` [PATCH v3 05/15] x86/msr-index: Add MSRs for Key Locker internal wrapping key Chang S. Bae
2021-11-24 20:06 ` [PATCH v3 06/15] x86/keylocker: Define Key Locker CPUID leaf Chang S. Bae
2021-11-24 20:06 ` [PATCH v3 07/15] x86/cpu/keylocker: Load an internal wrapping key at boot-time Chang S. Bae
2021-11-24 20:06 ` [PATCH v3 08/15] x86/power/keylocker: Restore internal wrapping key from the ACPI S3/4 sleep states Chang S. Bae
2021-11-30  3:30   ` Eric Biggers
2021-11-30  6:31     ` [PATCH v3-fix " Chang S. Bae
2021-11-30  6:56     ` [PATCH v3 " Bae, Chang Seok
2021-11-24 20:06 ` [PATCH v3 09/15] x86/cpu: Add a configuration and command line option for Key Locker Chang S. Bae
2021-11-24 20:06 ` [PATCH v3 10/15] crypto: x86/aes - Prepare for a new AES implementation Chang S. Bae
2021-11-24 20:06 ` [PATCH v3 11/15] crypto: x86/aes-kl - Support AES algorithm using Key Locker instructions Chang S. Bae
2021-11-30  3:48   ` Eric Biggers
2021-11-30  6:57     ` Bae, Chang Seok
2021-11-30  7:03       ` Dan Williams
2021-12-06 22:14       ` Ard Biesheuvel
2021-12-06 22:59         ` Bae, Chang Seok
2021-12-02 14:21   ` Peter Zijlstra
2021-12-06 21:32     ` Bae, Chang Seok
2021-11-24 20:06 ` [PATCH v3 12/15] crypto: x86/aes-kl - Support ECB mode Chang S. Bae
2021-11-24 20:06 ` [PATCH v3 13/15] crypto: x86/aes-kl - Support CBC mode Chang S. Bae
2021-11-24 20:06 ` [PATCH v3 14/15] crypto: x86/aes-kl - Support CTR mode Chang S. Bae
2021-11-24 20:07 ` [PATCH v3 15/15] crypto: x86/aes-kl - Support XTS mode Chang S. Bae
2021-11-30  3:27 ` [PATCH v3 00/15] x86: Support Key Locker Eric Biggers
2021-11-30  6:36   ` Bae, Chang Seok
2021-11-30  7:23     ` Eric Biggers
2021-11-30  7:34       ` Bae, Chang Seok

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