On Mon, May 18, 2026 at 05:33:08AM -0400, Michael S. Tsirkin wrote:
>On Mon, May 18, 2026 at 11:18:24AM +0200, Stefano Garzarella wrote:
>> 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.
>
>the patch is upstream now, right? So pretty much have to be patches on
>top.
If those are actual issues, then yes. TBH, I didn’t look into that
aspect and left it as it was before. We should take a closer look at how
MSG_ZEROCOPY is handled.
David, if you think it needs fixing and you have time, feel free to send
patches on top.
