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

Nikolay Kuratov
2025-07-18T11:04:33Z

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>
---
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