Re: KASAN: use-after-free Read in hci_cmd_timeout

[syzbot <syzbot+19a9f729f05272857487@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following crash on:

HEAD commit:    eca94432 Bluetooth: Fix faulty expression for minimum encr..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1006cc8ba00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f6451f0da3d42d53
dashboard link: https://syzkaller.appspot.com/bug?extid=19a9f729f05272857487
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=125b7999a00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=176deefba00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+19a9f729f05272857487@syzkaller.appspotmail.com

Bluetooth: hci0: command 0xfc11 tx timeout
==================================================================
BUG: KASAN: use-after-free in hci_cmd_timeout+0x1fe/0x220  
net/bluetooth/hci_core.c:2614
Read of size 8 at addr ffff88809e8a3c48 by task kworker/0:5/9461

CPU: 0 PID: 9461 Comm: kworker/0:5 Not tainted 5.2.0-rc7+ #40
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: events hci_cmd_timeout
Call Trace:
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0x172/0x1f0 lib/dump_stack.c:113
  print_address_description.cold+0x7c/0x20d mm/kasan/report.c:188
  __kasan_report.cold+0x1b/0x40 mm/kasan/report.c:317
  kasan_report+0x12/0x20 mm/kasan/common.c:614
  __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:132
  hci_cmd_timeout+0x1fe/0x220 net/bluetooth/hci_core.c:2614
  process_one_work+0x989/0x1790 kernel/workqueue.c:2269
  worker_thread+0x98/0xe40 kernel/workqueue.c:2415
  kthread+0x354/0x420 kernel/kthread.c:255
  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Allocated by task 9446:
  save_stack+0x23/0x90 mm/kasan/common.c:71
  set_track mm/kasan/common.c:79 [inline]
  __kasan_kmalloc mm/kasan/common.c:489 [inline]
  __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:462
  kasan_slab_alloc+0xf/0x20 mm/kasan/common.c:497
  slab_post_alloc_hook mm/slab.h:437 [inline]
  slab_alloc mm/slab.c:3326 [inline]
  kmem_cache_alloc+0x11a/0x6f0 mm/slab.c:3488
  skb_clone+0x154/0x3d0 net/core/skbuff.c:1321
  hci_cmd_work+0xe0/0x2a0 net/bluetooth/hci_core.c:4495
  process_one_work+0x989/0x1790 kernel/workqueue.c:2269
  worker_thread+0x98/0xe40 kernel/workqueue.c:2415
  kthread+0x354/0x420 kernel/kthread.c:255
  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Freed by task 1501:
  save_stack+0x23/0x90 mm/kasan/common.c:71
  set_track mm/kasan/common.c:79 [inline]
  __kasan_slab_free+0x102/0x150 mm/kasan/common.c:451
  kasan_slab_free+0xe/0x10 mm/kasan/common.c:459
  __cache_free mm/slab.c:3432 [inline]
  kmem_cache_free+0x86/0x260 mm/slab.c:3698
  kfree_skbmem net/core/skbuff.c:620 [inline]
  kfree_skbmem+0xc5/0x150 net/core/skbuff.c:614
  __kfree_skb net/core/skbuff.c:677 [inline]
  kfree_skb net/core/skbuff.c:694 [inline]
  kfree_skb+0xf0/0x390 net/core/skbuff.c:688
  hci_dev_do_open+0xb20/0x1760 net/bluetooth/hci_core.c:1550
  hci_power_on+0x10d/0x580 net/bluetooth/hci_core.c:2171
  process_one_work+0x989/0x1790 kernel/workqueue.c:2269
  worker_thread+0x98/0xe40 kernel/workqueue.c:2415
  kthread+0x354/0x420 kernel/kthread.c:255
  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

The buggy address belongs to the object at ffff88809e8a3b80
  which belongs to the cache skbuff_head_cache of size 224
The buggy address is located 200 bytes inside of
  224-byte region [ffff88809e8a3b80, ffff88809e8a3c60)
The buggy address belongs to the page:
page:ffffea00027a28c0 refcount:1 mapcount:0 mapping:ffff88821baabb40  
index:0x0
flags: 0x1fffc0000000200(slab)
raw: 01fffc0000000200 ffffea00027b2d08 ffffea00021b83c8 ffff88821baabb40
raw: 0000000000000000 ffff88809e8a3040 000000010000000c 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
  ffff88809e8a3b00: fb fb fb fb fc fc fc fc fc fc fc fc fc fc fc fc
  ffff88809e8a3b80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff88809e8a3c00: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
                                               ^
  ffff88809e8a3c80: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
  ffff88809e8a3d00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================