[PATCH v2 2/2] KVM: nVMX: Don't use vmcs01.GUEST_CR3 to snapshot L1's CR3 when EPT is disabled

[Sean Christopherson <seanjc@google.com>]

Add a dedicated field in "struct nested_vmx" to track L1's pre-VM-Enter CR3
instead of using vmcs01.GUEST_CR3, which isn't anywhere near as safe as the
comment purports it to be.  E.g. in addition to the warn_on_missed_cc bug
(that was fixed by relocating the consistency check), if getting vmcs12
pages (during actual nested VM-Entry) fails and EPT is disabled (in KVM),
KVM will return control to userspace with vmcs01.GUEST_CR3 holding a guest-
controlled value.

Alternatively, KVM could force a reload of vmcs01.GUEST_CR3 by resetting
the MMU context in the error path, but as above, the safety of the vmcs01
approach is extremely questionable, e.g. it took all of ~4 months for the
code to break.

Fixes: 671ddc700fd0 ("KVM: nVMX: Don't leak L1 MMIO regions to L2")
Cc: stable@vger.kernel.org
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/vmx/nested.c | 21 ++++++++-------------
 arch/x86/kvm/vmx/vmx.h    |  7 +++++++
 2 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 199b866072c0..7a2251061bfa 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3669,19 +3669,14 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
 				    &vmx->nested.pre_vmenter_ssp_tbl);
 
 	/*
-	 * Overwrite vmcs01.GUEST_CR3 with L1's CR3 if EPT is disabled.  In the
-	 * event of a "late" VM-Fail, i.e. a VM-Fail detected by hardware but
-	 * not KVM, KVM must unwind its software model to the pre-VM-Entry host
-	 * state.  When EPT is disabled, GUEST_CR3 holds KVM's shadow CR3, not
-	 * L1's "real" CR3, which causes nested_vmx_restore_host_state() to
-	 * corrupt vcpu->arch.cr3.  Stuffing vmcs01.GUEST_CR3 results in the
-	 * unwind naturally setting arch.cr3 to the correct value.  Smashing
-	 * vmcs01.GUEST_CR3 is safe because nested VM-Exits, and the unwind,
-	 * reset KVM's MMU, i.e. vmcs01.GUEST_CR3 is guaranteed to be
-	 * overwritten with a shadow CR3 prior to re-entering L1.
+	 * Stash L1's CR3, so that in the event of a "late" VM-Fail, i.e. a
+	 * VM-Fail detected by hardware but not KVM, KVM can unwind its
+	 * software model to the pre-VM-Entry host state.  When EPT is
+	 * disabled, GUEST_CR3 holds KVM's shadow CR3, not L1's "real" CR3,
+	 * and so simply restoring from vmcs01.GUEST_CR3 would corrupt
+	 * vcpu->arch.cr3.
 	 */
-	if (!enable_ept)
-		vmcs_writel(GUEST_CR3, vcpu->arch.cr3);
+	vmx->nested.pre_vmenter_cr3 = kvm_read_cr3(vcpu);
 
 	vmx_switch_vmcs(vcpu, &vmx->nested.vmcs02);
 
@@ -4993,7 +4988,7 @@ static void nested_vmx_restore_host_state(struct kvm_vcpu *vcpu)
 	vmx_set_cr4(vcpu, vmcs_readl(CR4_READ_SHADOW));
 
 	nested_ept_uninit_mmu_context(vcpu);
-	vcpu->arch.cr3 = vmcs_readl(GUEST_CR3);
+	vcpu->arch.cr3 = vmx->nested.pre_vmenter_cr3;
 	kvm_register_mark_available(vcpu, VCPU_REG_CR3);
 
 	/*
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index de9de0d2016c..dc8517f15bc4 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -159,6 +159,13 @@ struct nested_vmx {
 	bool has_preemption_timer_deadline;
 	bool preemption_timer_expired;
 
+	/*
+	 * Used to restore L1's CR3 if hardware detects a VM-Fail Consistency
+	 * Check that KVM does not, in which case KVM needs to unwind CR3 back
+	 * to its pre-VM-Enter state, NOT to vmcs01.HOST_CR3.
+	 */
+	unsigned long pre_vmenter_cr3;
+
 	/*
 	 * Used to snapshot MSRs that are conditionally loaded on VM-Enter in
 	 * order to propagate the guest's pre-VM-Enter value into vmcs02.  For
-- 
2.54.0.1136.gdb2ca164c4-goog