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

[Bandan Das <bsd@redhat.com>]

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>
---
 arch/x86/kvm/x86.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 73537ec..56327a6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5907,6 +5907,19 @@ 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)) {
+		/*
+		 * TODO/FIXME: We are calling check_nested_events again
+		 * here to avoid a race condition. We should really be
+		 * setting KVM_REQ_EVENT only on certain events
+		 * and not unconditionally.
+		 * See https://lkml.org/lkml/2014/7/2/60 for discussion
+		 * about this proposal and current concerns
+		 */
+		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);
-- 
1.8.3.1