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

[Maxim Levitsky]

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@xxxxxxxxx>
> > ---
> > 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.

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