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

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

On Sun, Dec 13, 2020 at 10:30 PM Jason Wang <jasowang@redhat.com> wrote:
>
>
> On 2020/12/14 上午9:32, Willem de Bruijn wrote:
> > On Sat, Dec 12, 2020 at 7:18 PM Willem de Bruijn
> > <willemdebruijn.kernel@gmail.com> wrote:
> >>>>> 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?
> >>> We can not call kfree_skb() until the skb was created.
> >>>
> >>>> Or alternatively clear the destructor in drop:
> >>> The uarg->callback() is called immediately after we decide do datacopy
> >>> even if caller want to do zerocopy. If another error occurs later, the vhost
> >>> handle_tx() will try to decrease it again.
> >> Oh right, I missed the else branch in this path:
> >>
> >>          /* 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);
> >>          }
> >>
> >> So if handle_tx_zerocopy calls tun_sendmsg with ubuf_info (and thus a
> >> reference to release), there are these five options:
> >>
> >> 1. tun_sendmsg succeeds, ubuf_info is associated with skb.
> >>       reference released from kfree_skb calling vhost_zerocopy_callback later
> >>
> >> 2. tun_sendmsg succeeds, ubuf_info is released immediately, as skb is
> >> not zerocopy.
> >>
> >> 3. tun_sendmsg fails before creating skb, handle_tx_zerocopy correctly
> >> cleans up on receiving error from tun_sendmsg.
> >>
> >> 4. tun_sendmsg fails after creating skb, but with copying: decremented
> >> at branch shown above + again in handle_tx_zerocopy
> >>
> >> 5. tun_sendmsg fails after creating skb, with zerocopy: decremented at
> >> kfree_skb in drop: + again in handle_tx_zerocopy
> >>
> >> Since handle_tx_zerocopy has no idea whether on error 3, 4 or 5
> >> occurred,
> > Actually, it does. If sendmsg returns an error, it can test whether
> > vq->heads[nvq->upend_idx].len != VHOST_DMA_IN_PROGRESS.
>
>
> Just to make sure I understand this. Any reason for it can't be
> VHOST_DMA_IN_PROGRESS here?

It can be, and it will be if tun_sendmsg returns EINVAL before
assigning the skb destructor.

Only if tun_sendmsg released the zerocopy state through
kfree_skb->vhost_zerocopy_callback will it have been updated to
VHOST_DMA_DONE_LEN. And only then must the caller not try to release
the state again.