[PATCH net v5] ipv6: addrconf: bail out of dad_failure when state is no longer POSTDAD

[Linmao Li]

addrconf_dad_failure() transitions ifp->state from DAD to POSTDAD
via addrconf_dad_end(), which drops ifp->lock on return.  The lock
is re-acquired after net_info_ratelimited().  A concurrent
ipv6_del_addr() can take 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

Fold the addrconf_dad_end() logic into addrconf_dad_failure() under
a single ifp->lock critical section.  The STABLE_PRIVACY branch
temporarily drops ifp->lock around address regeneration, so at
lock_errdad: verify the state is still POSTDAD before transitioning
to ERRDAD; bail out otherwise to avoid overwriting a state set by
another path while the lock was released.

Fixes: c15b1ccadb32 ("ipv6: move DAD and addrconf_verify processing to workqueue")
Signed-off-by: Linmao Li <lilinmao@xxxxxxxxxx>
---
v5: At lock_errdad, check for any state other than POSTDAD instead of
    only DEAD, to avoid overwriting a state set while the lock was
    released (suggested by Ido Schimmel).
    Update subject and last commit message paragraph accordingly.
v4: Move the bail-out from errdad: to right after lock_errdad:
    re-acquires ifp->lock, since DAD failure path needs the
    state transition to ERRDAD (suggested by Paolo Abeni).
v3: Fold addrconf_dad_end() into addrconf_dad_failure() under a
    single ifp->lock critical section, instead of adding a check
    at errdad: with a racy state inspection (suggested by Sabrina
    Dubroca).
v2: Add Fixes: tag.
v1: https://lore.kernel.org/netdev/20260420032842.1063277-1-lilinmao@xxxxxxxxxx/

 net/ipv6/addrconf.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 5476b6536eb7..da0f07e21e3f 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -2166,16 +2166,18 @@ void addrconf_dad_failure(struct sk_buff *skb, struct inet6_ifaddr *ifp)
 	struct net *net = dev_net(idev->dev);
 	int max_addresses;
 
-	if (addrconf_dad_end(ifp)) {
+	spin_lock_bh(&ifp->lock);
+
+	if (ifp->state != INET6_IFADDR_STATE_DAD) {
+		spin_unlock_bh(&ifp->lock);
 		in6_ifa_put(ifp);
 		return;
 	}
+	ifp->state = INET6_IFADDR_STATE_POSTDAD;
 
 	net_info_ratelimited("%s: IPv6 duplicate address %pI6c used by %pM detected!\n",
 			     ifp->idev->dev->name, &ifp->addr, eth_hdr(skb)->h_source);
 
-	spin_lock_bh(&ifp->lock);
-
 	if (ifp->flags & IFA_F_STABLE_PRIVACY) {
 		struct in6_addr new_addr;
 		struct inet6_ifaddr *ifp2;
@@ -2223,6 +2225,11 @@ void addrconf_dad_failure(struct sk_buff *skb, struct inet6_ifaddr *ifp)
 		in6_ifa_put(ifp2);
 lock_errdad:
 		spin_lock_bh(&ifp->lock);
+		if (ifp->state != INET6_IFADDR_STATE_POSTDAD) {
+			spin_unlock_bh(&ifp->lock);
+			in6_ifa_put(ifp);
+			return;
+		}
 	}
 
 errdad:
-- 
2.25.1