Re: [PATCH v19 076/130] KVM: TDX: Finalize VM initialization

Adrian Hunter
2024-04-11T16:39:21Z

On 26/02/24 10:26, isaku.yamahata@intel.com wrote:
> From: Isaku Yamahata <isaku.yamahata@intel.com>
>
> To protect the initial contents of the guest TD, the TDX module measures
> the guest TD during the build process as SHA-384 measurement. The
> measurement of the guest TD contents needs to be completed to make the
> guest TD ready to run.
>
> Add a new subcommand, KVM_TDX_FINALIZE_VM, for VM-scoped
> KVM_MEMORY_ENCRYPT_OP to finalize the measurement and mark the TDX VM ready
> to run.

Perhaps a spruced up commit message would be:

<BEGIN>
Add a new VM-scoped KVM_MEMORY_ENCRYPT_OP IOCTL subcommand,
KVM_TDX_FINALIZE_VM, to perform TD Measurement Finalization.

Documentation for the API is added in another patch:
"Documentation/virt/kvm: Document on Trust Domain Extensions(TDX)"

For the purpose of attestation, a measurement must be made of the TDX VM
initial state. This is referred to as TD Measurement Finalization, and
uses SEAMCALL TDH.MR.FINALIZE, after which:
1. The VMM adding TD private pages with arbitrary content is no longer
allowed
2. The TDX VM is runnable
<END>

History:

This code is essentially unchanged from V1, as below.
Except for V5, the code has never had any comments.
Paolo's comment from then still appears unaddressed.

V19: Unchanged
V18: Undoes change of V17
V17: Also change tools/arch/x86/include/uapi/asm/kvm.h
V16: Unchanged
V15: Undoes change of V10
V11-V14: Unchanged
V10: Adds a hack (related to TDH_MEM_TRACK)
that was later removed in V15
V6-V9: Unchanged
V5 Broke out the code into a separate patch and
received its only comments, which were from Paolo:

"Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Note however that errors should be passed back in the struct."

This presumably refers to struct kvm_tdx_cmd which has an "error"
member, but that is not updated by tdx_td_finalizemr()

V4 was a cut-down series and the code was not present
V3 introduced WARN_ON_ONCE for the error condition
V2 accommodated renaming the seamcall function and ID

Outstanding:

1. Address Paolo's comment about the error code
2. Is WARN_ON sensible?

Final note:

It might be possible to make TD Measurement Finalization
transparent to the user space VMM and forego another API, but it seems
doubtful that would really make anything much simpler.

>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
>
> ---
> v18:
> - Remove the change of tools/arch/x86/include/uapi/asm/kvm.h.
>
> v14 -> v15:
> - removed unconditional tdx_track() by tdx_flush_tlb_current() that
> does tdx_track().
>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> ---
> arch/x86/include/uapi/asm/kvm.h | 1 +
> arch/x86/kvm/vmx/tdx.c | 21 +++++++++++++++++++++
> 2 files changed, 22 insertions(+)
>
> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> index 34167404020c..c160f60189d1 100644
> --- a/arch/x86/include/uapi/asm/kvm.h
> +++ b/arch/x86/include/uapi/asm/kvm.h
> @@ -573,6 +573,7 @@ enum kvm_tdx_cmd_id {
> KVM_TDX_INIT_VM,
> KVM_TDX_INIT_VCPU,
> KVM_TDX_EXTEND_MEMORY,
> + KVM_TDX_FINALIZE_VM,
>
> KVM_TDX_CMD_NR_MAX,
> };
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 3cfba63a7762..6aff3f7e2488 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -1400,6 +1400,24 @@ static int tdx_extend_memory(struct kvm *kvm, struct kvm_tdx_cmd *cmd)
> return ret;
> }
>
> +static int tdx_td_finalizemr(struct kvm *kvm)
> +{
> + struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> + u64 err;
> +
> + if (!is_hkid_assigned(kvm_tdx) || is_td_finalized(kvm_tdx))
> + return -EINVAL;
> +
> + err = tdh_mr_finalize(kvm_tdx->tdr_pa);
> + if (WARN_ON_ONCE(err)) {

Is a failed SEAMCALL really something to WARN over?

> + pr_tdx_error(TDH_MR_FINALIZE, err, NULL);

As per Paolo, error code is not returned in struct kvm_tdx_cmd

> + return -EIO;
> + }
> +
> + kvm_tdx->finalized = true;
> + return 0;
> +}
> +
> int tdx_vm_ioctl(struct kvm *kvm, void __user *argp)
> {
> struct kvm_tdx_cmd tdx_cmd;
> @@ -1422,6 +1440,9 @@ int tdx_vm_ioctl(struct kvm *kvm, void __user *argp)
> case KVM_TDX_EXTEND_MEMORY:
> r = tdx_extend_memory(kvm, &tdx_cmd);
> break;
> + case KVM_TDX_FINALIZE_VM:
> + r = tdx_td_finalizemr(kvm);
> + break;
> default:
> r = -EINVAL;
> goto out;