> On 6 Sep 2019, at 21:59, Jim Mattson <jmattson@xxxxxxxxxx> wrote:
>
> If the "virtualize APIC accesses" VM-execution control is set in the
> VMCS, the APIC virtualization hardware is triggered when a page walk
> in VMX non-root mode terminates at a PTE wherein the address of the 4k
> page frame matches the APIC-access address specified in the VMCS. On
> hardware, the APIC-access address may be any valid 4k-aligned physical
> address.
>
> KVM's nVMX implementation enforces the additional constraint that the
> APIC-access address specified in the vmcs12 must be backed by
> cacheable memory in L1. If not, L0 will simply clear the "virtualize
> APIC accesses" VM-execution control in the vmcs02.
>
> The problem with this approach is that the L1 guest has arranged the
> vmcs12 EPT tables--or shadow page tables, if the "enable EPT"
> VM-execution control is clear in the vmcs12--so that the L2 guest
> physical address(es)--or L2 guest linear address(es)--that reference
> the L2 APIC map to the APIC-access address specified in the
> vmcs12. Without the "virtualize APIC accesses" VM-execution control in
> the vmcs02, the APIC accesses in the L2 guest will directly access the
> APIC-access page in L1.
>
> When L0 has no mapping whatsoever for the APIC-access address in L1,
> the L2 VM just loses the intended APIC virtualization. However, when
> the L2 APIC-access address is mapped to an MMIO region in L1, the L2
> guest gets direct access to the L1 MMIO device. For example, if the
> APIC-access address specified in the vmcs12 is 0xfee00000, then L2
> gets direct access to L1's APIC.
>
> Fixing this correctly is complicated. Since this vmcs12 configuration
> is something that KVM cannot faithfully emulate, the appropriate
> response is to exit to userspace with
> KVM_INTERNAL_ERROR_EMULATION. Sadly, the kvm-unit-tests fail, so I'm
> posting this as an RFC.
>
> Note that the 'Code' line emitted by qemu in response to this error
> shows the guest %rip two instructions after the
> vmlaunch/vmresume. Hmmm.
>
> Fixes: fe3ef05c7572 ("KVM: nVMX: Prepare vmcs02 from vmcs01 and vmcs12")
> Reported-by: Dan Cross <dcross@xxxxxxxxxx>
> Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
> Reviewed-by: Marc Orr <marcorr@xxxxxxxxxx>
> Reviewed-by: Peter Shier <pshier@xxxxxxxxxx>
> Reviewed-by: Dan Cross <dcross@xxxxxxxxxx>
The idea of the patch and the functionality of it seems correct to me.
However, I have some small code comments below.
> ---
> arch/x86/include/asm/kvm_host.h | 2 +-
> arch/x86/kvm/vmx/nested.c | 55 ++++++++++++++++++++++-----------
> arch/x86/kvm/x86.c | 9 ++++--
> 3 files changed, 45 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 74e88e5edd9cf..e95acf8c82b47 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1191,7 +1191,7 @@ struct kvm_x86_ops {
> int (*set_nested_state)(struct kvm_vcpu *vcpu,
> struct kvm_nested_state __user *user_kvm_nested_state,
> struct kvm_nested_state *kvm_state);
> - void (*get_vmcs12_pages)(struct kvm_vcpu *vcpu);
> + int (*get_vmcs12_pages)(struct kvm_vcpu *vcpu);
>
> int (*smi_allowed)(struct kvm_vcpu *vcpu);
> int (*pre_enter_smm)(struct kvm_vcpu *vcpu, char *smstate);
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index ced9fba32598d..bdf5a11816fa4 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2871,7 +2871,7 @@ static int nested_vmx_check_vmentry_hw(struct kvm_vcpu *vcpu)
> static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
> struct vmcs12 *vmcs12);
>
> -static void nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> +static int nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> {
> struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -2891,19 +2891,31 @@ static void nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> vmx->nested.apic_access_page = NULL;
> }
> page = kvm_vcpu_gpa_to_page(vcpu, vmcs12->apic_access_addr);
> - /*
> - * If translation failed, no matter: This feature asks
> - * to exit when accessing the given address, and if it
> - * can never be accessed, this feature won't do
> - * anything anyway.
> - */
> if (!is_error_page(page)) {
> vmx->nested.apic_access_page = page;
> hpa = page_to_phys(vmx->nested.apic_access_page);
> vmcs_write64(APIC_ACCESS_ADDR, hpa);
> } else {
> - secondary_exec_controls_clearbit(vmx,
> - SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES);
> + /*
> + * Since there is no backing page, we can't
> + * just rely on the usual L1 GPA -> HPA
> + * translation mechanism to do the right
> + * thing. We'd have to assign an appropriate
> + * HPA for the L1 APIC-access address, and
> + * then we'd have to modify the MMU to ensure
> + * that the L1 APIC-access address is mapped
> + * to the assigned HPA if and only if an L2 VM
> + * with that APIC-access address and the
> + * "virtualize APIC accesses" VM-execution
> + * control set in the vmcs12 is running. For
> + * now, just admit defeat.
> + */
> + pr_warn_ratelimited("Unsupported vmcs12 APIC-access address\n");
> + vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> + vcpu->run->internal.suberror =
> + KVM_INTERNAL_ERROR_EMULATION;
> + vcpu->run->internal.ndata = 0;
I think it is wise to pass here vmcs12->apic_access_addr value to userspace.
In addition, also print it in pr_warn_ratelimited() call.
To aid debugging.
> + return -ENOTSUPP;
> }
> }
>
> @@ -2948,6 +2960,7 @@ static void nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> exec_controls_setbit(vmx, CPU_BASED_USE_MSR_BITMAPS);
> else
> exec_controls_clearbit(vmx, CPU_BASED_USE_MSR_BITMAPS);
> + return 0;
> }
>
> /*
> @@ -2986,11 +2999,12 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
> /*
> * If from_vmentry is false, this is being called from state restore (either RSM
> * or KVM_SET_NESTED_STATE). Otherwise it's called from vmlaunch/vmresume.
> -+ *
> -+ * Returns:
> -+ * 0 - success, i.e. proceed with actual VMEnter
> -+ * 1 - consistency check VMExit
> -+ * -1 - consistency check VMFail
> + *
> + * Returns:
> + * < 0 - error
> + * 0 - success, i.e. proceed with actual VMEnter
> + * 1 - consistency check VMExit
> + * 2 - consistency check VMFail
Whenever we start having non-trivial return values, I believe adding an enum
is in place. It makes code easier to read and less confusing.
> */
> int nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu, bool from_vmentry)
> {
> @@ -2999,6 +3013,7 @@ int nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu, bool from_vmentry)
> bool evaluate_pending_interrupts;
> u32 exit_reason = EXIT_REASON_INVALID_STATE;
> u32 exit_qual;
> + int r;
>
> evaluate_pending_interrupts = exec_controls_get(vmx) &
> (CPU_BASED_VIRTUAL_INTR_PENDING | CPU_BASED_VIRTUAL_NMI_PENDING);
> @@ -3035,11 +3050,13 @@ int nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu, bool from_vmentry)
> prepare_vmcs02_early(vmx, vmcs12);
>
> if (from_vmentry) {
> - nested_get_vmcs12_pages(vcpu);
> + r = nested_get_vmcs12_pages(vcpu);
> + if (unlikely(r))
It makes sense to also mark !is_error_page(page) as likely() in nested_get_vmcs12_pages().
> + return r;
>
> if (nested_vmx_check_vmentry_hw(vcpu)) {
> vmx_switch_vmcs(vcpu, &vmx->vmcs01);
> - return -1;
> + return 2;
> }
>
> if (nested_vmx_check_guest_state(vcpu, vmcs12, &exit_qual))
> @@ -3200,9 +3217,11 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
> vmx->nested.nested_run_pending = 1;
> ret = nested_vmx_enter_non_root_mode(vcpu, true);
> vmx->nested.nested_run_pending = !ret;
> - if (ret > 0)
> + if (ret < 0)
> + return 0;
> + if (ret == 1)
> return 1;
> - else if (ret)
> + if (ret)
All these checks for of "ret" are not really readable.
They also implicitly define any ret value which is >1 as consistency check VMFail instead of just ret==2.
I prefer to have something like:
switch (ret) {
case VMEXIT_ON_INVALID_STATE:
return 1;
case VMFAIL_ON_INVALID_STATE:
return nested_vmx_failValid(vcpu, VMXERR_ENTRY_INVALID_CONTROL_FIELD);
default:
/* Return to userspace on error */
if (ret < 0)
return 0;
}
In addition, can you remind me why we call nested_vmx_failValid() at nested_vmx_run()
instead of inside nested_vmx_enter_non_root_mode() when nested_vmx_check_vmentry_hw() fails?
Then code could have indeed simply be:
if (ret < 0)
return 0;
if (ret)
return 1;
….
Which is easy to understand.
i.e. First case return to userspace on error, second report error to guest and rest continue as usual
now assuming vCPU is non-root mode.
-Liran
> return nested_vmx_failValid(vcpu,
> VMXERR_ENTRY_INVALID_CONTROL_FIELD);
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 290c3c3efb877..5ddbf16c8b108 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7803,8 +7803,13 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> bool req_immediate_exit = false;
>
> if (kvm_request_pending(vcpu)) {
> - if (kvm_check_request(KVM_REQ_GET_VMCS12_PAGES, vcpu))
> - kvm_x86_ops->get_vmcs12_pages(vcpu);
> + if (kvm_check_request(KVM_REQ_GET_VMCS12_PAGES, vcpu)) {
> + r = kvm_x86_ops->get_vmcs12_pages(vcpu);
> + if (unlikely(r)) {
> + r = 0;
> + goto out;
> + }
> + }
> if (kvm_check_request(KVM_REQ_MMU_RELOAD, vcpu))
> kvm_mmu_unload(vcpu);
> if (kvm_check_request(KVM_REQ_MIGRATE_TIMER, vcpu))
> --
> 2.23.0.187.g17f5b7556c-goog
>
