[PATCH 2/2] vsock/virtio: Fix message iterator handling on transmit path

[Will Deacon <will@kernel.org>]

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 <hdanton@sina.com>
Cc: Stefano Garzarella <sgarzare@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Reported-by: syzbot+b4d960daf7a3c7c2b7b1@syzkaller.appspotmail.com
Signed-off-by: Will Deacon <will@kernel.org>
---
 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