Commit 7ca29de2 authored Nov 30, 2016 by Ladi Prosek Committed by Paolo Bonzini Dec 08, 2016

KVM: nVMX: fix CR3 load if L2 uses PAE paging and EPT



KVM does not correctly handle L1 hypervisors that emulate L2 real mode with
PAE and EPT, such as Hyper-V. In this mode, the L1 hypervisor populates guest
PDPTE VMCS fields and leaves guest CR3 uninitialized because it is not used
(see 26.3.2.4 Loading Page-Directory-Pointer-Table Entries). KVM always
dereferences CR3 and tries to load PDPTEs if PAE is on. This leads to two
related issues:

1) On the first nested vmentry, the guest PDPTEs, as populated by L1, are
overwritten in ept_load_pdptrs because the registers are believed to have
been loaded in load_pdptrs as part of kvm_set_cr3. This is incorrect. L2 is
running with PAE enabled but PDPTRs have been set up by L1.

2) When L2 is about to enable paging and loads its CR3, we, again, attempt
to load PDPTEs in load_pdptrs called from kvm_set_cr3. There are no guarantees
that this will succeed (it's just a CR3 load, paging is not enabled yet) and
if it doesn't, kvm_set_cr3 returns early without persisting the CR3 which is
then lost and L2 crashes right after it enables paging.

This patch replaces the kvm_set_cr3 call with a simple register write if PAE
and EPT are both on. CR3 is not to be interpreted in this case.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

parent 5a6a9748

arch/x86/kvm/vmx.c

+14 −2

Original line number	Diff line number	Diff line
		@@ -9981,6 +9981,7 @@ static void prepare_vmcs02(struct kvm_vcpu vcpu, struct vmcs12 vmcs12)
		{
		struct vcpu_vmx *vmx = to_vmx(vcpu);
		u32 exec_control;
		bool nested_ept_enabled = false;

		vmcs_write16(GUEST_ES_SELECTOR, vmcs12->guest_es_selector);
		vmcs_write16(GUEST_CS_SELECTOR, vmcs12->guest_cs_selector);
		@@ -10145,6 +10146,7 @@ static void prepare_vmcs02(struct kvm_vcpu vcpu, struct vmcs12 vmcs12)
		vmcs12->guest_intr_status);
		}

		nested_ept_enabled = (exec_control & SECONDARY_EXEC_ENABLE_EPT) != 0;
		vmcs_write32(SECONDARY_VM_EXEC_CONTROL, exec_control);
		}

		@@ -10295,8 +10297,18 @@ static void prepare_vmcs02(struct kvm_vcpu vcpu, struct vmcs12 vmcs12)
		/* Note: modifies VM_ENTRY/EXIT_CONTROLS and GUEST/HOST_IA32_EFER */
		vmx_set_efer(vcpu, vcpu->arch.efer);

		/* shadow page tables on either EPT or shadow page tables */
		/*
		* Shadow page tables on either EPT or shadow page tables.
		* If PAE and EPT are both on, CR3 is not used by the CPU and must not
		* be dereferenced.
		*/
		if (is_pae(vcpu) && is_paging(vcpu) && !is_long_mode(vcpu) &&
		nested_ept_enabled) {
		vcpu->arch.cr3 = vmcs12->guest_cr3;
		__set_bit(VCPU_EXREG_CR3, (ulong *)&vcpu->arch.regs_avail);
		} else
		kvm_set_cr3(vcpu, vmcs12->guest_cr3);

		kvm_mmu_reset_context(vcpu);

		if (!enable_ept)

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