linux-kernel - Re: [PATCH] KVM: nVMX: Fix L2 guest hang if shadow page tables on EPT

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Message-ID: <CABdb734qbaDh51DK2xhAC0kRgqtgY-mb6YvstM-5D6XXqhuQqA@mail.gmail.com>
Date:   Fri, 17 Mar 2017 18:28:09 +0100
From:   Ladi Prosek <lprosek@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, KVM list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH] KVM: nVMX: Fix L2 guest hang if shadow page tables on EPT

On Fri, Mar 17, 2017 at 3:41 PM, Wanpeng Li <kernellwp@...il.com> wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
>
> The L2 guest hang if shadow page tables on EPT, the trace on L1 shows that
> L2 kvm_exit reason EXCEPTION_NMI and page fault repeatedly:
>
> qemu-system-x86-2821  [003] d..2    45.848814: kvm_entry: vcpu 0
> qemu-system-x86-2821  [003] ...1    45.848827: kvm_exit: reason EXCEPTION_NMI rip 0xe05b info fe05b 80000b0e
> qemu-system-x86-2821  [003] ...1    45.848827: kvm_page_fault: address fe05b error_code 14
>
> Commit 7ca29de21362 (KVM: nVMX: fix CR3 load if L2 uses PAE paging and EPT)
> prevents to load L2's PDPTRs according to dereferencing L2's CR3 since it is
> uninitialized in real mode. Hyper-V L1 will emulate L2 real mode with PAE
> paging and EPT enabled. However, there is a progress to switch from Legacy
> mode's such-mode Protected mode to Long mode during system boot, the check
> in nested_vmx_load_cr3() will prevent to load PDPTRs if it is still in
> Protected mode w/ PAE paging and nested EPT/shadow page tables on EPT. Actually
> the original commit should just intended to prevent to dereference L2's CR3
> if the L1 hypervisor emulates L2's real mode through vm8086.
>
> This patch fixes it by allowing load PDPTRs if PAE paing, EPT enabled and
> !vm86_active.
>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Ladi Prosek <lprosek@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  arch/x86/kvm/vmx.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index c664365..2b2a05f 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -9933,7 +9933,7 @@ static bool nested_cr3_valid(struct kvm_vcpu *vcpu, unsigned long val)
>  static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool nested_ept,
>                                u32 *entry_failure_code)
>  {
> -       if (cr3 != kvm_read_cr3(vcpu) || (!nested_ept && pdptrs_changed(vcpu))) {
> +       if (cr3 != kvm_read_cr3(vcpu) || pdptrs_changed(vcpu)) {
>                 if (!nested_cr3_valid(vcpu, cr3)) {
>                         *entry_failure_code = ENTRY_FAIL_DEFAULT;
>                         return 1;
> @@ -9944,7 +9944,7 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool ne
>                  * must not be dereferenced.
>                  */
>                 if (!is_long_mode(vcpu) && is_pae(vcpu) && is_paging(vcpu) &&
> -                   !nested_ept) {
> +                   !(nested_ept && to_vmx(vcpu)->rmode.vm86_active)) {

This change breaks Hyper-V on KVM. L2 hangs on start-up, same symptoms
as before 7ca29de21362.

I'll take a closer look next week. Is there an easy way for me to
reproduce the issue you're seeing?

>                         if (!load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3)) {
>                                 *entry_failure_code = ENTRY_FAIL_PDPTE;
>                                 return 1;
> --
> 2.7.4
>