net: usb: net1080: validate packet_len before pad-byte access in rx_fixup

net: usb: net1080: validate packet_len before pad-byte access in rx_fixup

For an even packet_len, net1080_rx_fixup() reads the pad byte at
skb->data[packet_len] before the skb->len != packet_len check further
down, and packet_len is only bounded against NC_MAX_PACKET. A malicious
NetChip 1080 device can send a short frame advertising a large even
packet_len (e.g. 0x4000), so the pad-byte read lands past the end of the
skb:

  BUG: KASAN: slab-out-of-bounds in net1080_rx_fixup
  Read of size 1 at addr ffff8880106c83c6 by task ksoftirqd/0/14
   ...
   net1080_rx_fixup (drivers/net/usb/net1080.c:384)
   usbnet_bh (drivers/net/usb/usbnet.c:1589)
   process_one_work (kernel/workqueue.c:3322)
   bh_worker (kernel/workqueue.c:3708)
   tasklet_action (kernel/softirq.c:965)
   handle_softirqs (kernel/softirq.c:622)
   ...

Reject the frame when packet_len >= skb->len before reading.

Fixes: 904813cd8a0b ("[PATCH] USB: usbnet (4/9) module for net1080 cables")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <xmei5@asu.edu>
Link: https://patch.msgid.link/20260630045121.1565324-1-xmei5@asu.edu
Signed-off-by: Paolo Abeni <pabeni@redhat.com>