Bluetooth: add a mutex lock to avoid UAF in do_enale_set

In the case we set or free the global value listen_chan in
different threads, we can encounter the UAF problems because
the method is not protected by any lock, add one to avoid
this bug.

BUG: KASAN: use-after-free in l2cap_chan_close+0x48/0x990
net/bluetooth/l2cap_core.c:730
Read of size 8 at addr ffff888096950000 by task kworker/1:102/2868

CPU: 1 PID: 2868 Comm: kworker/1:102 Not tainted 5.5.0-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+0x1fb/0x318 lib/dump_stack.c:118
 print_address_description+0x74/0x5c0 mm/kasan/report.c:374
 __kasan_report+0x149/0x1c0 mm/kasan/report.c:506
 kasan_report+0x26/0x50 mm/kasan/common.c:641
 __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:135
 l2cap_chan_close+0x48/0x990 net/bluetooth/l2cap_core.c:730
 do_enable_set+0x660/0x900 net/bluetooth/6lowpan.c:1074
 process_one_work+0x7f5/0x10f0 kernel/workqueue.c:2264
 worker_thread+0xbbc/0x1630 kernel/workqueue.c:2410
 kthread+0x332/0x350 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Allocated by task 2870:
 save_stack mm/kasan/common.c:72 [inline]
 set_track mm/kasan/common.c:80 [inline]
 __kasan_kmalloc+0x118/0x1c0 mm/kasan/common.c:515
 kasan_kmalloc+0x9/0x10 mm/kasan/common.c:529
 kmem_cache_alloc_trace+0x221/0x2f0 mm/slab.c:3551
 kmalloc include/linux/slab.h:555 [inline]
 kzalloc include/linux/slab.h:669 [inline]
 l2cap_chan_create+0x50/0x320 net/bluetooth/l2cap_core.c:446
 chan_create net/bluetooth/6lowpan.c:640 [inline]
 bt_6lowpan_listen net/bluetooth/6lowpan.c:959 [inline]
 do_enable_set+0x6a4/0x900 net/bluetooth/6lowpan.c:1078
 process_one_work+0x7f5/0x10f0 kernel/workqueue.c:2264
 worker_thread+0xbbc/0x1630 kernel/workqueue.c:2410
 kthread+0x332/0x350 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Freed by task 2870:
 save_stack mm/kasan/common.c:72 [inline]
 set_track mm/kasan/common.c:80 [inline]
 kasan_set_free_info mm/kasan/common.c:337 [inline]
 __kasan_slab_free+0x12e/0x1e0 mm/kasan/common.c:476
 kasan_slab_free+0xe/0x10 mm/kasan/common.c:485
 __cache_free mm/slab.c:3426 [inline]
 kfree+0x10d/0x220 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+0x170/0x190 net/bluetooth/l2cap_core.c:498
 do_enable_set+0x66c/0x900 net/bluetooth/6lowpan.c:1075
 process_one_work+0x7f5/0x10f0 kernel/workqueue.c:2264
 worker_thread+0xbbc/0x1630 kernel/workqueue.c:2410
 kthread+0x332/0x350 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

The buggy address belongs to the object at ffff888096950000
 which belongs to the cache kmalloc-2k of size 2048
The buggy address is located 0 bytes inside of
 2048-byte region [ffff888096950000, ffff888096950800)
The buggy address belongs to the page:
page:ffffea00025a5400 refcount:1 mapcount:0 mapping:ffff8880aa400e00 index:0x0
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea00027d1548 ffffea0002397808 ffff8880aa400e00
raw: 0000000000000000 ffff888096950000 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88809694ff00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff88809694ff80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff888096950000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff888096950080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888096950100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

Reported-by: syzbot+96414aa0033c363d8458@syzkaller.appspotmail.com
Signed-off-by: Lihong Kou <koulihong@huawei.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/6lowpan.c

@@ -50,6 +50,7 @@ static bool enable_6lowpan;
 /* We are listening incoming connections via this channel
  */
 static struct l2cap_chan *listen_chan;
+static DEFINE_MUTEX(set_lock);
 
 struct lowpan_peer {
 	struct list_head list;
@@ -1078,12 +1079,14 @@ static void do_enable_set(struct work_struct *work)
 
 	enable_6lowpan = set_enable->flag;
 
+	mutex_lock(&set_lock);
 	if (listen_chan) {
 		l2cap_chan_close(listen_chan, 0);
 		l2cap_chan_put(listen_chan);
 	}
 
 	listen_chan = bt_6lowpan_listen();
+	mutex_unlock(&set_lock);
 
 	kfree(set_enable);
 }
@@ -1135,11 +1138,13 @@ static ssize_t lowpan_control_write(struct file *fp,
 		if (ret == -EINVAL)
 			return ret;
 
+		mutex_lock(&set_lock);
 		if (listen_chan) {
 			l2cap_chan_close(listen_chan, 0);
 			l2cap_chan_put(listen_chan);
 			listen_chan = NULL;
 		}
+		mutex_unlock(&set_lock);
 
 		if (conn) {
 			struct lowpan_peer *peer;