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Secure Encrypted Virtualization (SEV) is a feature found on AMD processors.
SEV is an extension to the AMD-V architecture which supports running encrypted
virtual machines (VMs) under the control of KVM. Encrypted VMs have their pages
(code and data) secured such that only the guest itself has access to the
unencrypted version. Each encrypted VM is associated with a unique encryption
key; if its data is accessed by a different entity using a different key the
encrypted guests data will be incorrectly decrypted, leading to unintelligible
data.
Key management for this feature is handled by a separate processor known as the
AMD secure processor (AMD-SP), which is present in AMD SOCs. Firmware running
inside the AMD-SP provides commands to support a common VM lifecycle. This
includes commands for launching, snapshotting, migrating and debugging the
encrypted guest. These SEV commands can be issued via KVM_MEMORY_ENCRYPT_OP
ioctls.
Secure Encrypted Virtualization - Encrypted State (SEV-ES) builds on the SEV
support to additionally protect the guest register state. In order to allow a
hypervisor to perform functions on behalf of a guest, there is architectural
support for notifying a guest's operating system when certain types of VMEXITs
are about to occur. This allows the guest to selectively share information with
the hypervisor to satisfy the requested function.
Launching
---------
Boot images (such as bios) must be encrypted before a guest can be booted. The
MEMORY_ENCRYPT_OP ioctl provides commands to encrypt the images: LAUNCH_START,
LAUNCH_UPDATE_DATA, LAUNCH_MEASURE and LAUNCH_FINISH. These four commands
together generate a fresh memory encryption key for the VM, encrypt the boot
images and provide a measurement than can be used as an attestation of a
successful launch.
For a SEV-ES guest, the LAUNCH_UPDATE_VMSA command is also used to encrypt the
guest register state, or VM save area (VMSA), for all of the guest vCPUs.
LAUNCH_START is called first to create a cryptographic launch context within
the firmware. To create this context, guest owner must provide a guest policy,
its public Diffie-Hellman key (PDH) and session parameters. These inputs
should be treated as a binary blob and must be passed as-is to the SEV firmware.
The guest policy is passed as plaintext. A hypervisor may choose to read it,
but should not modify it (any modification of the policy bits will result
in bad measurement). The guest policy is a 4-byte data structure containing
several flags that restricts what can be done on a running SEV guest.
See KM Spec section 3 and 6.2 for more details.
The guest policy can be provided via the 'policy' property (see below)
# ${QEMU} \
sev-guest,id=sev0,policy=0x1...\
Setting the "SEV-ES required" policy bit (bit 2) will launch the guest as a
SEV-ES guest (see below)
# ${QEMU} \
sev-guest,id=sev0,policy=0x5...\
The guest owner provided DH certificate and session parameters will be used to
establish a cryptographic session with the guest owner to negotiate keys used
for the attestation.
The DH certificate and session blob can be provided via the 'dh-cert-file' and
'session-file' properties (see below)
# ${QEMU} \
sev-guest,id=sev0,dh-cert-file=<file1>,session-file=<file2>
LAUNCH_UPDATE_DATA encrypts the memory region using the cryptographic context
created via the LAUNCH_START command. If required, this command can be called
multiple times to encrypt different memory regions. The command also calculates
the measurement of the memory contents as it encrypts.
LAUNCH_UPDATE_VMSA encrypts all the vCPU VMSAs for a SEV-ES guest using the
cryptographic context created via the LAUNCH_START command. The command also
calculates the measurement of the VMSAs as it encrypts them.
LAUNCH_MEASURE can be used to retrieve the measurement of encrypted memory and,
for a SEV-ES guest, encrypted VMSAs. This measurement is a signature of the
memory contents and, for a SEV-ES guest, the VMSA contents, that can be sent
to the guest owner as an attestation that the memory and VMSAs were encrypted
correctly by the firmware. The guest owner may wait to provide the guest
confidential information until it can verify the attestation measurement.
Since the guest owner knows the initial contents of the guest at boot, the
attestation measurement can be verified by comparing it to what the guest owner
expects.
LAUNCH_FINISH finalizes the guest launch and destroys the cryptographic
context.
See SEV KM API Spec [1] 'Launching a guest' usage flow (Appendix A) for the
complete flow chart.
To launch a SEV guest
# ${QEMU} \
-machine ...,confidential-guest-support=sev0 \
-object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=1
To launch a SEV-ES guest
# ${QEMU} \
-machine ...,confidential-guest-support=sev0 \
-object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=1,policy=0x5
An SEV-ES guest has some restrictions as compared to a SEV guest. Because the
guest register state is encrypted and cannot be updated by the VMM/hypervisor,
a SEV-ES guest:
- Does not support SMM - SMM support requires updating the guest register
state.
- Does not support reboot - a system reset requires updating the guest register
state.
- Requires in-kernel irqchip - the burden is placed on the hypervisor to
manage booting APs.
Debugging
-----------
Since the memory contents of a SEV guest are encrypted, hypervisor access to
the guest memory will return cipher text. If the guest policy allows debugging,
then a hypervisor can use the DEBUG_DECRYPT and DEBUG_ENCRYPT commands to access
the guest memory region for debug purposes. This is not supported in QEMU yet.
Snapshot/Restore
-----------------
TODO
Live Migration
----------------
TODO
References
-----------------
AMD Memory Encryption whitepaper:
https://developer.amd.com/wordpress/media/2013/12/AMD_Memory_Encryption_Whitepaper_v7-Public.pdf
Secure Encrypted Virtualization Key Management:
[1] http://developer.amd.com/wordpress/media/2017/11/55766_SEV-KM-API_Specification.pdf
KVM Forum slides:
http://www.linux-kvm.org/images/7/74/02x08A-Thomas_Lendacky-AMDs_Virtualizatoin_Memory_Encryption_Technology.pdf
https://www.linux-kvm.org/images/9/94/Extending-Secure-Encrypted-Virtualization-with-SEV-ES-Thomas-Lendacky-AMD.pdf
AMD64 Architecture Programmer's Manual:
http://support.amd.com/TechDocs/24593.pdf
SME is section 7.10
SEV is section 15.34
SEV-ES is section 15.35
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