KVM: nVMX: Preserve exception priority irrespective of exiting behavior

Short circuit vmx_check_nested_events() if an exception is pending and
needs to be injected into L2, priority between coincident events is not
dependent on exiting behavior.  This fixes a bug where a single-step #DB
that is not intercepted by L1 is incorrectly dropped due to servicing a
VMX Preemption Timer VM-Exit.

Injected exceptions also need to be blocked if nested VM-Enter is
pending or an exception was already injected, otherwise injecting the
exception could overwrite an existing event injection from L1.
Technically, this scenario should be impossible, i.e. KVM shouldn't
inject its own exception during nested VM-Enter.  This will be addressed
in a future patch.

Note, event priority between SMI, NMI and INTR is incorrect for L2, e.g.
SMI should take priority over VM-Exit on NMI/INTR, and NMI that is
injected into L2 should take priority over VM-Exit INTR.  This will also
be addressed in a future patch.

Fixes: b6b8a1451fc4 ("KVM: nVMX: Rework interception of IRQs and NMIs")
Reported-by: Jim Mattson <jmattson@google.com>
Cc: Oliver Upton <oupton@google.com>
Cc: Peter Shier <pshier@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200423022550.15113-2-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index b644bbf85460d41d5c20d1f6c1eb24191ef7bf26..de678be3c92d69f26c29e5e2b6efa1e4f2324d94 100644 (file)
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3716,11 +3716,11 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
        /*
         * Process any exceptions that are not debug traps before MTF.
         */
-       if (vcpu->arch.exception.pending &&
-           !vmx_pending_dbg_trap(vcpu) &&
-           nested_vmx_check_exception(vcpu, &exit_qual)) {
+       if (vcpu->arch.exception.pending && !vmx_pending_dbg_trap(vcpu)) {
                if (block_nested_events)
                        return -EBUSY;
+               if (!nested_vmx_check_exception(vcpu, &exit_qual))
+                       goto no_vmexit;
                nested_vmx_inject_exception_vmexit(vcpu, exit_qual);
                return 0;
        }
@@ -3733,10 +3733,11 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
                return 0;
        }
 
-       if (vcpu->arch.exception.pending &&
-           nested_vmx_check_exception(vcpu, &exit_qual)) {
+       if (vcpu->arch.exception.pending) {
                if (block_nested_events)
                        return -EBUSY;
+               if (!nested_vmx_check_exception(vcpu, &exit_qual))
+                       goto no_vmexit;
                nested_vmx_inject_exception_vmexit(vcpu, exit_qual);
                return 0;
        }
@@ -3771,6 +3772,7 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
                return 0;
        }
 
+no_vmexit:
        vmx_complete_nested_posted_interrupt(vcpu);
        return 0;
 }