On Sat, May 16, 2026 at 12:53:29PM +0100, David Laight wrote:
> On Thu, 14 May 2026 11:29:48 +0200
> Stefano Garzarella <[email protected]> wrote:
>
> > From: Stefano Garzarella <[email protected]>
> >
> > When a large message is fragmented into multiple skbs, the zerocopy
> > uarg is only allocated and attached to the last skb in the loop.
> > Non-final skbs carry pinned user pages with no completion tracking,
> > so the kernel has no way to notify userspace when those pages are safe
> > to reuse. If the loop breaks early the uarg is never allocated at all,
> > leaking pinned pages with no completion notification.
> >
> > Fix this by following the approach used by TCP: allocate the zerocopy
> > uarg (if not provided by the caller) before the send loop and attach
> > it to every skb via skb_zcopy_set(), which takes a reference per skb.
> > Each skb's completion properly decrements the refcount, and the
> > notification only fires after the last skb is freed.
> > On failure, if no data was sent, the uarg is cleanly aborted via
> > net_zcopy_put_abort().
> >
> > This issue was initially discovered by sashiko while reviewing commit
> > 1cb36e252211 ("vsock/virtio: fix MSG_ZEROCOPY pinned-pages accounting")
> > but was pre-existing.
> >
> > Fixes: 581512a6dc93 ("vsock/virtio: MSG_ZEROCOPY flag support")
> > Cc: Arseniy Krasnov <[email protected]>
> > Closes:
https://sashiko.dev/#/patchset/20260420132051.217589-1-sgarzare%40redhat.com
> > Reported-by: Maher Azzouzi <[email protected]>
> > Signed-off-by: Stefano Garzarella <[email protected]>
> > ---
> > net/vmw_vsock/virtio_transport_common.c | 83 ++++++++++---------------
> > 1 file changed, 34 insertions(+), 49 deletions(-)
> >
> > diff --git a/net/vmw_vsock/virtio_transport_common.c
b/net/vmw_vsock/virtio_transport_common.c
> > index 989cc252d3d3..1e3409d28164 100644
> > --- a/net/vmw_vsock/virtio_transport_common.c
> > +++ b/net/vmw_vsock/virtio_transport_common.c
> > @@ -70,34 +70,6 @@ static bool virtio_transport_can_zcopy(const struct
virtio_transport *t_ops,
> > return true;
> > }
> >
> > -static int virtio_transport_init_zcopy_skb(struct vsock_sock *vsk,
> > - struct sk_buff *skb,
> > - struct msghdr *msg,
> > - size_t pkt_len,
> > - bool zerocopy)
> > -{
> > - struct ubuf_info *uarg;
> > -
> > - if (msg->msg_ubuf) {
> > - uarg = msg->msg_ubuf;
> > - net_zcopy_get(uarg);
> > - } else {
> > - struct ubuf_info_msgzc *uarg_zc;
> > -
> > - uarg = msg_zerocopy_realloc(sk_vsock(vsk),
> > - pkt_len, NULL, false);
> > - if (!uarg)
> > - return -1;
> > -
> > - uarg_zc = uarg_to_msgzc(uarg);
> > - uarg_zc->zerocopy = zerocopy ? 1 : 0;
> > - }
> > -
> > - skb_zcopy_init(skb, uarg);
> > -
> > - return 0;
> > -}
> > -
> > static int virtio_transport_fill_skb(struct sk_buff *skb,
> > struct virtio_vsock_pkt_info *info,
> > size_t len,
> > @@ -317,8 +289,10 @@ static int virtio_transport_send_pkt_info(struct
vsock_sock *vsk,
> > u32 src_cid, src_port, dst_cid, dst_port;
> > const struct virtio_transport *t_ops;
> > struct virtio_vsock_sock *vvs;
> > + struct ubuf_info *uarg = NULL;
> > u32 pkt_len = info->pkt_len;
> > bool can_zcopy = false;
> > + bool have_uref = false;
> > u32 rest_len;
> > int ret;
> >
> > @@ -360,6 +334,25 @@ static int virtio_transport_send_pkt_info(struct
vsock_sock *vsk,
> > if (can_zcopy)
> > max_skb_len = min_t(u32,
VIRTIO_VSOCK_MAX_PKT_BUF_SIZE,
> > (MAX_SKB_FRAGS * PAGE_SIZE));
> > +
> > + if (info->msg->msg_flags & MSG_ZEROCOPY &&
> > + info->op == VIRTIO_VSOCK_OP_RW) {
> > + uarg = info->msg->msg_ubuf;
> > +
> > + if (!uarg) {
> > + uarg = msg_zerocopy_realloc(sk_vsock(vsk),
> > + pkt_len, NULL,
false);
> > + if (!uarg) {
> > + virtio_transport_put_credit(vvs,
pkt_len);
> > + return -ENOMEM;
> > + }
> > +
> > + if (!can_zcopy)
> > + uarg_to_msgzc(uarg)->zerocopy = 0;
> > +
> > + have_uref = true;
> > + }
> > + }
>
> Surely that block should only be done if can_zcopy is true?
> And shouldn't something unset it if info->op != VIRTIO_VSOCK_OP_RW ?
> If the msg_zerocopy_realloc() fails then can't you just set can_zcopy to
false.
>
> It info->msg->msg_buf is already set then I think you have to disable
zero-copy.
> The caller has already requested a callback - and you can't add another.
>
> In any case by the end of this can_zcopy and have_uref are really the same
flag.
I kept the same approach we had before, trying to make as few changes as
possible.
All these potential issues seem to be pre-existing and should be eventually
addressed in other patches IMHO. This patch one only resolves the main issue
of calling `skb_zcopy_set()` for every skb to avoid leaking pages, etc.
