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SubjectRe: [PATCH] KVM: nVMX: Fix L2 guest hang if shadow page tables on EPT
On Sat, Mar 18, 2017 at 7:37 AM, Wanpeng Li <kernellwp@gmail.com> wrote:
> 2017-03-18 1:28 GMT+08:00 Ladi Prosek <lprosek@redhat.com>:
>> On Fri, Mar 17, 2017 at 3:41 PM, Wanpeng Li <kernellwp@gmail.com> wrote:
>>> From: Wanpeng Li <wanpeng.li@hotmail.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@redhat.com>
>>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>>> Cc: Ladi Prosek <lprosek@redhat.com>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.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.
>
> Hmm, I miss the function pdptrs_changed() will also dereference CR3.
> How about something like this:
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index c664365..d7ebf03 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -9933,7 +9933,9 @@ 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) ||
> + (!(nested_ept && to_vmx(vcpu)->rmode.vm86_active) &&
> + pdptrs_changed(vcpu))) {
> if (!nested_cr3_valid(vcpu, cr3)) {
> *entry_failure_code = ENTRY_FAIL_DEFAULT;
> return 1;
> @@ -9944,7 +9946,7 @@ 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)) {
> if (!load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3)) {
> *entry_failure_code = ENTRY_FAIL_PDPTE;
> return 1;

Still the same, Hyper-V is broken. The problem is not in real vs.
protected mode. The way nested_ept_enabled is computed is incorrect.

I can run both Hyper-V and KVM with EPT = 0 in L1 with this patch. Can
you please give it a try?

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 98e82ee..9145c94 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -10121,7 +10121,7 @@ static int prepare_vmcs02(struct kvm_vcpu
*vcpu, struct vmcs12 *vmcs12,
vmcs12->guest_intr_status);
}

- nested_ept_enabled = (exec_control &
SECONDARY_EXEC_ENABLE_EPT) != 0;
+ nested_ept_enabled =
(vmcs12->secondary_vm_exec_control & SECONDARY_EXEC_ENABLE_EPT) != 0;

/*
* Write an illegal value to APIC_ACCESS_ADDR. Later,
Thanks!
Ladi

>> I'll take a closer look next week. Is there an easy way for me to
>> reproduce the issue you're seeing?
>
> L1 KVM w/ EPT = 0 and L0 KVM w/ EPT = 1.
>
> Regards,
> Wanpeng Li
>
>>
>>> if (!load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3)) {
>>> *entry_failure_code = ENTRY_FAIL_PDPTE;
>>> return 1;
>>> --
>>> 2.7.4
>>>

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