libvirt/docs/manpages/virt-qemu-sev-validate.rst

virt-qemu-sev-validate

validate a domain AMD SEV launch measurement

Manual section

1

Manual group

Virtualization Support

SYNOPSIS

virt-qemu-sev-validate [OPTIONS]

DESCRIPTION

This program validates the reported measurement for a domain launched with AMD SEV. If the program exits with a status of zero, the guest owner can be confident that their guest OS is running under the protection offered by the SEV / SEV-ES platform.

Note that the level of protection varies depending on the AMD SEV platform generation and describing the differences is outside the scope of this document.

For the results of this program to be considered trustworthy, it is required to be run on a machine that is already trusted by the guest owner. This could be a machine that the guest owner has direct physical control over, or it could be another virtual machine protected by AMD SEV that has already had its launch measurement validated. Running this program on the virtualization host will not produce an answer that can be trusted.

If told to connect to libvirt, it will refuse to use a libvirt connection that is local to the machine, since that cannot be trusted. For the sake of testing or demonstration purposes, however, it can be forced to run in this scenario using the --insecure flag. The result will, of course, still not be trustworthy.

OPTIONS

Common options

-h, --help

Display command line help usage then exit.

-d, --debug

Show debug information while running

-q, --quiet

Don't print information about the attestation result.

Guest state options

These options provide information about the state of the guest that needs its boot attested.

--measurement BASE64-STRING

The launch measurement reported by the hypervisor of the domain to be validated. The measurement must be 48 bytes of binary data encoded as a base64 string.

--api-major VERSION

The SEV API major version of the hypervisor the domain is running on.

--api-minor VERSION

The SEV API major version of the hypervisor the domain is running on.

--build-id ID

The SEV build ID of the hypervisor the domain is running on.

--policy POLiCY

The policy bitmask associated with the session launch data of the domain to be validated.

Guest config options

These options provide items needed to calculate the expected domain launch measurement. This will then be compared to the reported launch measurement.

-f PATH, --firmware=PATH

Path to the firmware loader binary. This is the EDK2 build that knows how to initialize AMD SEV. For the validation to be trustworthy it important that the firmware build used has no support for loading non-volatile variables from NVRAM, even if NVRAM is expose to the guest.

-k PATH, --kernel=PATH

Path to the kernel binary if doing direct kernel boot.

-r PATH, --initrd=PATH

Path to the initrd binary if doing direct kernel boot. Defaults to zero length content if omitted.

-e STRING, --cmdline=STRING

String containing any kernel command line parameters used during boot of the domain. Defaults to the empty string if omitted.

-n COUNT, --num-cpus=COUNT

The number of virtual CPUs for the domain. This is required when the domain policy is set to require SEV-ES.

-0 PATH, --vmsa-cpu0=PATH

Path to the VMSA initial state for the boot CPU. This is required when the domain policy is set to require SEV-ES. The file contents must be exactly 4096 bytes in length.

-1 PATH, --vmsa-cpu1=PATH

Path to the VMSA initial state for the non-boot CPU. This is required when the domain policy is set to require SEV-ES and the domain has more than one CPU present. The file contents must be exactly 4096 bytes in length.

--tik PATH

TIK file for domain. This file must be exactly 16 bytes in size and contains the unique transport integrity key associated with the domain session launch data. This is mutually exclusive with the --tk argument.

--tek PATH

TEK file for domain. This file must be exactly 16 bytes in size and contains the unique transport encryption key associated with the domain session launch data. This is mutually exclusive with the --tk argument.

--tk PATH

TEK/TIK combined file for the domain. This file must be exactly 32 bytes in size, with the first 16 bytes containing the TEK and the last 16 bytes containing the TIK. This is mutually exclusive with the --tik and --tek arguments.

Libvirt options

These options are used when connecting to libvirt to automatically obtain state and configuration information about the domain to be attested.

-c, --connect URI

Libvirt connection URI. For the validation to be trustworthy this must be a URI resolving to a remote virtualization host. This requirement can be overridden using the --insecure argument.

-o, --domain ID|NAME|UUID

Domain ID, or domain name or domain UUID. Used to identify which libvirt domain is to have its launch measured. The domain must be running, and would usually have been started in a paused state, to allow validation to be performed before guest CPUs begin execution.

-i, --insecure

Proceed even if usage scenario is known to be insecure. This allows the program to connect to a local libvirt hypervisor and rely on file content from the virtualization host. It also allows the validation to proceed even if the virtual machine CPUs are not in the initial paused state. The result of the validation must not be trusted.

-g, --ignore-config

Do not attempt to sanity check the domain config. The default behaviour is to print out errors if identifying configuration elements in the guest XML that would invalidate the launch measurement. This can help the guest owner to understand any configuration mistakes that have been made. If the --ignore-config argument is given, this sanity checking of configuration will be skipped. The result is that the validation will likely be reported as failed.

Secret injection options

These options provide a way to inject a secret if validation of the launch measurement passes.

--inject-secret ALIAS-OR-GUID:PATH

Path to a file containing a secret to inject into the guest OS. Typical usage would be to supply a password for unlocking the root filesystem full disk encryption. ALIAS can be one of the well known secrets:

• luks-key - bytes to use as a key for unlocking a LUKS key slot. GUID of 736869e5-84f0-4973-92ec-06879ce3da0b.

Alternatively GUID refers to an arbitrary UUID of the callers choosing. The contents of PATH are defined by the requirements of the associated GUID, and will used as-is without modification. In particular be aware:

• Avoid unwanted trailing newline characters in PATH unless mandated by the GUID.
• Any trailing NUL byte must be explicitly included in PATH if mandated by the GUID.

This argument can be repeated multiple times, provided a different GUID is given for each instance.

--secret-header PATH

Path to a file in which the injected secret header will be written in base64 format and later injected into the domain. This is required if there is no connection to libvirt, otherwise the secret will be directly injected.

--secret-payload PATH

Path to a file in which the injected secret payload will be written in base64 format and later injected into the domain. This is required if there is no connection to libvirt, otherwise the secret will be directly injected.

EXAMPLES

Fully offline execution

This scenario allows a measurement to be securely validated in a completely offline state without any connection to the hypervisor host. All required data items must be provided as command line parameters. This usage model is considered secure, because all input data is provided by the user.

Validate the measurement of a SEV guest booting from disk:

# virt-qemu-sev-validate \
    --firmware OVMF.sev.fd \
    --tk this-guest-tk.bin \
    --measurement Zs2pf19ubFSafpZ2WKkwquXvACx9Wt/BV+eJwQ/taO8jhyIj/F8swFrybR1fZ2ID \
    --api-major 0 \
    --api-minor 24 \
    --build-id 13 \
    --policy 3

Validate the measurement of a SEV guest with direct kernel boot:

# virt-qemu-sev-validate \
    --firmware OVMF.sev.fd \
    --kernel vmlinuz-5.11.12 \
    --initrd initramfs-5.11.12 \
    --cmdline "root=/dev/vda1" \
    --tk this-guest-tk.bin \
    --measurement Zs2pf19ubFSafpZ2WKkwquXvACx9Wt/BV+eJwQ/taO8jhyIj/F8swFrybR1fZ2ID \
    --api-major 0 \
    --api-minor 24 \
    --build-id 13 \
    --policy 3

Validate the measurement of a SEV-ES SMP guest booting from disk:

# virt-qemu-sev-validate \
    --firmware OVMF.sev.fd \
    --num-cpus 2 \
    --vmsa-cpu0 vmsa0.bin \
    --vmsa-cpu1 vmsa1.bin \
    --tk this-guest-tk.bin \
    --measurement Zs2pf19ubFSafpZ2WKkwquXvACx9Wt/BV+eJwQ/taO8jhyIj/F8swFrybR1fZ2ID \
    --api-major 0 \
    --api-minor 24 \
    --build-id 13 \
    --policy 7

Validate the measurement of a SEV-ES SMP guest booting from disk, with automatically constructed VMSA:

# virt-qemu-sev-validate \
    --firmware OVMF.sev.fd \
    --num-cpus 2 \
    --cpu-family 23 \
    --cpu-model 49 \
    --cpu-stepping 0 \
    --tk this-guest-tk.bin \
    --measurement Zs2pf19ubFSafpZ2WKkwquXvACx9Wt/BV+eJwQ/taO8jhyIj/F8swFrybR1fZ2ID \
    --api-major 0 \
    --api-minor 24 \
    --build-id 13 \
    --policy 7

Validate the measurement of a SEV guest booting from disk and inject a disk password on success:

# virt-qemu-sev-validate \
    --firmware OVMF.sev.fd \
    --tk this-guest-tk.bin \
    --measurement Zs2pf19ubFSafpZ2WKkwquXvACx9Wt/BV+eJwQ/taO8jhyIj/F8swFrybR1fZ2ID \
    --api-major 0 \
    --api-minor 24 \
    --build-id 13 \
    --policy 3 \
    --inject-secret 736869e5-84f0-4973-92ec-06879ce3da0b:passwd.txt \
    --secret-header secret-header.b64 \
    --secret-payload secret-payload.b64

The secret-header.b64 and secret-payload.b64 files can now be sent to the virtualization host for injection.

Fetch from remote libvirt

This scenario allows fetching certain data from a remote hypervisor via a connection to libvirt. It will aid in debugging by analysing the guest configuration and reporting anything that could invalidate the measurement of the guest. This usage model is considered secure, because the limited information obtained from the untrusted hypervisor cannot be used to change the result.

Validate the measurement of a SEV guest booting from disk:

# virt-qemu-sev-validate \
    --connect qemu+ssh://root@some.remote.host/system \
    --firmware OVMF.sev.fd \
    --tk this-guest-tk.bin \
    --domain fedora34x86_64

Validate the measurement of a SEV guest with direct kernel boot:

# virt-qemu-sev-validate \
    --connect qemu+ssh://root@some.remote.host/system \
    --firmware OVMF.sev.fd \
    --kernel vmlinuz-5.11.12 \
    --initrd initramfs-5.11.12 \
    --cmdline "root=/dev/vda1" \
    --tk this-guest-tk.bin \
    --domain fedora34x86_64

Validate the measurement of a SEV-ES SMP guest booting from disk:

# virt-qemu-sev-validate \
    --connect qemu+ssh://root@some.remote.host/system \
    --firmware OVMF.sev.fd \
    --num-cpus 2 \
    --vmsa-cpu0 vmsa0.bin \
    --vmsa-cpu1 vmsa1.bin \
    --tk this-guest-tk.bin \
    --domain fedora34x86_64

Validate the measurement of a SEV-ES SMP guest booting from disk, with automatically constructed VMSA:

# virt-qemu-sev-validate \
    --connect qemu+ssh://root@some.remote.host/system \
    --firmware OVMF.sev.fd \
    --cpu-family 23 \
    --cpu-model 49 \
    --cpu-stepping 0 \
    --tk this-guest-tk.bin \
    --domain fedora34x86_64

Validate the measurement of a SEV guest booting from disk and inject a disk password on success:

# virt-qemu-sev-validate \
    --connect qemu+ssh://root@some.remote.host/system \
    --firmware OVMF.sev.fd \
    --tk this-guest-tk.bin \
    --domain fedora34x86_64 \
    --inject-secret 736869e5-84f0-4973-92ec-06879ce3da0b:passwd.txt

Fetch from local libvirt

This scenario allows fetching all data from the local hypervisor via a connection to libvirt. It is only to be used for the purpose of testing, debugging, or demonstrations, because running on the local hypervisor is not a secure scenario. To enable this usage, the --insecure flag must be specified. Given a pointer to the libvirt guest to validate, all information needed to perform a validation, except the TIK/TEK pair can be acquired automatically.

Validate the measurement of a SEV guest booting from disk:

# virt-qemu-sev-validate \
    --insecure \
    --tk this-guest-tk.bin \
    --domain fedora34x86_64

Validate the measurement of a SEV guest with direct kernel boot:

# virt-qemu-sev-validate \
    --insecure \
    --tk this-guest-tk.bin \
    --domain fedora34x86_64

Validate the measurement of a SEV-ES SMP guest booting from disk:

# virt-qemu-sev-validate \
    --insecure \
    --vmsa-cpu0 vmsa0.bin \
    --vmsa-cpu1 vmsa1.bin \
    --tk this-guest-tk.bin \
    --domain fedora34x86_64

Validate the measurement of a SEV-ES SMP guest booting from disk, with automatically constructed VMSA:

# virt-qemu-sev-validate \
    --insecure \
    --tk this-guest-tk.bin \
    --domain fedora34x86_64

Validate the measurement of a SEV guest booting from disk and inject a disk password on success:

# virt-qemu-sev-validate \
    --insecure \
    --tk this-guest-tk.bin \
    --domain fedora34x86_64 \
    --inject-secret 736869e5-84f0-4973-92ec-06879ce3da0b:passwd.txt

COMMON MISTAKES CHECKLIST

The complexity of configuring a guest and validating its boot measurement means it is very likely to see the failure:

ERROR: Measurement does not match, VM is not trustworthy

This error message assumes the worst, but in most cases will failure will be a result of either mis-configuring the guest, or passing the wrong information when trying to validate it. The following information is a guide for what items to check in order to stand the best chance of diagnosing the problem

• Check the VM configuration for the DH certificate and session blob in the libvirt guest XML.

  The content for these fields should be in base64 format, which is what sevctl session generates. Other tools may generate the files in binary format, so ensure it has been correctly converted to base64.

• Check the VM configuration policy value matches the session blob

  The <policy> value in libvirt guest XML has to match the value passed to the sevctl session command. If this is mismatched then the guest will not even start, and QEMU will show an error such as:

  sev_launch_start: LAUNCH_START ret=1 fw_error=11 'Bad measurement'

• Check the correct TIK/TEK keypair are passed

  The TIK/TEK keypair are uniquely tied to each DH cert and session blob. Make sure that the TIK/TEK keypair passed to this program the ones matched to the DH cert and session blob configured for the libvirt guest XML. This is one of the most common mistakes. Further ensure that the TIK and TEK files are not swapped.

• Check the firmware binary matches the one used to boot

  The firmware binary content is part of the data covered by the launch measurement. Ensure that the firmware binary passed to this program matches the one used to launch the guest. The hypervisor host will periodically get software updates which introduce a new firmware binary version.

• Check the kernel, initrd and cmdline match the one used to boot

  If the guest is configured to use direct kernel boot, check that the kernel, initrd and cmdline passed to this program match the ones used to boot the guest. In the kernel cmdline whitespace must be preserved exactly, including any leading or trailing spaces.

• Check whether the kernel hash measurement is enabled

  The kernelHashes property in the libvirt guest XML controls whether hashes of the kernel, initrd and cmdline content are covered by the boot measurement. If enabled, then the matching content must be passed to this program. UIf disabled, then the content must NOT be passed.

• Check that the correct measurement hash is passed

  The measurement hash includes a nonce, so it will be different on every boot attempt. Thus when validating the measuremnt it is important ensure the most recent measurement is used.

• Check the correct VMSA blobs / CPU SKU values for the host are used

  The VMSA blobs provide the initial register state for the boot CPU and any additional CPUs. One of the registers encodes the CPU SKU (family, model, stepping) of the physical host CPU. Make sure that the VMSA blob used for validation is one that matches the SKU of the host the guest is booted on. Passing the CPU SKU values directly to the tool can reduce the likelihood of using the wrong ones.

• Check the CPU count is correct

  When passing VMSA blobs for SEV-ES guests, the number of CPUs present will influence the measurement result. Ensure that the correct vCPU count is used corresponding to the guest boot attempt.

Best practice is to run this tool in completely offline mode and pass all information as explicit command line parameters. When debugging failures, however, it can be useful to tell it to connect to libvirt and fetch information. If connecting to a remote libvirt instance, it will fetch any information that can be trusted, which is the basic VM launch state data. It will also sanity check the XML configuration to identify some common mistakes. If the --insecure flag is passed it can extract some configuration information and use that for the attestation process.

If the mistake still can't be identified, then this tool can be run on the virtualization host. In that scenario the only three command line parameters required are for the TIK, TEK and libvirt domain name. It should be able to automatically determine all the other information required. If it still reports a failure, this points very strongly to the TIK/TEK pair not matching the configured DH certificate and session blob.

The --debug flag will display hashes and/or hex dumps for various pieces of information used in the attestation process. Comparing the --debug output from running on the hypervisor host, against that obtained when running in offline mode can give further guidance to which parameter is inconsistent.

As mentioned earlier in this document, bear in mind that in general any attestation answers obtained from running on the hypervisor host should not be trusted. So if a configuration mistake is identified it is strongly recommended to re-run the attestation in offline mode on a trusted machine.

EXIT STATUS

Upon successful attestation of the launch measurement, an exit status of 0 will be set.

Upon failure to attest the launch measurement one of the following codes will be set:

• 1 - Guest measurement did not validate

  Assuming the inputs to this program are correct, the virtual machine launch has been compromised and it should not be trusted henceforth.

• 2 - Usage scenario cannot be supported

  The way in which this program has been invoked prevent it from being able to validate the launch measurement.

• 3 - Usage scenario is not secure

  The way in which this program has been invoked means that the result of any launch measurement validation will not be secure.

  The program can be reinvoked with --insecure argument to force a validation, however, the results of this should not be trusted. This should only be used for testing, debugging or demonstration purposes, never in a production deployment.

• 4 - Domain has incorrect configuration to be measured

  The way in which the guest has been configured prevent this program from being able to validate the launch measurement. Note that in general the guest configuration reported by the hypervisor is not trustworthy, so it is possible this error could be a false positive designed to cause a denial of service.

  This program can be reinvoked with the --ignore-config argument to skip the sanity checks on the domain XML. This will likely result in it failing with an exit code of 1 indicating the measurement is invalid

• 5 - Domain is in incorrect state to be measured

  The domain has to be running in order to validate a launch measurement.

• 6 - unexpected error occurred in the code

  A logic flaw in this program means it is unable to complete the validation of the measurement. This is a bug which should be reported to the maintainers.

AUTHOR

Daniel P. Berrangé

BUGS

Please report all bugs you discover. This should be done via either:

1. the mailing list

   https://libvirt.org/contact.html

2. the bug tracker

   https://libvirt.org/bugs.html

Alternatively, you may report bugs to your software distributor / vendor.

COPYRIGHT

Copyright (C) 2022 by Red Hat, Inc.

LICENSE

virt-qemu-sev-validate is distributed under the terms of the GNU LGPL v2.1+. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE

SEE ALSO

virsh(1), SEV launch security usage, https://www.libvirt.org/