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.

Fixes: b7b1bfce ("ipv6: split duplicate address detection and router solicitation timer")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -318,9 +318,8 @@ static void addrconf_del_dad_work(struct inet6_ifaddr *ifp)
 static void addrconf_mod_rs_timer(struct inet6_dev *idev,
 				  unsigned long when)
 {
-	if (!timer_pending(&idev->rs_timer))
+	if (!mod_timer(&idev->rs_timer, jiffies + when))
 		in6_dev_hold(idev);
-	mod_timer(&idev->rs_timer, jiffies + when);
 }
 
 static void addrconf_mod_dad_work(struct inet6_ifaddr *ifp,