[Devel] [PATCH 1/2] KVM: nVMX: fix guest CR4 loading when emulating L2 to L1 exit

[Jan Dakinevich]

From: Haozhong Zhang <haozhong.zhang at intel.com>

When KVM emulates an exit from L2 to L1, it loads L1 CR4 into the
guest CR4. Before this CR4 loading, the guest CR4 refers to L2
CR4. Because these two CR4's are in different levels of guest, we
should vmx_set_cr4() rather than kvm_set_cr4() here. The latter, which
is used to handle guest writes to its CR4, checks the guest change to
CR4 and may fail if the change is invalid.

The failure may cause trouble. Consider we start
  a L1 guest with non-zero L1 PCID in use,
     (i.e. L1 CR4.PCIDE == 1 && L1 CR3.PCID != 0)
and
  a L2 guest with L2 PCID disabled,
     (i.e. L2 CR4.PCIDE == 0)
and following events may happen:

1. If kvm_set_cr4() is used in load_vmcs12_host_state() to load L1 CR4
   into guest CR4 (in VMCS01) for L2 to L1 exit, it will fail because
   of PCID check. As a result, the guest CR4 recorded in L0 KVM (i.e.
   vcpu->arch.cr4) is left to the value of L2 CR4.

2. Later, if L1 attempts to change its CR4, e.g., clearing VMXE bit,
   kvm_set_cr4() in L0 KVM will think L1 also wants to enable PCID,
   because the wrong L2 CR4 is used by L0 KVM as L1 CR4. As L1
   CR3.PCID != 0, L0 KVM will inject GP to L1 guest.

Fixes: 4704d0befb072 ("KVM: nVMX: Exiting from L2 to L1")
Cc: qemu-stable at nongnu.org
Signed-off-by: Haozhong Zhang <haozhong.zhang at intel.com>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>

https://jira.sw.ru/browse/PSBM-83005
(cherry picked from commit 8eb3f87d903168bdbd1222776a6b1e281f50513e)
Signed-off-by: Jan Dakinevich <jan.dakinevich at virtuozzo.com>
---
 arch/x86/kvm/vmx.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 8ab2b3d..5ac9221 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -10454,7 +10454,7 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
 	 * (KVM doesn't change it)- no reason to call set_cr4_guest_host_mask();
 	 */
 	vcpu->arch.cr4_guest_owned_bits = ~vmcs_readl(CR4_GUEST_HOST_MASK);
-	kvm_set_cr4(vcpu, vmcs12->host_cr4);
+	vmx_set_cr4(vcpu, vmcs12->host_cr4);
 
 	nested_ept_uninit_mmu_context(vcpu);
 
-- 
2.1.4