[PATCH v3] KVM: nVMX: Fix losing NMI blocking state

From: Wanpeng Li
Date: Tue Jul 25 2017 - 07:30:39 EST


Run kvm-unit-tests/eventinj.flat in L1 w/ ept=0 on both L0 and L1:

Before NMI IRET test
Sending NMI to self
NMI isr running stack 0x461000
Sending nested NMI to self
After nested NMI to self
Nested NMI isr running rip=40038e
After iret
After NMI to self
FAIL: NMI

Reference SDM 31.7.1.2:

If the âvirtual NMIsâ VM-execution control is 1, bit 12 of the VM-exit
interruption-information field indicates that the VM exit was due to a fault
encountered during an execution of the IRET instruction that removed virtual-NMI
blocking. In particular, it provides this indication if the following are both
true:

- Bit 31 (valid) in the IDT-vectoring information field is 0.
- The value of bits 7:0 (vector) of the VM-exit interruption-information
field is not 8 (the VM exit is not due to a double-fault exception).

If both are true and bit 12 of the VM-exit interruption-information field is 1,
there was virtual-NMI blocking before guest software executed the IRET instruction
that caused the fault that caused the VM exit. The VMM should set bit 3 (blocking
by NMI) in the interruptibility-state field (using VMREAD and VMWRITE) before
resuming guest software.

Commit 4c4a6f790ee862 (KVM: nVMX: track NMI blocking state separately for each VMCS)
tracks NMI blocking state separately for vmcs01 and vmcs02. However it is not enough:

- The L2 (kvm-unit-tests/eventinj.flat) generates NMI that will fault
on IRET, so the L2 can generate #PF which can be intercepted by L0.
- L0 walks L1's guest page table and sees the mapping is invalid, it
resumes the L1 guest and injects the #PF into L1. At this point the
vmcs02 has nmi_known_unmasked=true.
- L1 sets set bit 3 (blocking by NMI) in the interruptibility-state field
of vmcs12 (and fixes the shadow page table) before resuming L2 guest.
- L1 executes VMRESUME to resume L2, causing a vmexit to L0
- during VMRESUME emulation, prepare_vmcs02 sets bit 3 in the
interruptibility-state field of vmcs02, but nmi_known_unmasked is
still true.
- on the next L2 exit to L0, nmi_known_unmasked is true so
vmx_recover_nmi_blocking does not do anything.

This patch fixes it by updating nmi_known_unmasked when preparing vmcs02 from vmcs12.

Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
arch/x86/kvm/vmx.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 29fd8af..bc999a1 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -10041,6 +10041,8 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
vmcs12->vm_entry_instruction_len);
vmcs_write32(GUEST_INTERRUPTIBILITY_INFO,
vmcs12->guest_interruptibility_info);
+ vmx->loaded_vmcs->nmi_known_unmasked =
+ !(vmcs12->guest_interruptibility_info & GUEST_INTR_STATE_NMI);
} else {
vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, 0);
}
--
2.7.4