On Fri, Sep 6, 2019 at 1:06 PM Liran Alon <liran.alon@xxxxxxxxxx> wrote:
>
>
>
> > 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.
You're not bothered by the fact that the vmx kvm-unit-test now dies
early? Should I just comment out the APIC-access address tests that
are currently xfail?
> > ---
> > 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.
SGTM
> > + 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.
Yeah. This is ugly, but look what I had to work with!
> > */
> > 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().
The use of static branch prediction hints in kvm seems pretty ad hoc
to me, but I'll go ahead and add one to the referenced code path.
> > + 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.
Previously, any value less than 0 was consistency check VMFail instead
of just ret==-1. :-)
> 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;
> }
You and me both. On the other hand, I also dislike code churn. It
complicates release engineering.
> 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?
I assume this is because of the call to
nested_vmx_enter_non_root_mode() from vmx_pre_leave_smm(), but we do
have that from_vmentry argument that I despise, so we may as well use
it.
> 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
> >
>
