in6_dev_get() reads dev->ip6_ptr under RCU and then unconditionally increments its refcount. Device teardown can clear the pointer and drop the last reference between these operations. The increment then resurrects an object whose RCU free has already been queued, so callers can use it after it is freed. Use refcount_inc_not_zero() and return NULL when the object has already reached zero. RCU keeps the memory accessible through the attempted reference acquisition, and a successful increment pins the object for the caller. Fixes: 8814c4b53381 ("[IPV6] ADDRCONF: Convert addrconf_lock to RCU.") Bug found and triaged by OpenAI Security Research and validated by Trail of Bits. Assisted-by: Codex:gpt-5.6-sol gpt-5.5-cyber Signed-off-by: Kyle Zeng --- Trail of Bits has a reproducer for this bug that triggers a KASAN use-after-free and can share if needed. include/net/addrconf.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/include/net/addrconf.h b/include/net/addrconf.h index 8ced27a82..e67642459 100644 --- a/include/net/addrconf.h +++ b/include/net/addrconf.h @@ -405,8 +405,8 @@ static inline struct inet6_dev *in6_dev_get(const struct net_device *dev) rcu_read_lock(); idev = rcu_dereference(dev->ip6_ptr); - if (idev) - refcount_inc(&idev->refcnt); + if (idev && !refcount_inc_not_zero(&idev->refcnt)) + idev = NULL; rcu_read_unlock(); return idev; }