AZL-102567

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

cuse: wait for pending RCU callbacks on module exit

Since commit 053fc4f755ad ("fuse: fix UAF in rcu pathwalks"), fuse_conn_put() frees the fuse_conn through call_rcu() rather than synchronously. For cuse, fc->release is cuse_fc_release(), which lives in the cuse module. If the module is removed before the RCU grace period ends, the callback jumps into freed module memory:

  userspace / module unload      |        RCU softirq

 close(/dev/cuse)                    |
  cuse_channel_release()             |
   fuse_dev_release()                |
    fuse_conn_put(fch->conn)         |
     call_rcu(delayed_release) ------+---> callback queued
                                     |
 rmmod cuse                          |
  cuse_exit()                        |
   cuse_channel_destroy()            |
   ...                               |
   return                            |
                                     |
 <module text freed>                 |
                                     |  rcu_do_batch()
                                     |   delayed_release()
                                     |    fc->release()
                                     |     -> cuse_fc_release()
                                     |        ^^^ freed text!

The freed module text is unmapped by vfree(), so the jump into the
stale callback triggers a page-fault Oops.  If the virtual address
is subsequently reused, the callback could execute unrelated code
(undefined behaviour).

Fix this by calling rcu_barrier() in cuse_exit() so that any pending
fuse_conn release callback completes before the module is removed.