[PATCH net] ipv6: addrconf: skip ERRDAD transition when address already DEAD

[Linmao Li <lilinmao@kylinos.cn>]

addrconf_dad_end() transitions ifp->state from DAD to POSTDAD under
ifp->lock and releases the lock.  addrconf_dad_failure() takes
ifp->lock again with the spin_lock_bh() following the
net_info_ratelimited() duplicate-address log.  A concurrent
ipv6_del_addr() can acquire the lock in that window, set ifp->state
to DEAD and run list_del_rcu(&ifp->if_list).

addrconf_dad_failure() then overwrites DEAD with ERRDAD at errdad:
and schedules a new dad_work.  The work calls ipv6_del_addr() again,
hitting the already-poisoned list entry:

  general protection fault: 0000 [#1] SMP NOPTI
  CPU: 4 PID: 217 Comm: kworker/4:1
  Workqueue: ipv6_addrconf addrconf_dad_work
  RIP: 0010:ipv6_del_addr+0xe9/0x280
  RAX: dead000000000122
  Call Trace:
   addrconf_dad_stop+0x113/0x140
   addrconf_dad_work+0x28c/0x430
   process_one_work+0x1eb/0x3b0
   worker_thread+0x4d/0x400
   kthread+0x104/0x140
   ret_from_fork+0x35/0x40

Bail out at errdad: when ifp->state is already DEAD. The existing
in6_ifa_put() releases the reference taken for this invocation.

Signed-off-by: Linmao Li <lilinmao@kylinos.cn>
---
 net/ipv6/addrconf.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 5476b6536eb7..14b1ab43da87 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -2227,6 +2227,12 @@ void addrconf_dad_failure(struct sk_buff *skb, struct inet6_ifaddr *ifp)
 
 errdad:
 	/* transition from _POSTDAD to _ERRDAD */
+	if (ifp->state == INET6_IFADDR_STATE_DEAD) {
+		/* ipv6_del_addr() already removed ifp while lock was dropped */
+		spin_unlock_bh(&ifp->lock);
+		in6_ifa_put(ifp);
+		return;
+	}
 	ifp->state = INET6_IFADDR_STATE_ERRDAD;
 	spin_unlock_bh(&ifp->lock);
 
-- 
2.25.1