rsi_get_* functions rely on an offset variable from usb
input. The size of usb input is RSI_MAX_RX_USB_PKT_SIZE(3000),
while 2-byte offset can be up to 0xFFFF. Thus a large offset
can cause out-of-bounds read.

The patch adds a bound checking condition when rcv_pkt_len is 0,
indicating it's USB. It's unclear whether this is triggerable
from other type of bus. The following check might help in that case.
offset > rcv_pkt_len - FRAME_DESC_SZ

The bug is trigerrable with conpromised/malfunctioning USB devices.
I tested the patch with the crashing input and got no more bug report.

Attached is the KASAN report from fuzzing.

BUG: KASAN: slab-out-of-bounds in rsi_read_pkt+0x42e/0x500 [rsi_91x]
Read of size 2 at addr ffff888019439fdb by task RX-Thread/227

CPU: 0 PID: 227 Comm: RX-Thread Not tainted 5.6.0 #66
Call Trace:
 dump_stack+0x76/0xa0
 print_address_description.constprop.0+0x16/0x200
 ? rsi_read_pkt+0x42e/0x500 [rsi_91x]
 ? rsi_read_pkt+0x42e/0x500 [rsi_91x]
 __kasan_report.cold+0x37/0x7c
 ? rsi_read_pkt+0x42e/0x500 [rsi_91x]
 kasan_report+0xe/0x20
 rsi_read_pkt+0x42e/0x500 [rsi_91x]
 rsi_usb_rx_thread+0x1b1/0x2fc [rsi_usb]
 ? rsi_probe+0x16a0/0x16a0 [rsi_usb]
 ? _raw_spin_lock_irqsave+0x7b/0xd0
 ? _raw_spin_trylock_bh+0x120/0x120
 ? __wake_up_common+0x10b/0x520
 ? rsi_probe+0x16a0/0x16a0 [rsi_usb]
 kthread+0x2b5/0x3b0
 ? kthread_create_on_node+0xd0/0xd0
 ret_from_fork+0x22/0x40
Reported-by: Brendan Dolan-Gavitt <brendandg@nyu.edu>
Signed-off-by: Zekun Shen <bruceshenzk@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/YXxXS4wgu2OsmlVv@10-18-43-117.dynapool.wireless.nyu.edu