vsock/virtio: fix MSG_ZEROCOPY pinned-pages accounting

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: 581512a ("vsock/virtio: MSG_ZEROCOPY flag support")
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260420132051.217589-1-sgarzare@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: stefano-garzarella

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;