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

ipv6: prevent in6_dev_get() from resurrecting inet6_dev

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.

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

  refcount_t: addition on 0; use-after-free.
  ip6_mc_source+0xef4/0x17e0

It was followed by the corresponding reference underflow in
ip6_mc_source(). 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 mutex_lock+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.

https://git.kernel.org/stable/c/0e243671bc7b8eaf00f83dd2f4367436dc0cff98

https://git.kernel.org/stable/c/145812b678de9f3b59780173be3c0d22ed60dd93

https://git.kernel.org/stable/c/14e812ab41df0cac033479da835ec9a5de633404

https://git.kernel.org/stable/c/1c206d461c680c3151daa3c89fc26eaf5bf98a7f

https://git.kernel.org/stable/c/680fbd7942185448eadb990a3d10a53eb946b702

https://git.kernel.org/stable/c/785d908f8d21c8bc78b6fb2c2932ab662bf6918a

https://git.kernel.org/stable/c/aedcfefdb5b7ed7f8a6196a3e68a25bdbe51d2f8

https://git.kernel.org/stable/c/cc5bd568f9b7683e60841b6fd02c10d64535bd6e

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