Re: [PATCH net] vsock/virtio: fix MSG_ZEROCOPY pinned-pages accounting

[Bobby Eshleman <bobbyeshleman@gmail.com>]

On Mon, Apr 20, 2026 at 03:20:51PM +0200, Stefano Garzarella wrote:
> From: Stefano Garzarella <sgarzare@redhat.com>
> 
> virtio_transport_init_zcopy_skb() uses iter->count as the size argument
> for msg_zerocopy_realloc(), which in turn passes it to
> mm_account_pinned_pages() for RLIMIT_MEMLOCK accounting. However, this
> function is called after virtio_transport_fill_skb() has already consumed
> the iterator via __zerocopy_sg_from_iter(), so on the last skb, iter->count
> will be 0, skipping the RLIMIT_MEMLOCK enforcement.
> 
> Pass pkt_len (the total bytes being sent) as an explicit parameter to
> virtio_transport_init_zcopy_skb() instead of reading the already-consumed
> iter->count.
> 
> This matches TCP and UDP, which both call msg_zerocopy_realloc() with
> the original message size.
> 
> Fixes: 581512a6dc93 ("vsock/virtio: MSG_ZEROCOPY flag support")
> Reported-by: Yiming Qian <yimingqian591@gmail.com>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
>  net/vmw_vsock/virtio_transport_common.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 0742091beae7..416d533f493d 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -73,6 +73,7 @@ static bool virtio_transport_can_zcopy(const struct virtio_transport *t_ops,
>  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;
> @@ -81,12 +82,10 @@ static int virtio_transport_init_zcopy_skb(struct vsock_sock *vsk,
>  		uarg = msg->msg_ubuf;
>  		net_zcopy_get(uarg);
>  	} else {
> -		struct iov_iter *iter = &msg->msg_iter;
>  		struct ubuf_info_msgzc *uarg_zc;
>  
>  		uarg = msg_zerocopy_realloc(sk_vsock(vsk),
> -					    iter->count,
> -					    NULL, false);
> +					    pkt_len, NULL, false);
>  		if (!uarg)
>  			return -1;
>  
> @@ -398,11 +397,17 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>  		 * 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;
> -- 
> 2.53.0
> 

Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>