net: usb: rtl8150: fix use-after-free in rtl8150_start_xmit()

net: usb: rtl8150: fix use-after-free in rtl8150_start_xmit()

syzbot reported a KASAN slab-use-after-free read in rtl8150_start_xmit()
when accessing skb->len for tx statistics after usb_submit_urb() has
been called:

  BUG: KASAN: slab-use-after-free in rtl8150_start_xmit+0x71f/0x760
    drivers/net/usb/rtl8150.c:712
  Read of size 4 at addr ffff88810eb7a930 by task kworker/0:4/5226

The URB completion handler write_bulk_callback() frees the skb via
dev_kfree_skb_irq(dev->tx_skb). The URB may complete on another CPU
in softirq context before usb_submit_urb() returns in the submitter,
so by the time the submitter reads skb->len the skb has already been
queued to the per-CPU completion_queue and freed by net_tx_action():

  CPU A (xmit)                      CPU B (USB completion softirq)
  ------------                      ------------------------------
  dev->tx_skb = skb;
  usb_submit_urb()      --+
                          |-------> write_bulk_callback()
                          |           dev_kfree_skb_irq(dev->tx_skb)
                          |         net_tx_action()
                          |           napi_skb_cache_put()   <-- free
  netdev->stats.tx_bytes  |
    += skb->len;          <-- UAF read

Fix it by caching skb->len before submitting the URB and using the
cached value when updating the tx_bytes counter.

The pre-existing tx_bytes semantics are preserved: the counter tracks
the original frame length (skb->len), not the ETH_ZLEN/USB-alignment
padded "count" value that is handed to the device.  Changing that
would be a user-visible accounting change and is out of scope for
this UAF fix.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+3f46c095ac0ca048cb71@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69e69ee7.050a0220.24bfd3.002b.GAE@google.com/
Closes: https://syzkaller.appspot.com/bug?extid=3f46c095ac0ca048cb71
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Zhan Jun <zhanjun@uniontech.com>
Link: https://patch.msgid.link/809895186B866C10+20260423004913.136655-1-zhangdandan@uniontech.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>