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

net: qrtr: fix refcount saturation and potential UAF in qrtr_port_remove

In qrtr_port_remove(), the socket reference count is decremented via
__sock_put() before the port is removed from the qrtr_ports XArray and
before the RCU grace period elapses.

This breaks the fundamental RCU update paradigm. It exposes a race
window where a concurrent RCU reader (such as qrtr_reset_ports() or
qrtr_port_lookup()) can obtain a pointer to the socket from the XArray,
and attempt to call sock_hold() on a socket whose reference count has
already dropped to zero.

This exact race condition was hit during syzkaller fuzzing, leading to
the following refcount saturation warning and a potential Use-After-Free:

  refcount_t: saturated; leaking memory.
  WARNING: CPU: 3 PID: 1273 at lib/refcount.c:22 refcount_warn_saturate+0xae/0x1d0
  Modules linked in: qrtr(+) bochs drm_shmem_helper ...
  Call Trace:
   <TASK>
   qrtr_reset_ports net/qrtr/af_qrtr.c:768 [inline] [qrtr]
   __qrtr_bind.isra.0+0x48b/0x570 net/qrtr/af_qrtr.c:805 [qrtr]
   qrtr_bind+0x17d/0x210 net/qrtr/af_qrtr.c:901 [qrtr]
   kernel_bind+0xe4/0x120 net/socket.c:3592
   qrtr_ns_init+0x1a6/0x380 net/qrtr/ns.c:715 [qrtr]
   qrtr_proto_init+0x3b/0xff0 net/qrtr/af_qrtr.c:169 [qrtr]
   do_one_initcall+0xf5/0x5e0 init/main.c:1283
   ...
   </TASK>

Fix this by deferring the reference count decrement until after the
xa_erase() and the synchronize_rcu() complete.

(Note: The v1 of this patch incorrectly replaced __sock_put() with
sock_put(). As Simon Horman pointed out, the callers of qrtr_port_remove()
still hold a reference to the socket, so freeing the socket memory here
would lead to a subsequent UAF in the caller. Thus, the __sock_put() is
kept, but only repositioned to close the RCU race.)