Bug 2395360 (CVE-2023-53189)

In the Linux kernel, the following vulnerability has been resolved:

ipv6/addrconf: fix a potential refcount underflow for idev

Now in addrconf_mod_rs_timer(), reference idev depends on whether
rs_timer is not pending. Then modify rs_timer timeout.

There is a time gap in [1], during which if the pending rs_timer
becomes not pending. It will miss to hold idev, but the rs_timer
is activated. Thus rs_timer callback function addrconf_rs_timer()
will be executed and put idev later without holding idev. A refcount
underflow issue for idev can be caused by this.

	if (!timer_pending(&idev->rs_timer))
		in6_dev_hold(idev);
		  <--------------[1]
	mod_timer(&idev->rs_timer, jiffies + when);

To fix the issue, hold idev if mod_timer() return 0.