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

From: Bandan Das
Date: Tue Jul 08 2014 - 00:30:37 EST

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@xxxxxxxxxx>
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)
} 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),

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