CVE-2026-74630: ipv6: prevent in6_dev_get() from resurrecting inet6_dev

In the Linux kernel, the following vulnerability has been resolved:

ipv6: prevent in6devget() from resurrecting inet6dev

in6devget() reads dev->ip6ptr 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 refcountincnotzero() 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.

An independent run on the exact unpatched 6f5156d7a31a (v7.2-rc3) kernel reproduced the invalid reference acquisition as UID 1000:

refcountt: addition on 0; use-after-free. ip6mcsource+0xef4/0x17e0

It was followed by the corresponding reference underflow in ip6mcsource(). The supplied trace from the same unpatched revision additionally shows the access after the RCU read-side section ends:

BUG: KASAN: slab-use-after-free in mutexlock+0x76/0xe0 Write of size 8 at addr ffff888015b50240 by task poc/1219

Bug found and triaged by OpenAI Security Research and validated by Trail of Bits.