ipv6: Fix idev->addr_list corruption

addrconf_ifdown() removes elements from the idev->addr_list without
holding the idev->lock.

If this happens while the loop in __ipv6_dev_get_saddr() is handling the
same element, that function ends up in an infinite loop:

  NMI watchdog: BUG: soft lockup - CPU#1 stuck for 23s! [test:1719]
  Call Trace:
   ipv6_get_saddr_eval+0x13c/0x3a0
   __ipv6_dev_get_saddr+0xe4/0x1f0
   ipv6_dev_get_saddr+0x1b4/0x204
   ip6_dst_lookup_tail+0xcc/0x27c
   ip6_dst_lookup_flow+0x38/0x80
   udpv6_sendmsg+0x708/0xba8
   sock_sendmsg+0x18/0x30
   SyS_sendto+0xb8/0xf8
   syscall_common+0x34/0x58

Fixes: 6a92393 (Revert "ipv6: Revert optional address flusing on ifdown.")
Signed-off-by: Rabin Vincent <rabinv@axis.com>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -3626,14 +3626,19 @@ static int addrconf_ifdown(struct net_device *dev, int how)
 	INIT_LIST_HEAD(&del_list);
 	list_for_each_entry_safe(ifa, tmp, &idev->addr_list, if_list) {
 		struct rt6_info *rt = NULL;
+		bool keep;
 
 		addrconf_del_dad_work(ifa);
 
+		keep = keep_addr && (ifa->flags & IFA_F_PERMANENT) &&
+			!addr_is_local(&ifa->addr);
+		if (!keep)
+			list_move(&ifa->if_list, &del_list);
+
 		write_unlock_bh(&idev->lock);
 		spin_lock_bh(&ifa->lock);
 
-		if (keep_addr && (ifa->flags & IFA_F_PERMANENT) &&
-		    !addr_is_local(&ifa->addr)) {
+		if (keep) {
 			/* set state to skip the notifier below */
 			state = INET6_IFADDR_STATE_DEAD;
 			ifa->state = 0;
@@ -3645,8 +3650,6 @@ static int addrconf_ifdown(struct net_device *dev, int how)
 		} else {
 			state = ifa->state;
 			ifa->state = INET6_IFADDR_STATE_DEAD;
-
-			list_move(&ifa->if_list, &del_list);
 		}
 
 		spin_unlock_bh(&ifa->lock);