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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 machine (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 to a different entity using a different key the
encrypted guests data will be incorrectly decrypted, leading to unintelligible
data.
The key management of this feature is handled by separate processor known as
AMD secure processor (AMD-SP) which is present in AMD SOCs. Firmware running
inside the AMD-SP provide commands to support common VM lifecycle. This
includes commands for launching, snapshotting, migrating and debugging the
encrypted guest. Those SEV command can be issued via KVM_MEMORY_ENCRYPT_OP
ioctls.
Launching
---------
Boot images (such as bios) must be encrypted before guest can be booted.
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 the
successful launch.
LAUNCH_START is called first to create a cryptographic launch context within
the firmware. To create this context, guest owner must provides guest policy,
its public Diffie-Hellman key (PDH) and session parameters. These inputs
should be treated as binary blob and must be passed as-is to the SEV firmware.
The guest policy is passed as plaintext and hypervisor may able 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 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...\
Guest owners 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 'dh-cert-file' and
'session-file' property (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 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_MEASURE command can be used to retrieve the measurement of encrypted
memory. This measurement is a signature of the memory contents that can be
sent to the guest owner as an attestation that the memory was 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 command finalizes the guest launch and destroy's 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
Debugging
-----------
Since memory contents of SEV guest is encrypted hence hypervisor access to the
guest memory will get a cipher text. If guest policy allows debugging, then
hypervisor can use DEBUG_DECRYPT and DEBUG_ENCRYPT commands 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
AMD64 Architecture Programmer's Manual:
http://support.amd.com/TechDocs/24593.pdf
SME is section 7.10
SEV is section 15.34
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