On Mon, 18 May 2026 11:54:19 +0200 Stefano Garzarella <[email protected]> wrote:
> 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. I'm not fully sure how it all works. Especially the paths where msg->msg_ubuf is non-NULL, I suspect it should be added to all the skb even if the ZEROCOPY flag isn't set. I was just reading the one function. But there did look like some very dodgy conditionals. -- David > > Thanks, > Stefano > > > > >> @Arseniy can you help on this? > >> > >> > > >> > > } > >> > > > >> > > rest_len = pkt_len; > >> > > @@ -378,27 +371,7 @@ static int virtio_transport_send_pkt_info(struct > >> > > vsock_sock *vsk, > >> > > break; > >> > > } > >> > > > >> > > - /* We process buffer part by part, allocating skb on > >> > > - * each iteration. If this is last skb for this buffer > >> > > - * and MSG_ZEROCOPY mode is in use - we must allocate > >> > > - * completion for the current syscall. > >> > > - * > >> > > - * Pass pkt_len because msg iter is already consumed > >> > > - * by virtio_transport_fill_skb(), so iter->count > >> > > - * can not be used for RLIMIT_MEMLOCK pinned-pages > >> > > - * accounting done by msg_zerocopy_realloc(). > >> > > - */ > >> > > - if (info->msg && info->msg->msg_flags & MSG_ZEROCOPY && > >> > > - skb_len == rest_len && info->op == > >> > > VIRTIO_VSOCK_OP_RW) { > >> > > - if (virtio_transport_init_zcopy_skb(vsk, skb, > >> > > - info->msg, > >> > > - pkt_len, > >> > > - can_zcopy)) > >> > > { > >> > > - kfree_skb(skb); > >> > > - ret = -ENOMEM; > >> > > - break; > >> > > - } > >> > > - } > >> > > + skb_zcopy_set(skb, uarg, NULL); > >> > > > >> > > virtio_transport_inc_tx_pkt(vvs, skb); > >> > > > >> > > @@ -422,6 +395,18 @@ static int virtio_transport_send_pkt_info(struct > >> > > vsock_sock *vsk, > >> > > > >> > > virtio_transport_put_credit(vvs, rest_len); > >> > > > >> > > + /* msg_zerocopy_realloc() initializes the ubuf_info refcnt to 1. > >> > > + * skb_zcopy_set() increases it for each skb, so we can drop > >> > > that > >> > ^ must > >> > > >> > > + * initial reference to keep it balanced. > >> > > + */ > >> > > + if (have_uref) { > >> > > + if (rest_len == pkt_len) > >> > > + /* No data sent, abort the notification. */ > >> > > + net_zcopy_put_abort(uarg, true); > >> > > >> > Is it worth optimising for the 'nothing sent' case ? > >> > >> What do you suggest doing? > >> > >> I followed what TCP does. > >> > >> Thanks, > >> Stefano > >> > >> > > >> > -- David > >> > > >> > > + else > >> > > + net_zcopy_put(uarg); > >> > > + } > >> > > + > >> > > /* Return number of bytes, if any data has been sent. */ > >> > > if (rest_len != pkt_len) > >> > > ret = pkt_len - rest_len; > >> > > > >
