[PATCH] KVM: nVMX: Clear nested.smm.guest_mode on a failed RSM-to-L2

[Amirmohammad Eftekhar <amirmohammad.eftekhar@cispa.de>]

vmx_leave_smm() consumes nested.smm.vmxon into nested.vmxon up front, but
clears nested.smm.guest_mode only on the success path.  On either failure
arm (nested_vmx_check_restored_vmcs12() < 0, or
nested_vmx_enter_non_root_mode() != NVMX_VMENTRY_SUCCESS) it returns early
with nested.smm.guest_mode still true while nested.smm.vmxon is already
false.

em_rsm() turns that non-zero return into a triple fault, but the vCPU is
left with the internal pair (smm.guest_mode=true, smm.vmxon=false).  That
pair is not representable in migration state: vmx_get_nested_state()
serializes KVM_STATE_NESTED_SMM_GUEST_MODE without
KVM_STATE_NESTED_SMM_VMXON, and vmx_set_nested_state() rejects exactly
SMM_GUEST_MODE && !SMM_VMXON with -EINVAL.  KVM thus serializes a state it
refuses to restore -- a save/restore (live-migration) round-trip break.

The leak is sticky: free_nested() and vCPU reset do not clear
nested.smm.guest_mode (kvm_vcpu_reset() gates the nested unwind on
is_guest_mode(), false here), so a later VMXON re-exposes the bad
KVM_GET_NESTED_STATE with no SMI/RSM involved, for the life of the vCPU.

Clear nested.smm.guest_mode before the consistency check and re-entry, so
it is cleared on every exit from the block.  nested.vmxon is left true,
which is correct (L1 does have VMX on) and round-trips cleanly on its own.
Not a memory-safety issue; fidelity / migration only.

Cc: stable@vger.kernel.org
Signed-off-by: Amirmohammad Eftekhar <amirmohammad.eftekhar@cispa.de>
---
 arch/x86/kvm/vmx/vmx.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 1f802cdbe..56332a85c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -8381,6 +8381,18 @@ int vmx_leave_smm(struct kvm_vcpu *vcpu, const union kvm_smram *smram)
 	}
 
 	if (vmx->nested.smm.guest_mode) {
+		/*
+		 * Clear smm.guest_mode up front so the failure arms below, which
+		 * triple-fault the vCPU, cannot leave the internal pair
+		 * (smm.guest_mode=true, smm.vmxon=false): smm.vmxon was already
+		 * consumed into nested.vmxon above, and KVM_GET_NESTED_STATE
+		 * would then serialize SMM_GUEST_MODE without SMM_VMXON, which
+		 * KVM_SET_NESTED_STATE rejects (-EINVAL), breaking the
+		 * save/restore round-trip.  The flag has served its only purpose
+		 * (bringing the vCPU into this re-entry) by now.
+		 */
+		vmx->nested.smm.guest_mode = false;
+
 		/* Triple fault if the state is invalid.  */
 		if (nested_vmx_check_restored_vmcs12(vcpu) < 0)
 			return 1;
@@ -8390,7 +8402,6 @@ int vmx_leave_smm(struct kvm_vcpu *vcpu, const union kvm_smram *smram)
 			return 1;
 
 		vcpu->arch.nested_run_pending = KVM_NESTED_RUN_PENDING;
-		vmx->nested.smm.guest_mode = false;
 	}
 	return 0;
 }
-- 
2.54.0 (Apple Git-157)