Commit 6693731487a8 ("vsock/virtio: Allocate nonlinear SKBs for handling large transmit buffers") converted the virtio vsock transmit path to utilise nonlinear SKBs when handling large buffers. As part of this change, virtio_transport_fill_skb() was updated to call skb_copy_datagram_from_iter() instead of memcpy_from_msg() as the latter expects a single destination buffer and cannot handle nonlinear SKBs correctly. Unfortunately, during this conversion, I overlooked the error case when the copying function returns -EFAULT due to a fault on the input buffer in userspace. In this case, memcpy_from_msg() reverts the iterator to its initial state thanks to copy_from_iter_full() whereas skb_copy_datagram_from_iter() leaves the iterator partially advanced. This results in a WARN_ONCE() from the vsock code, which expects the iterator to stay in sync with the number of bytes transmitted so that virtio_transport_send_pkt_info() can return -EFAULT when it is called again: ------------[ cut here ]------------ 'send_pkt()' returns 0, but 65536 expected WARNING: CPU: 0 PID: 5503 at net/vmw_vsock/virtio_transport_common.c:428 virtio_transport_send_pkt_info+0xd11/0xf00 net/vmw_vsock/virtio_transport_common.c:426 Modules linked in: CPU: 0 UID: 0 PID: 5503 Comm: syz.0.17 Not tainted 6.16.0-syzkaller-12063-g37816488247d #0 PREEMPT(full) Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014 Call virtio_transport_fill_skb_full() to restore the previous iterator behaviour. Cc: Hillf Danton Cc: Stefano Garzarella Cc: Stefan Hajnoczi Cc: "Michael S. Tsirkin" Cc: Jason Wang Reported-by: syzbot+b4d960daf7a3c7c2b7b1@syzkaller.appspotmail.com Signed-off-by: Will Deacon --- net/vmw_vsock/virtio_transport_common.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c index fe92e5fa95b4..dcc8a1d5851e 100644 --- a/net/vmw_vsock/virtio_transport_common.c +++ b/net/vmw_vsock/virtio_transport_common.c @@ -105,12 +105,14 @@ static int virtio_transport_fill_skb(struct sk_buff *skb, size_t len, bool zcopy) { + struct msghdr *msg = info->msg; + if (zcopy) - return __zerocopy_sg_from_iter(info->msg, NULL, skb, - &info->msg->msg_iter, len, NULL); + return __zerocopy_sg_from_iter(msg, NULL, skb, + &msg->msg_iter, len, NULL); virtio_vsock_skb_put(skb, len); - return skb_copy_datagram_from_iter(skb, 0, &info->msg->msg_iter, len); + return skb_copy_datagram_from_iter_full(skb, 0, &msg->msg_iter, len); } static void virtio_transport_init_hdr(struct sk_buff *skb, -- 2.51.0.rc1.167.g924127e9c0-goog