On 6/3/23 08:22, Chang S. Bae wrote: > Document the overview of the feature along with relevant consideration > when provisioning dm-crypt volumes with AES-KL instead of AES-NI. > > --- > --- > 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 > > 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 Upon the event of a ... > +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 data volume > +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, Is it logged anywhere? i.e., a kernel log message? > + 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. > + -- ~Randy