KASAN: use-after-free Write in refcount_warn_saturate

[syzbot <syzbot+7dd7f2f77a7a01d1dc14@syzkaller.appspotmail.com>]

Hello,

syzbot found the following crash on:

HEAD commit:    f8788d86 Linux 5.6-rc3
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13005fd9e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=9833e26bab355358
dashboard link: https://syzkaller.appspot.com/bug?extid=7dd7f2f77a7a01d1dc14
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
userspace arch: i386
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17e3ebede00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16a9f8f9e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+7dd7f2f77a7a01d1dc14@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in atomic_set include/asm-generic/atomic-instrumented.h:44 [inline]
BUG: KASAN: use-after-free in refcount_set include/linux/refcount.h:123 [inline]
BUG: KASAN: use-after-free in refcount_warn_saturate+0x1f/0x1f0 lib/refcount.c:15
Write of size 4 at addr ffff888090eb4018 by task kworker/1:24/2888

CPU: 1 PID: 2888 Comm: kworker/1:24 Not tainted 5.6.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events do_enable_set
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x197/0x210 lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
 __kasan_report.cold+0x1b/0x32 mm/kasan/report.c:506
 kasan_report+0x12/0x20 mm/kasan/common.c:641
 check_memory_region_inline mm/kasan/generic.c:185 [inline]
 check_memory_region+0x134/0x1a0 mm/kasan/generic.c:192
 __kasan_check_write+0x14/0x20 mm/kasan/common.c:101
 atomic_set include/asm-generic/atomic-instrumented.h:44 [inline]
 refcount_set include/linux/refcount.h:123 [inline]
 refcount_warn_saturate+0x1f/0x1f0 lib/refcount.c:15
 refcount_sub_and_test include/linux/refcount.h:261 [inline]
 refcount_dec_and_test include/linux/refcount.h:281 [inline]
 kref_put include/linux/kref.h:64 [inline]
 l2cap_chan_put+0x1d9/0x240 net/bluetooth/l2cap_core.c:498
 do_enable_set+0x54b/0x960 net/bluetooth/6lowpan.c:1075
 process_one_work+0xa05/0x17a0 kernel/workqueue.c:2264
 worker_thread+0x98/0xe40 kernel/workqueue.c:2410
 kthread+0x361/0x430 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Allocated by task 2888:
 save_stack+0x23/0x90 mm/kasan/common.c:72
 set_track mm/kasan/common.c:80 [inline]
 __kasan_kmalloc mm/kasan/common.c:515 [inline]
 __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:488
 kasan_kmalloc+0x9/0x10 mm/kasan/common.c:529
 kmem_cache_alloc_trace+0x158/0x790 mm/slab.c:3551
 kmalloc include/linux/slab.h:555 [inline]
 kzalloc include/linux/slab.h:669 [inline]
 l2cap_chan_create+0x45/0x3a0 net/bluetooth/l2cap_core.c:446
 chan_create+0x10/0xe0 net/bluetooth/6lowpan.c:640
 bt_6lowpan_listen net/bluetooth/6lowpan.c:959 [inline]
 do_enable_set+0x583/0x960 net/bluetooth/6lowpan.c:1078
 process_one_work+0xa05/0x17a0 kernel/workqueue.c:2264
 worker_thread+0x98/0xe40 kernel/workqueue.c:2410
 kthread+0x361/0x430 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Freed by task 2975:
 save_stack+0x23/0x90 mm/kasan/common.c:72
 set_track mm/kasan/common.c:80 [inline]
 kasan_set_free_info mm/kasan/common.c:337 [inline]
 __kasan_slab_free+0x102/0x150 mm/kasan/common.c:476
 kasan_slab_free+0xe/0x10 mm/kasan/common.c:485
 __cache_free mm/slab.c:3426 [inline]
 kfree+0x10a/0x2c0 mm/slab.c:3757
 l2cap_chan_destroy net/bluetooth/l2cap_core.c:484 [inline]
 kref_put include/linux/kref.h:65 [inline]
 l2cap_chan_put+0x1b7/0x240 net/bluetooth/l2cap_core.c:498
 do_enable_set+0x54b/0x960 net/bluetooth/6lowpan.c:1075
 process_one_work+0xa05/0x17a0 kernel/workqueue.c:2264
 worker_thread+0x98/0xe40 kernel/workqueue.c:2410
 kthread+0x361/0x430 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

The buggy address belongs to the object at ffff888090eb4000
 which belongs to the cache kmalloc-2k of size 2048
The buggy address is located 24 bytes inside of
 2048-byte region [ffff888090eb4000, ffff888090eb4800)
The buggy address belongs to the page:
page:ffffea000243ad00 refcount:1 mapcount:0 mapping:ffff8880aa400e00 index:0x0
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea0002694048 ffffea000241e648 ffff8880aa400e00
raw: 0000000000000000 ffff888090eb4000 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff888090eb3f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff888090eb3f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff888090eb4000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                            ^
 ffff888090eb4080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888090eb4100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


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