KVM: selftests: Extend x86's sync_regs_test to check for event vector races

Attempt to modify the to-be-injected exception vector to an illegal value
_after_ the sanity checks performed by KVM_CAP_SYNC_REGS's
arch/x86/kvm/x86.c:kvm_vcpu_ioctl_x86_set_vcpu_events().  Buggy KVM
versions will eventually yells loudly about attempting to inject a bogus
vector, e.g.

  WARNING: CPU: 0 PID: 1107 at arch/x86/kvm/x86.c:547 kvm_check_and_inject_events+0x4a0/0x500 [kvm]

  arch/x86/kvm/x86.c:exception_type():
  WARN_ON(vector > 31 || vector == NMI_VECTOR)
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20230728001606.2275586-3-mhal@rbox.co
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/x86_64/sync_regs_test.c

@@ -81,6 +81,27 @@ static void compare_vcpu_events(struct kvm_vcpu_events *left,
 #define TEST_SYNC_FIELDS   (KVM_SYNC_X86_REGS|KVM_SYNC_X86_SREGS|KVM_SYNC_X86_EVENTS)
 #define INVALID_SYNC_FIELD 0x80000000
 
+/*
+ * Set an invalid exception vector while KVM is processing events.  KVM is
+ * supposed to reject any vector >= 32, as well as NMIs (vector 2).
+ */
+static void *race_events_exc(void *arg)
+{
+	struct kvm_run *run = (struct kvm_run *)arg;
+	struct kvm_vcpu_events *events = &run->s.regs.events;
+
+	for (;;) {
+		WRITE_ONCE(run->kvm_dirty_regs, KVM_SYNC_X86_EVENTS);
+		WRITE_ONCE(events->flags, 0);
+		WRITE_ONCE(events->exception.pending, 1);
+		WRITE_ONCE(events->exception.nr, 255);
+
+		pthread_testcancel();
+	}
+
+	return NULL;
+}
+
 /*
  * Toggle CR4.PAE while KVM is processing SREGS, EFER.LME=1 with CR4.PAE=0 is
  * illegal, and KVM's MMU heavily relies on vCPU state being valid.
@@ -289,6 +310,7 @@ int main(int argc, char *argv[])
 	kvm_vm_free(vm);
 
 	race_sync_regs(race_sregs_cr4);
+	race_sync_regs(race_events_exc);
 
 	return 0;
 }