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

Michael S. Tsirkin
2025-07-20T16:13:21Z

On Sat, Jul 19, 2025 at 07:03:23AM +0800, Hillf Danton wrote:
> 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)

I like that.

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MST