Re: [PATCH v2] vhost/net: Replace wait_queue with completion in ubufs reference

Hillf Danton
2025-07-18T23:04:10Z

On Fri, 18 Jul 2025 14:03:55 +0300 Nikolay Kuratov wrote:
> When operating on struct vhost_net_ubuf_ref, the following execution
> sequence is theoretically possible:
> CPU0 is finalizing DMA operation CPU1 is doing VHOST_NET_SET_BACKEND
> // &ubufs->refcount == 2
> vhost_net_ubuf_put() vhost_net_ubuf_put_wait_and_free(oldubufs)
> vhost_net_ubuf_put_and_wait()
> vhost_net_ubuf_put()
> int r = atomic_sub_return(1, &ubufs->refcount);
> // r = 1
> int r = atomic_sub_return(1, &ubufs->refcount);
> // r = 0
> wait_event(ubufs->wait, !atomic_read(&ubufs->refcount));
> // no wait occurs here because condition is already true
> kfree(ubufs);
> if (unlikely(!r))
> wake_up(&ubufs->wait); // use-after-free
>
> This leads to use-after-free on ubufs access. This happens because CPU1
> skips waiting for wake_up() when refcount is already zero.
>
> To prevent that use a completion instead of wait_queue as the ubufs
> notification mechanism. wait_for_completion() guarantees that there will
> be complete() call prior to its return.
>
Alternatively rcu helps.

--- x/drivers/vhost/net.c
+++ y/drivers/vhost/net.c
@@ -96,6 +96,7 @@ struct vhost_net_ubuf_ref {
atomic_t refcount;
wait_queue_head_t wait;
struct vhost_virtqueue *vq;
+ struct rcu_head rcu;
};

#define VHOST_NET_BATCH 64
@@ -247,9 +248,13 @@ vhost_net_ubuf_alloc(struct vhost_virtqu

static int vhost_net_ubuf_put(struct vhost_net_ubuf_ref *ubufs)
{
- int r = atomic_sub_return(1, &ubufs->refcount);
+ int r;
+
+ rcu_read_lock();
+ r = atomic_sub_return(1, &ubufs->refcount);
if (unlikely(!r))
wake_up(&ubufs->wait);
+ rcu_read_unlock();
return r;
}

@@ -262,7 +267,7 @@ static void vhost_net_ubuf_put_and_wait(
static void vhost_net_ubuf_put_wait_and_free(struct vhost_net_ubuf_ref *ubufs)
{
vhost_net_ubuf_put_and_wait(ubufs);
- kfree(ubufs);
+ kfree_rcu(ubufs, rcu);
}

static void vhost_net_clear_ubuf_info(struct vhost_net *n)