Virtualization - Dec 2020 - [PATCH net v5 1/2] vhost_net: fix ubuf refcount incorrectly when sendmsg fails

On Fri, Dec 25, 2020 at 03:24:25PM +0800, wangyunjian
wrote:
> From: Yunjian Wang <wangyunjian at huawei.com>
> 
> Currently the vhost_zerocopy_callback() maybe be called to decrease
> the refcount when sendmsg fails in tun. The error handling in vhost
> handle_tx_zerocopy() will try to decrease the same refcount again.
> This is wrong. To fix this issue, we only call vhost_net_ubuf_put()
> when vq->heads[nvq->desc].len == VHOST_DMA_IN_PROGRESS.
> 
> Fixes: 0690899b4d45 ("tun: experimental zero copy tx support")
Are you sure about this tag? the patch in question only affects
tun, while the fix only affects vhost.
> Signed-off-by: Yunjian Wang <wangyunjian at huawei.com>
> Acked-by: Willem de Bruijn <willemb at google.com>
> Acked-by: Michael S. Tsirkin <mst at redhat.com>
> Acked-by: Jason Wang <jasowang at redhat.com>
> ---
>  drivers/vhost/net.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 531a00d703cd..c8784dfafdd7 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -863,6 +863,7 @@ static void handle_tx_zerocopy(struct vhost_net *net,
struct socket *sock)
>  	size_t len, total_len = 0;
>  	int err;
>  	struct vhost_net_ubuf_ref *ubufs;
> +	struct ubuf_info *ubuf;
>  	bool zcopy_used;
>  	int sent_pkts = 0;
>  
> @@ -895,9 +896,7 @@ static void handle_tx_zerocopy(struct vhost_net *net,
struct socket *sock)
>  
>  		/* use msg_control to pass vhost zerocopy ubuf info to skb */
>  		if (zcopy_used) {
> -			struct ubuf_info *ubuf;
>  			ubuf = nvq->ubuf_info + nvq->upend_idx;
> -
>  			vq->heads[nvq->upend_idx].id = cpu_to_vhost32(vq, head);
>  			vq->heads[nvq->upend_idx].len = VHOST_DMA_IN_PROGRESS;
>  			ubuf->callback = vhost_zerocopy_callback;
> @@ -927,7 +926,8 @@ static void handle_tx_zerocopy(struct vhost_net *net,
struct socket *sock)
>  		err = sock->ops->sendmsg(sock, &msg, len);
>  		if (unlikely(err < 0)) {
>  			if (zcopy_used) {
> -				vhost_net_ubuf_put(ubufs);
> +				if (vq->heads[ubuf->desc].len == VHOST_DMA_IN_PROGRESS)
> +					vhost_net_ubuf_put(ubufs);
>  				nvq->upend_idx = ((unsigned)nvq->upend_idx - 1)
>  					% UIO_MAXIOV;
>  			}
> -- 
> 2.23.0

On Sun, Dec 27, 2020 at 6:26 AM Michael S. Tsirkin <mst at redhat.com>
wrote:
>
> On Fri, Dec 25, 2020 at 03:24:25PM +0800, wangyunjian wrote:
> > From: Yunjian Wang <wangyunjian at huawei.com>
> >
> > Currently the vhost_zerocopy_callback() maybe be called to decrease
> > the refcount when sendmsg fails in tun. The error handling in vhost
> > handle_tx_zerocopy() will try to decrease the same refcount again.
> > This is wrong. To fix this issue, we only call vhost_net_ubuf_put()
> > when vq->heads[nvq->desc].len == VHOST_DMA_IN_PROGRESS.
> >
> > Fixes: 0690899b4d45 ("tun: experimental zero copy tx
support")
>
> Are you sure about this tag? the patch in question only affects
> tun, while the fix only affects vhost.
That was my suggestion. But you're right. Perhaps better:

Fixes: bab632d69ee4 ("vhost: vhost TX zero-copy support")

That introduces the actual block that releases the buffer on error:

"
                err = sock->ops->sendmsg(NULL, sock, &msg, len);
                if (unlikely(err < 0)) {
+                       if (zcopy) {
+                               if (ubufs)
+                                       vhost_ubuf_put(ubufs);
+                               vq->upend_idx = ((unsigned)vq->upend_idx -
1) %
+                                       UIO_MAXIOV;
+                       }
"