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@xxxxxxxxx> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> Cc: Jonathan Corbet <corbet@xxxxxxx> Cc: x86@xxxxxxxxxx Cc: linux-doc@xxxxxxxxxxxxxxx Cc: linux-kernel@xxxxxxxxxxxxxxx --- 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
