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Message-ID: <CAPpAL=xE4ZCyAhc+fkZwREo-cDHS4CG4fq4+sebazJgRzZoDHg@mail.gmail.com>
Date: Fri, 18 Jul 2025 16:31:52 +0800
From: Lei Yang <leiyang@...hat.com>
To: Nikolay Kuratov <kniv@...dex-team.ru>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	virtualization@...ts.linux.dev, kvm@...r.kernel.org, 
	"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Eugenio Pérez <eperezma@...hat.com>, stable@...r.kernel.org, 
	Andrey Ryabinin <arbn@...dex-team.com>, Andrey Smetanin <asmetanin@...dex-team.ru>
Subject: Re: [PATCH] vhost/net: Replace wait_queue with completion in ubufs reference

Tested this patch with virtio-net regression tests, everything works fine.

Tested-by: Lei Yang <leiyang@...hat.com>


On Thu, Jul 17, 2025 at 12:24 AM Nikolay Kuratov <kniv@...dex-team.ru> 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 because refcnt == 0 does not mean
> freeing in case of vhost_net_flush() - it then sets refcnt back to 1.
> AFAIK concurrent calls to vhost_net_ubuf_put_and_wait() with the same
> ubufs object aren't possible since those calls (through vhost_net_flush()
> or vhost_net_set_backend()) are protected by the device mutex.
> So reinit_completion() right after wait_for_completion() should be fine.
>
> Cc: stable@...r.kernel.org
> Fixes: 0ad8b480d6ee9 ("vhost: fix ref cnt checking deadlock")
> Reported-by: Andrey Ryabinin <arbn@...dex-team.com>
> Suggested-by: Andrey Smetanin <asmetanin@...dex-team.ru>
> Signed-off-by: Nikolay Kuratov <kniv@...dex-team.ru>
> ---
>  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..454d179fffeb 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 +249,15 @@ 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);
> +       reinit_completion(&ubufs->wait);
>  }
>
>  static void vhost_net_ubuf_put_wait_and_free(struct vhost_net_ubuf_ref *ubufs)
> --
> 2.34.1
>
>


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