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

Michael S. Tsirkin
2025-08-05T10:02:12Z

On Fri, Jul 18, 2025 at 02:03:55PM +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.
>
> We also need to reinit completion in vhost_net_flush(), because
> refcnt == 0 does not mean freeing in that case.
>
> Cc: stable@vger.kernel.org
> Fixes: 0ad8b480d6ee9 ("vhost: fix ref cnt checking deadlock")
> Reported-by: Andrey Ryabinin <arbn@yandex-team.com>
> Suggested-by: Andrey Smetanin <asmetanin@yandex-team.ru>
> Suggested-by: Hillf Danton <hdanton@sina.com>
> Tested-by: Lei Yang <leiyang@redhat.com> (v1)
> Signed-off-by: Nikolay Kuratov <kniv@yandex-team.ru>

Nikolay should I expect v3?

> ---
> v2:
> * move reinit_completion() into vhost_net_flush(), thanks
> to Hillf Danton
> * add Tested-by: Lei Yang
> * check that usages of put_and_wait() are consistent across
> LTS kernels
>
> drivers/vhost/net.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 7cbfc7d718b3..69e1bfb9627e 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -94,7 +94,7 @@ struct vhost_net_ubuf_ref {
> * >1: outstanding ubufs
> */
> atomic_t refcount;
> - wait_queue_head_t wait;
> + struct completion wait;
> struct vhost_virtqueue *vq;
> };
>
> @@ -240,7 +240,7 @@ vhost_net_ubuf_alloc(struct vhost_virtqueue *vq, bool zcopy)
> if (!ubufs)
> return ERR_PTR(-ENOMEM);
> atomic_set(&ubufs->refcount, 1);
> - init_waitqueue_head(&ubufs->wait);
> + init_completion(&ubufs->wait);
> ubufs->vq = vq;
> return ubufs;
> }
> @@ -249,14 +249,14 @@ static int vhost_net_ubuf_put(struct vhost_net_ubuf_ref *ubufs)
> {
> int r = atomic_sub_return(1, &ubufs->refcount);
> if (unlikely(!r))
> - wake_up(&ubufs->wait);
> + complete_all(&ubufs->wait);
> return r;
> }
>
> static void vhost_net_ubuf_put_and_wait(struct vhost_net_ubuf_ref *ubufs)
> {
> vhost_net_ubuf_put(ubufs);
> - wait_event(ubufs->wait, !atomic_read(&ubufs->refcount));
> + wait_for_completion(&ubufs->wait);
> }
>
> static void vhost_net_ubuf_put_wait_and_free(struct vhost_net_ubuf_ref *ubufs)
> @@ -1381,6 +1381,7 @@ static void vhost_net_flush(struct vhost_net *n)
> mutex_lock(&n->vqs[VHOST_NET_VQ_TX].vq.mutex);
> n->tx_flush = false;
> atomic_set(&n->vqs[VHOST_NET_VQ_TX].ubufs->refcount, 1);
> + reinit_completion(&n->vqs[VHOST_NET_VQ_TX].ubufs->wait);
> mutex_unlock(&n->vqs[VHOST_NET_VQ_TX].vq.mutex);
> }
> }
> --
> 2.34.1