linux-kernel - Re: [PATCH 20/21] KVM: TDX: Finalize VM initialization

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Message-ID: <88eb6d03-2fb4-49cf-944a-6ec64bf83ac8@intel.com>
Date: Tue, 10 Sep 2024 14:54:54 +0300
From: Adrian Hunter <adrian.hunter@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>, seanjc@...gle.com,
 kvm@...r.kernel.org
Cc: kai.huang@...el.com, dmatlack@...gle.com, isaku.yamahata@...il.com,
 yan.y.zhao@...el.com, nik.borisov@...e.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 20/21] KVM: TDX: Finalize VM initialization

On 10/09/24 13:25, Paolo Bonzini wrote:
> On 9/4/24 05:07, Rick Edgecombe wrote:
>> From: Isaku Yamahata <isaku.yamahata@...el.com>
>>
>> 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
>>
>> Co-developed-by: Adrian Hunter <adrian.hunter@...el.com>
>> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
>> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
>> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
>> ---
>> TDX MMU part 2 v1:
>>   - Added premapped check.
>>   - Update for the wrapper functions for SEAMCALLs. (Sean)
>>   - Add check if nr_premapped is zero.  If not, return error.
>>   - Use KVM_BUG_ON() in tdx_td_finalizer() for consistency.
>>   - Change tdx_td_finalizemr() to take struct kvm_tdx_cmd *cmd and return error
>>     (Adrian)
>>   - Handle TDX_OPERAND_BUSY case (Adrian)
>>   - Updates from seamcall overhaul (Kai)
>>   - Rename error->hw_error
>>
>> v18:
>>   - Remove the change of tools/arch/x86/include/uapi/asm/kvm.h.
>>
>> v15:
>>   - removed unconditional tdx_track() by tdx_flush_tlb_current() that
>>     does tdx_track().
>> ---
>>   arch/x86/include/uapi/asm/kvm.h |  1 +
>>   arch/x86/kvm/vmx/tdx.c          | 28 ++++++++++++++++++++++++++++
>>   2 files changed, 29 insertions(+)
>>
>> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
>> index 789d1d821b4f..0b4827e39458 100644
>> --- a/arch/x86/include/uapi/asm/kvm.h
>> +++ b/arch/x86/include/uapi/asm/kvm.h
>> @@ -932,6 +932,7 @@ enum kvm_tdx_cmd_id {
>>       KVM_TDX_INIT_VM,
>>       KVM_TDX_INIT_VCPU,
>>       KVM_TDX_INIT_MEM_REGION,
>> +    KVM_TDX_FINALIZE_VM,
>>       KVM_TDX_GET_CPUID,
>>         KVM_TDX_CMD_NR_MAX,
>> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
>> index 796d1a495a66..3083a66bb895 100644
>> --- a/arch/x86/kvm/vmx/tdx.c
>> +++ b/arch/x86/kvm/vmx/tdx.c
>> @@ -1257,6 +1257,31 @@ void tdx_flush_tlb_current(struct kvm_vcpu *vcpu)
>>       ept_sync_global();
>>   }
>>   +static int tdx_td_finalizemr(struct kvm *kvm, struct kvm_tdx_cmd *cmd)
>> +{
>> +    struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
>> +
>> +    if (!is_hkid_assigned(kvm_tdx) || is_td_finalized(kvm_tdx))
>> +        return -EINVAL;
>> +    /*
>> +     * Pages are pending for KVM_TDX_INIT_MEM_REGION to issue
>> +     * TDH.MEM.PAGE.ADD().
>> +     */
>> +    if (atomic64_read(&kvm_tdx->nr_premapped))
>> +        return -EINVAL;
> 
> I suggest moving all of patch 16, plus the
> 
> +    WARN_ON_ONCE(!atomic64_read(&kvm_tdx->nr_premapped));
> +    atomic64_dec(&kvm_tdx->nr_premapped);
> 
> lines of patch 19, into this patch.
> 
>> +    cmd->hw_error = tdh_mr_finalize(kvm_tdx);
>> +    if ((cmd->hw_error & TDX_SEAMCALL_STATUS_MASK) == TDX_OPERAND_BUSY)
>> +        return -EAGAIN;
>> +    if (KVM_BUG_ON(cmd->hw_error, kvm)) {
>> +        pr_tdx_error(TDH_MR_FINALIZE, cmd->hw_error);
>> +        return -EIO;
>> +    }
>> +
>> +    kvm_tdx->finalized = true;
>> +    return 0;
> 
> This should also set pre_fault_allowed to true.

Ideally, need to ensure it is not possible for another CPU
to see kvm_tdx->finalized==false and pre_fault_allowed==true

Perhaps also, to document the dependency, return an error if
pre_fault_allowed is true in tdx_mem_page_record_premap_cnt().