Message-ID: <2f14ae85-6ec7-2a37-a54c-ab9a68bbc40a@jm0.eu>
Date:   Wed, 8 Jul 2020 19:36:00 +0200
From:   "Ing. Josua Mayer" <josua.mayer@....eu>
To:     Kees Cook <keescook@...omium.org>,
        Jukka Rissanen <jukka.rissanen@...ux.intel.com>,
        Johan Hedberg <johan.hedberg@...el.com>
Cc:     davem@...emloft.net, kuba@...nel.org,
        linux-bluetooth@...r.kernel.org, linux-kernel@...r.kernel.org,
        marcel@...tmann.org, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com,
        syzbot <syzbot+7dd7f2f77a7a01d1dc14@...kaller.appspotmail.com>,
        Michael Scott <mike@...ndries.io>,
        Josua Mayer <josua.mayer@....eu>
Subject: Re: use-after-free in Bluetooth: 6lowpan (was Re: KASAN:
 use-after-free Write in refcount_warn_saturate)

Greetings everybody,
Am 06.07.20 um 05:32 schrieb Kees Cook:
> On Thu, Feb 27, 2020 at 11:50:11PM -0800, syzbot wrote:
>> 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@...kaller.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 free and use-after-free are both via the same do_enable_set() path
> which implies there are racing writes to the debugfs write handler. It
> seems locking is missing for both listen_chan and enable_6lowpan. The
> latter seems misused in is_bt_6lowpan(), which should likely just be
> checking for chan->ops == &bt_6lowpan_chan_ops, I think?
> 
> I have no way to actually test changes to this code...
> 

I do have limited hardware for testing bluetooth 6lowpan connections,
but am a little busy these weeks. So some time by the end of this month
I should be able to at least verify if my use-case would be broken by a
change, if somebody were to propose one ;)

br
Josua Mayer

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