KVM: only allow one gsi per fd

Looks like repeatedly binding same fd to multiple gsi's with irqfd can
use up a ton of kernel memory for irqfd structures.

A simple fix is to allow each fd to only trigger one gsi: triggering a
storm of interrupts in guest is likely useless anyway, and we can do it
by binding a single gsi to many interrupts if we really want to.

Cc: stable@kernel.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Acked-by: Gregory Haskins <ghaskins@novell.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

virt/kvm/eventfd.c

@@ -166,7 +166,7 @@ irqfd_ptable_queue_proc(struct file *file, wait_queue_head_t *wqh,
 static int
 kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
 {
-	struct _irqfd *irqfd;
+	struct _irqfd *irqfd, *tmp;
 	struct file *file = NULL;
 	struct eventfd_ctx *eventfd = NULL;
 	int ret;
@@ -203,9 +203,20 @@ kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
 	init_waitqueue_func_entry(&irqfd->wait, irqfd_wakeup);
 	init_poll_funcptr(&irqfd->pt, irqfd_ptable_queue_proc);
 
+	spin_lock_irq(&kvm->irqfds.lock);
+
+	ret = 0;
+	list_for_each_entry(tmp, &kvm->irqfds.items, list) {
+		if (irqfd->eventfd != tmp->eventfd)
+			continue;
+		/* This fd is used for another irq already. */
+		ret = -EBUSY;
+		spin_unlock_irq(&kvm->irqfds.lock);
+		goto fail;
+	}
+
 	events = file->f_op->poll(file, &irqfd->pt);
 
-	spin_lock_irq(&kvm->irqfds.lock);
 	list_add_tail(&irqfd->list, &kvm->irqfds.items);
 	spin_unlock_irq(&kvm->irqfds.lock);