[PATCH] ipv6: prevent in6_dev_get() from resurrecting inet6_dev

[David Lee <david.lee@trailofbits.com>]

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 <kylebot@openai.com>
---
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;
 }