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

nstree: check listing permission before taking a namespace reference

legitimize_ns() takes a reference on the candidate namespace before
may_list_ns() has decided whether the caller may see it. The
__free(ns_put) cleanup on the denied path can drop the last reference to a
mount namespace while we still hold the rcu read lock, and put_mnt_ns()
may sleep there. This is the same problem commit 2ec2aff3c8e2 ("ns: make
sure reference are dropped outside of rcu lock") fixed for the put_user()
path. Neither ns_requested() nor may_list_ns() needs a reference, both
only look at the namespace type and at the caller's own namespaces, so do
the checks first and take the reference last.

Splat:

  Voluntary context switch within RCU read-side critical section!
  WARNING: kernel/rcu/tree_plugin.h:332 at rcu_note_context_switch+0x238/0x2a0, CPU#5: a/3442
  CPU: 5 UID: 1000 PID: 3442 Comm: a Not tainted 7.0.0-30-generic #30-Ubuntu PREEMPT(lazy)
  RIP: 0010:rcu_note_context_switch+0x238/0x2a0
  Call Trace:
   <TASK>
   __schedule+0xcf/0x650
   schedule+0x27/0x90
   schedule_preempt_disabled+0x15/0x30
   __mutex_lock.constprop.0+0x550/0xaf0
   __mutex_lock_slowpath+0x13/0x20
   mutex_lock+0x3b/0x50
   exp_funnel_lock+0xb2/0x260
   synchronize_rcu_expedited+0xe7/0x220
   namespace_unlock+0x26a/0x320
   put_mnt_ns+0xd3/0x120
   mntns_put+0xe/0x20
   do_listns+0x13e/0x560
   __do_sys_listns+0x126/0x2d0
   __x64_sys_listns+0x20/0x30
   x64_sys_call+0x2366/0x2390
   do_syscall_64+0x105/0x5a0
   entry_SYSCALL_64_after_hwframe+0x76/0x7e
   </TASK>