linux-kernel - BUG: KASAN: use-after-free in dec_rlimit

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Date:   Wed, 17 Nov 2021 17:00:07 -0500
From:   Qian Cai <quic_qiancai@...cinc.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexey Gladkov <legion@...nel.org>
CC:     Yu Zhao <yuzhao@...gle.com>, <linux-kernel@...r.kernel.org>
Subject: BUG: KASAN: use-after-free in dec_rlimit_ucounts

Hi there, I can still reproduce this quickly on today's linux-next and all
the way back to 5.15-rc6 by running a syscall fuzzer for a while. The trace
points out to this line,

        for (iter = ucounts; iter; iter = iter->ns->ucounts) {

It looks KASAN indicated that that "ns" had already been freed. Is that
possible or perhaps this is more of refcount issue? 

 BUG: KASAN: use-after-free in dec_rlimit_ucounts
 Read of size 8 at addr ffff0008c0739860 by task trinity-c27/10924

 CPU: 27 PID: 10924 Comm: trinity-c27 Not tainted 5.15.0-next-20211115-dirty #192
 Hardware name: MiTAC RAPTOR EV-883832-X3-0001/RAPTOR, BIOS 1.6 06/28/2020
 Call trace:
  dump_backtrace
  show_stack
  dump_stack_lvl
  print_address_description.constprop.0
  kasan_report
  __asan_report_load8_noabort
  dec_rlimit_ucounts
  dec_rlimit_ucounts at kernel/ucount.c:284
  mqueue_evict_inode
  mqueue_evict_inode at ipc/mqueue.c:544
  evict
  iput.part.0
  iput
  __arm64_sys_mq_unlink
  invoke_syscall
  el0_svc_common.constprop.0
  do_el0_svc
  el0_svc
  el0t_64_sync_handler
  el0t_64_sync

 Allocated by task 10615:
  kasan_save_stack
  __kasan_slab_alloc
  slab_post_alloc_hook
  kmem_cache_alloc
  create_user_ns
  unshare_userns
  ksys_unshare
  __arm64_sys_unshare
  invoke_syscall
  el0_svc_common.constprop.0
  do_el0_svc
  el0_svc
  el0t_64_sync_handler
  el0t_64_sync

 Freed by task 8660:
  kasan_save_stack
  kasan_set_track
  kasan_set_free_info
  __kasan_slab_free
  slab_free_freelist_hook
  kmem_cache_free
  free_user_ns
  process_one_work
  worker_thread
  kthread
  ret_from_fork

 Last potentially related work creation:
  kasan_save_stack
  __kasan_record_aux_stack
  kasan_record_aux_stack_noalloc
  insert_work
  __queue_work
  queue_work_on
  __put_user_ns
  put_cred_rcu
  rcu_do_batch
  rcu_core
  rcu_core_si
  __do_softirq

 The buggy address belongs to the object at ffff0008c07395e8
  which belongs to the cache user_namespace of size 768
 The buggy address is located 632 bytes inside of
  768-byte region [ffff0008c07395e8, ffff0008c07398e8)
 The buggy address belongs to the page:
 page:fffffc002301ce00 refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff0008c073cec8 pfn:0x940738
 head:fffffc002301ce00 order:3 compound_mapcount:0 compound_pincount:0
 memcg:ffff0008b9b5f101
 flags: 0xbfffc0000010200(slab|head|node=0|zone=2|lastcpupid=0xffff)
 raw: 0bfffc0000010200 ffff000800f3e9c8 ffff000800f3e9c8 ffff000802e69b80
 raw: ffff0008c073cec8 00000000001d0012 00000001ffffffff ffff0008b9b5f101
 page dumped because: kasan: bad access detected

 Memory state around the buggy address:
  ffff0008c0739700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
  ffff0008c0739780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 >ffff0008c0739800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                        ^
  ffff0008c0739880: fb fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc
  ffff0008c0739900: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc