linux-kernel - Re: [PATCH v6 25/25] KVM: nVMX: Enable CET support for nested guest

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Date:   Wed, 15 Nov 2023 16:56:27 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, Chao Gao <chao.gao@...el.com>
CC:     <seanjc@...gle.com>, <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <dave.hansen@...el.com>,
        <peterz@...radead.org>, <rick.p.edgecombe@...el.com>,
        <john.allen@....com>
Subject: Re: [PATCH v6 25/25] KVM: nVMX: Enable CET support for nested guest

On 11/1/2023 5:54 PM, Maxim Levitsky wrote:
> On Wed, 2023-11-01 at 10:09 +0800, Chao Gao wrote:
>> On Thu, Sep 14, 2023 at 02:33:25AM -0400, Yang Weijiang wrote:
>>> Set up CET MSRs, related VM_ENTRY/EXIT control bits and fixed CR4 setting
>>> to enable CET for nested VM.
>>>
>>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>>> ---
>>> arch/x86/kvm/vmx/nested.c | 27 +++++++++++++++++++++++++--
>>> arch/x86/kvm/vmx/vmcs12.c |  6 ++++++
>>> arch/x86/kvm/vmx/vmcs12.h | 14 +++++++++++++-
>>> arch/x86/kvm/vmx/vmx.c    |  2 ++
>>> 4 files changed, 46 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>>> index 78a3be394d00..2c4ff13fddb0 100644
>>> --- a/arch/x86/kvm/vmx/nested.c
>>> +++ b/arch/x86/kvm/vmx/nested.c
>>> @@ -660,6 +660,28 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
>>> 	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
>>> 					 MSR_IA32_FLUSH_CMD, MSR_TYPE_W);
>>>
>>> +	/* Pass CET MSRs to nested VM if L0 and L1 are set to pass-through. */
>>> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
>>> +					 MSR_IA32_U_CET, MSR_TYPE_RW);
>>> +
>>> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
>>> +					 MSR_IA32_S_CET, MSR_TYPE_RW);
>>> +
>>> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
>>> +					 MSR_IA32_PL0_SSP, MSR_TYPE_RW);
>>> +
>>> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
>>> +					 MSR_IA32_PL1_SSP, MSR_TYPE_RW);
>>> +
>>> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
>>> +					 MSR_IA32_PL2_SSP, MSR_TYPE_RW);
>>> +
>>> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
>>> +					 MSR_IA32_PL3_SSP, MSR_TYPE_RW);
>>> +
>>> +	nested_vmx_set_intercept_for_msr(vmx, msr_bitmap_l1, msr_bitmap_l0,
>>> +					 MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW);
>>> +
>>> 	kvm_vcpu_unmap(vcpu, &vmx->nested.msr_bitmap_map, false);
>>>
>>> 	vmx->nested.force_msr_bitmap_recalc = false;
>>> @@ -6794,7 +6816,7 @@ static void nested_vmx_setup_exit_ctls(struct vmcs_config *vmcs_conf,
>>> 		VM_EXIT_HOST_ADDR_SPACE_SIZE |
>>> #endif
>>> 		VM_EXIT_LOAD_IA32_PAT | VM_EXIT_SAVE_IA32_PAT |
>>> -		VM_EXIT_CLEAR_BNDCFGS;
>>> +		VM_EXIT_CLEAR_BNDCFGS | VM_EXIT_LOAD_CET_STATE;
>>> 	msrs->exit_ctls_high |=
>>> 		VM_EXIT_ALWAYSON_WITHOUT_TRUE_MSR |
>>> 		VM_EXIT_LOAD_IA32_EFER | VM_EXIT_SAVE_IA32_EFER |
>>> @@ -6816,7 +6838,8 @@ static void nested_vmx_setup_entry_ctls(struct vmcs_config *vmcs_conf,
>>> #ifdef CONFIG_X86_64
>>> 		VM_ENTRY_IA32E_MODE |
>>> #endif
>>> -		VM_ENTRY_LOAD_IA32_PAT | VM_ENTRY_LOAD_BNDCFGS;
>>> +		VM_ENTRY_LOAD_IA32_PAT | VM_ENTRY_LOAD_BNDCFGS |
>>> +		VM_ENTRY_LOAD_CET_STATE;
>>> 	msrs->entry_ctls_high |=
>>> 		(VM_ENTRY_ALWAYSON_WITHOUT_TRUE_MSR | VM_ENTRY_LOAD_IA32_EFER |
>>> 		 VM_ENTRY_LOAD_IA32_PERF_GLOBAL_CTRL);
>>> diff --git a/arch/x86/kvm/vmx/vmcs12.c b/arch/x86/kvm/vmx/vmcs12.c
>>> index 106a72c923ca..4233b5ca9461 100644
>>> --- a/arch/x86/kvm/vmx/vmcs12.c
>>> +++ b/arch/x86/kvm/vmx/vmcs12.c
>>> @@ -139,6 +139,9 @@ const unsigned short vmcs12_field_offsets[] = {
>>> 	FIELD(GUEST_PENDING_DBG_EXCEPTIONS, guest_pending_dbg_exceptions),
>>> 	FIELD(GUEST_SYSENTER_ESP, guest_sysenter_esp),
>>> 	FIELD(GUEST_SYSENTER_EIP, guest_sysenter_eip),
>>> +	FIELD(GUEST_S_CET, guest_s_cet),
>>> +	FIELD(GUEST_SSP, guest_ssp),
>>> +	FIELD(GUEST_INTR_SSP_TABLE, guest_ssp_tbl),
>> I think we need to sync guest states, e.g., guest_s_cet/guest_ssp/guest_ssp_tbl,
>> between vmcs02 and vmcs12 on nested VM entry/exit, probably in
>> sync_vmcs02_to_vmcs12() and prepare_vmcs12() or "_rare" variants of them.
>>
> Aha, this is why I suspected that nested support is incomplete,
> 100% agree.
>
> In particular, looking at Intel's SDM I see that:
>
> HOST_S_CET, HOST_SSP, HOST_INTR_SSP_TABLE needs to be copied from vmcb12 to vmcb02 but not vise versa
> because the CPU doesn't touch them.
>
> GUEST_S_CET, GUEST_SSP, GUEST_INTR_SSP_TABLE should be copied bi-directionally.

Yes, I'll make this part of code complete in next version, thanks!

> This of course depends on the corresponding vm entry and vm exit controls being set.
> That means that it is legal in theory to do VM entry/exit with CET enabled but not use
> VM_ENTRY_LOAD_CET_STATE and/or VM_EXIT_LOAD_CET_STATE,
> because for example nested hypervisor in theory can opt to save/load these itself.
>
> I think that this is all, but I also can't be 100% sure. This thing has to be tested well before
> we can be sure that it works.
>
> Best regards,
> 	Maxim Levitsky
>