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KVM: Introduce KVM_EXIT_SNP_REQ_CERTS for SNP certificate-fetching

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For SEV-SNP, the host can optionally provide a certificate table to the
guest when it issues an attestation request to firmware (see GHCB 2.0
specification regarding "SNP Extended Guest Requests"). This certificate
table can then be used to verify the endorsement key used by firmware to
sign the attestation report.

While it is possible for guests to obtain the certificates through other
means, handling it via the host provides more flexibility in being able
to keep the certificate data in sync with the endorsement key throughout
host-side operations that might resulting in the endorsement key
changing.

In the case of KVM, userspace will be responsible for fetching the
certificate table and keeping it in sync with any modifications to the
endorsement key by other userspace management tools. Define a new
KVM_EXIT_SNP_REQ_CERTS event where userspace is provided with the GPA of
the buffer the guest has provided as part of the attestation request so
that userspace can write the certificate data into it while relying on
filesystem-based locking to keep the certificates up-to-date relative to
the endorsement keys installed/utilized by firmware at the time the
certificates are fetched.

[Melody: Update the documentation scheme about how file locking is
         expected to happen.]

Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Tested-by: Liam Merwick <liam.merwick@oracle.com>
Tested-by: Dionna Glaze <dionnaglaze@google.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Melody Wang <huibo.wang@amd.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Link: https://patch.msgid.link/20260109231732.1160759-2-michael.roth@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Michael Roth
2026-01-09 17:17:32 -06:00
committed by Sean Christopherson
parent 55780d8a1d
commit fa9893fadb
4 changed files with 110 additions and 6 deletions
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44
Documentation/virt/kvm/api.rst
View File

@@ -7353,6 +7353,50 @@ Please note that the kernel is allowed to use the kvm_run structure as the
primary storage for certain register types. Therefore, the kernel may use the
values in kvm_run even if the corresponding bit in kvm_dirty_regs is not set.
::
/* KVM_EXIT_SNP_REQ_CERTS */
struct kvm_exit_snp_req_certs {
__u64 gpa;
__u64 npages;
__u64 ret;
};
KVM_EXIT_SNP_REQ_CERTS indicates an SEV-SNP guest with certificate-fetching
enabled (see KVM_SEV_SNP_ENABLE_REQ_CERTS) has generated an Extended Guest
Request NAE #VMGEXIT (SNP_GUEST_REQUEST) with message type MSG_REPORT_REQ,
i.e. has requested an attestation report from firmware, and would like the
certificate data corresponding to the attestation report signature to be
provided by the hypervisor as part of the request.
To allow for userspace to provide the certificate, the 'gpa' and 'npages'
are forwarded verbatim from the guest request (the RAX and RBX GHCB fields
respectively). 'ret' is not an "output" from KVM, and is always '0' on
exit. KVM verifies the 'gpa' is 4KiB aligned prior to exiting to userspace,
but otherwise the information from the guest isn't validated.
Upon the next KVM_RUN, e.g. after userspace has serviced the request (or not),
KVM will complete the #VMGEXIT, using the 'ret' field to determine whether to
signal success or failure to the guest, and on failure, what reason code will
be communicated via SW_EXITINFO2. If 'ret' is set to an unsupported value (see
the table below), KVM_RUN will fail with -EINVAL. For a 'ret' of 'ENOSPC', KVM
also consumes the 'npages' field, i.e. userspace can use the field to inform
the guest of the number of pages needed to hold all the certificate data.
The supported 'ret' values and their respective SW_EXITINFO2 encodings:
====== =============================================================
0 0x0, i.e. success. KVM will emit an SNP_GUEST_REQUEST command
to SNP firmware.
ENOSPC 0x0000000100000000, i.e. not enough guest pages to hold the
certificate table and certificate data. KVM will also set the
RBX field in the GHBC to 'npages'.
EAGAIN 0x0000000200000000, i.e. the host is busy and the guest should
retry the request.
EIO 0xffffffff00000000, for all other errors (this return code is
a KVM-defined hypervisor value, as allowed by the GHCB)
====== =============================================================
.. _cap_enable: