Re: [PATCH net v2] tun: fix ubuf refcount incorrectly on error path

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Wed, Dec 9, 2020 at 8:03 AM wangyunjian <wangyunjian@huawei.com> wrote:
>
> From: Yunjian Wang <wangyunjian@huawei.com>
>
> After setting callback for ubuf_info of skb, the callback
> (vhost_net_zerocopy_callback) will be called to decrease
> the refcount when freeing skb. But when an exception occurs

With exception, you mean if tun_get_user returns an error that
propagates to the sendmsg call in vhost handle_tx, correct?

> afterwards, the error handling in vhost handle_tx() will
> try to decrease the same refcount again. This is wrong and
> fix this by delay copying ubuf_info until we're sure
> there's no errors.

I think the right approach is to address this in the error paths,
rather than complicate the normal datapath.

Is it sufficient to suppress the call to vhost_net_ubuf_put in the
handle_tx sendmsg error path, given that vhost_zerocopy_callback
will be called on kfree_skb?

Or alternatively clear the destructor in drop:

>
> Fixes: 4477138fa0ae ("tun: properly test for IFF_UP")
> Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver")
>
> Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
> ---
> v2:
>    Updated code, fix by delay copying ubuf_info
> ---
>  drivers/net/tun.c | 29 +++++++++++++++++++----------
>  1 file changed, 19 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 2dc1988a8973..2ea822328e73 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1637,6 +1637,20 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>         return NULL;
>  }
>
> +/* copy ubuf_info for callback when skb has no error */
> +static inline void tun_copy_ubuf_info(struct sk_buff *skb, bool zerocopy, void *msg_control)
> +{
> +       if (zerocopy) {
> +               skb_shinfo(skb)->destructor_arg = msg_control;
> +               skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
> +               skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
> +       } else if (msg_control) {
> +               struct ubuf_info *uarg = msg_control;
> +
> +               uarg->callback(uarg, false);
> +       }
> +}
> +
>  /* Get packet from user space buffer */
>  static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>                             void *msg_control, struct iov_iter *from,
> @@ -1812,16 +1826,6 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>                 break;
>         }
>
> -       /* copy skb_ubuf_info for callback when skb has no error */
> -       if (zerocopy) {
> -               skb_shinfo(skb)->destructor_arg = msg_control;
> -               skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
> -               skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
> -       } else if (msg_control) {
> -               struct ubuf_info *uarg = msg_control;
> -               uarg->callback(uarg, false);
> -       }
> -
>         skb_reset_network_header(skb);
>         skb_probe_transport_header(skb);
>         skb_record_rx_queue(skb, tfile->queue_index);
> @@ -1830,6 +1834,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>                 struct bpf_prog *xdp_prog;
>                 int ret;
>
> +               tun_copy_ubuf_info(skb, zerocopy, msg_control);
>                 local_bh_disable();
>                 rcu_read_lock();
>                 xdp_prog = rcu_dereference(tun->xdp_prog);
> @@ -1881,6 +1886,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>                         return -ENOMEM;
>                 }
>
> +               tun_copy_ubuf_info(skb, zerocopy, msg_control);
>                 local_bh_disable();
>                 napi_gro_frags(&tfile->napi);
>                 local_bh_enable();
> @@ -1889,6 +1895,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>                 struct sk_buff_head *queue = &tfile->sk.sk_write_queue;
>                 int queue_len;
>
> +               tun_copy_ubuf_info(skb, zerocopy, msg_control);
>                 spin_lock_bh(&queue->lock);
>                 __skb_queue_tail(queue, skb);
>                 queue_len = skb_queue_len(queue);
> @@ -1899,8 +1906,10 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>
>                 local_bh_enable();
>         } else if (!IS_ENABLED(CONFIG_4KSTACKS)) {
> +               tun_copy_ubuf_info(skb, zerocopy, msg_control);
>                 tun_rx_batched(tun, tfile, skb, more);
>         } else {
> +               tun_copy_ubuf_info(skb, zerocopy, msg_control);
>                 netif_rx_ni(skb);
>         }
>         rcu_read_unlock();
> --
> 2.23.0
>