[PATCH net v4 2/4] ipv6: serialize address publication with device teardown

[Daehyeon Ko <4ncienth@gmail.com>]

ipv6_add_addr() checks idev state before allocating an ifaddr, but
publishes the object later. addrconf_ifdown() can mark and detach the
idev between the check and publication.

This happens when a non-loopback device MTU falls below IPV6_MIN_MTU. A
forced interleaving published an address on a dead idev, and
ipv6_get_ifaddr() still returned it after teardown. Later device deletion
remained blocked with:

  unregister_netdevice: waiting for ac0 to become free. Usage count = 3

Protect the dead indication with idev->lock and keep that lock across
both hash and device-list publication. Recheck dead before publishing;
if teardown wins the lock, reject the unpublished object.

Non-unregister teardown does not set dead. The preceding change covers an
add that misses the initial hash scan but reaches the per-device list
snapshot. Both changes are required when backporting this fix.

Fixes: 8814c4b53381 ("[IPV6] ADDRCONF: Convert addrconf_lock to RCU.")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/179161099408.434549.14449036924552933828@kernel.org
Link: https://lore.kernel.org/r/20261012145306.GB779501@shredder
Suggested-by: Ido Schimmel <idosch@nvidia.com>
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
 net/ipv6/addrconf.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 72d7df23f79674..3bede30d4e989b 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -1168,14 +1168,18 @@ ipv6_add_addr(struct inet6_dev *idev, struct ifa6_config *cfg,
 
 	rcu_read_lock();
 
-	err = ipv6_add_addr_hash(idev->dev, ifa);
+	write_lock_bh(&idev->lock);
+
+	if (idev->dead)
+		err = -ENODEV;
+	else
+		err = ipv6_add_addr_hash(idev->dev, ifa);
 	if (err < 0) {
+		write_unlock_bh(&idev->lock);
 		rcu_read_unlock();
 		goto out;
 	}
 
-	write_lock_bh(&idev->lock);
-
 	/* Add to inet6_dev unicast addr list. */
 	ipv6_link_dev_addr(idev, ifa);
 
@@ -3897,7 +3901,9 @@ static int addrconf_ifdown(struct net_device *dev, bool unregister)
 	 *	   Do not dev_put!
 	 */
 	if (unregister) {
+		write_lock_bh(&idev->lock);
 		WRITE_ONCE(idev->dead, 1);
+		write_unlock_bh(&idev->lock);
 
 		/* protected by rtnl_lock */
 		RCU_INIT_POINTER(dev->ip6_ptr, NULL);
-- 
2.55.0