KVM: x86: Check for nested events if there is an injectable interrupt

With commit b6b8a1451fc40412c57d1 that introduced
vmx_check_nested_events, checks for injectable interrupts happen
at different points in time for L1 and L2 that could potentially
cause a race. The regression occurs because KVM_REQ_EVENT is always
set when nested_run_pending is set even if there's no pending interrupt.
Consequently, there could be a small window when check_nested_events
returns without exiting to L1, but an interrupt comes through soon
after and it incorrectly, gets injected to L2 by inject_pending_event
Fix this by adding a call to check for nested events too when a check
for injectable interrupt returns true

Signed-off-by: Bandan Das <bsd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f6449334ec45..ef432f891d30 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5887,6 +5887,18 @@ static int inject_pending_event(struct kvm_vcpu *vcpu, bool req_int_win)
                         kvm_x86_ops->set_nmi(vcpu);
                 }
         } else if (kvm_cpu_has_injectable_intr(vcpu)) {
+                /*
+                 * Because interrupts can be injected asynchronously, we are
+                 * calling check_nested_events again here to avoid a race condition.
+                 * See https://lkml.org/lkml/2014/7/2/60 for discussion about this
+                 * proposal and current concerns.  Perhaps we should be setting
+                 * KVM_REQ_EVENT only on certain events and not unconditionally?
+                 */
+                if (is_guest_mode(vcpu) && kvm_x86_ops->check_nested_events) {
+                        r = kvm_x86_ops->check_nested_events(vcpu, req_int_win);
+                        if (r != 0)
+                                return r;
+                }
                 if (kvm_x86_ops->interrupt_allowed(vcpu)) {
                         kvm_queue_interrupt(vcpu, kvm_cpu_get_interrupt(vcpu),
                                             false);