USB: serial: quatech2: fix null-ptr-deref in qt2_process_read_urb()

stable inclusion
from stable-v6.6.75
commit 6377838560c03b36e1153a42ef727533def9b68f
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/IBLDIN
CVE: CVE-2025-21689

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6377838560c03b36e1153a42ef727533def9b68f



--------------------------------

commit 575a5adf48b06a2980c9eeffedf699ed5534fade upstream.

This patch addresses a null-ptr-deref in qt2_process_read_urb() due to
an incorrect bounds check in the following:

       if (newport > serial->num_ports) {
               dev_err(&port->dev,
                       "%s - port change to invalid port: %i\n",
                       __func__, newport);
               break;
       }

The condition doesn't account for the valid range of the serial->port
buffer, which is from 0 to serial->num_ports - 1. When newport is equal
to serial->num_ports, the assignment of "port" in the
following code is out-of-bounds and NULL:

       serial_priv->current_port = newport;
       port = serial->port[serial_priv->current_port];

The fix checks if newport is greater than or equal to serial->num_ports
indicating it is out-of-bounds.

Reported-by: syzbot <syzbot+506479ebf12fe435d01a@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=506479ebf12fe435d01a


Fixes: f7a33e60 ("USB: serial: add quatech2 usb to serial driver")
Cc: <stable@vger.kernel.org>      # 3.5
Signed-off-by: Qasim Ijaz <qasdev00@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ye Bin <yebin10@huawei.com>