KVM: Disallow binding multiple irqfds to an eventfd with a priority waiter

Disallow binding an irqfd to an eventfd that already has a priority waiter,
i.e. to an eventfd that already has an attached irqfd.  KVM always
operates in exclusive mode for EPOLL_IN (unconditionally returns '1'),
i.e. only the first waiter will be notified.

KVM already disallows binding multiple irqfds to an eventfd in a single
VM, but doesn't guard against multiple VMs binding to an eventfd.  Adding
the extra protection reduces the pain of a userspace VMM bug, e.g. if
userspace fails to de-assign before re-assigning when transferring state
for intra-host migration, then the migration will explicitly fail as
opposed to dropping IRQs on the destination VM.

Temporarily keep KVM's manual check on irqfds.items, but add a WARN, e.g.
to allow sanity checking the waitqueue enforcement.

Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: David Matlack <dmatlack@google.com>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250522235223.3178519-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

virt/kvm/eventfd.c

@@ -291,38 +291,57 @@ static void kvm_irqfd_register(struct file *file, wait_queue_head_t *wqh,
 	struct kvm_kernel_irqfd *tmp;
 	struct kvm *kvm = p->kvm;
 
+	/*
+	 * Note, irqfds.lock protects the irqfd's irq_entry, i.e. its routing,
+	 * and irqfds.items.  It does NOT protect registering with the eventfd.
+	 */
 	spin_lock_irq(&kvm->irqfds.lock);
 
-	list_for_each_entry(tmp, &kvm->irqfds.items, list) {
+	/*
-		if (irqfd->eventfd != tmp->eventfd)
+	 * Initialize the routing information prior to adding the irqfd to the
-			continue;
+	 * eventfd's waitqueue, as irqfd_wakeup() can be invoked as soon as the
-		/* This fd is used for another irq already. */
+	 * irqfd is registered.
-		p->ret = -EBUSY;
+	 */
-		spin_unlock_irq(&kvm->irqfds.lock);
-		return;
-	}
-
 	irqfd_update(kvm, irqfd);
 
-	list_add_tail(&irqfd->list, &kvm->irqfds.items);
-
 	/*
 	 * Add the irqfd as a priority waiter on the eventfd, with a custom
 	 * wake-up handler, so that KVM *and only KVM* is notified whenever the
-	 * underlying eventfd is signaled.  Temporarily lie to lockdep about
+	 * underlying eventfd is signaled.
-	 * holding irqfds.lock to avoid a false positive regarding potential
-	 * deadlock with irqfd_wakeup() (see irqfd_wakeup() for details).
 	 */
 	init_waitqueue_func_entry(&irqfd->wait, irqfd_wakeup);
 
+	/*
+	 * Temporarily lie to lockdep about holding irqfds.lock to avoid a
+	 * false positive regarding potential deadlock with irqfd_wakeup()
+	 * (see irqfd_wakeup() for details).
+	 *
+	 * Adding to the wait queue will fail if there is already a priority
+	 * waiter, i.e. if the eventfd is associated with another irqfd (in any
+	 * VM).  Note, kvm_irqfd_deassign() waits for all in-flight shutdown
+	 * jobs to complete, i.e. ensures the irqfd has been removed from the
+	 * eventfd's waitqueue before returning to userspace.
+	 */
 	spin_release(&kvm->irqfds.lock.dep_map, _RET_IP_);
-	irqfd->wait.flags |= WQ_FLAG_EXCLUSIVE;
+	p->ret = add_wait_queue_priority_exclusive(wqh, &irqfd->wait);
-	add_wait_queue_priority(wqh, &irqfd->wait);
 	spin_acquire(&kvm->irqfds.lock.dep_map, 0, 0, _RET_IP_);
+	if (p->ret)
+		goto out;
 
+	list_for_each_entry(tmp, &kvm->irqfds.items, list) {
+		if (irqfd->eventfd != tmp->eventfd)
+			continue;
+
+		WARN_ON_ONCE(1);
+		/* This fd is used for another irq already. */
+		p->ret = -EBUSY;
+		goto out;
+	}
+
+	list_add_tail(&irqfd->list, &kvm->irqfds.items);
+
+out:
 	spin_unlock_irq(&kvm->irqfds.lock);
-
-	p->ret = 0;
 }
 
 #if IS_ENABLED(CONFIG_HAVE_KVM_IRQ_BYPASS)