Bluetooth: l2cap: fix null-ptr-deref in l2cap_chan_timeout

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: l2cap: fix null-ptr-deref in l2cap_chan_timeout

There is a race condition between l2cap_chan_timeout() and

l2cap_chan_del(). When we use l2cap_chan_del() to delete the

channel, the chan->conn will be set to null. But the conn could

be dereferenced again in the mutex_lock() of l2cap_chan_timeout().

As a result the null pointer dereference bug will happen. The

KASAN report triggered by POC is shown below:

[ 472.074580] ==================================================================

[ 472.075284] BUG: KASAN: null-ptr-deref in mutex_lock+0x68/0xc0

[ 472.075308] Write of size 8 at addr 0000000000000158 by task kworker/0:0/7

[ 472.075308]

[ 472.075308] CPU: 0 PID: 7 Comm: kworker/0:0 Not tainted 6.9.0-rc5-00356-g78c0094a146b #36

[ 472.075308] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu4

[ 472.075308] Workqueue: events l2cap_chan_timeout

[ 472.075308] Call Trace:

[ 472.075308] <TASK>

[ 472.075308] dump_stack_lvl+0x137/0x1a0

[ 472.075308] print_report+0x101/0x250

[ 472.075308] ? __virt_addr_valid+0x77/0x160

[ 472.075308] ? mutex_lock+0x68/0xc0

[ 472.075308] kasan_report+0x139/0x170

[ 472.075308] ? mutex_lock+0x68/0xc0

[ 472.075308] kasan_check_range+0x2c3/0x2e0

[ 472.075308] mutex_lock+0x68/0xc0

[ 472.075308] l2cap_chan_timeout+0x181/0x300

[ 472.075308] process_one_work+0x5d2/0xe00

[ 472.075308] worker_thread+0xe1d/0x1660

[ 472.075308] ? pr_cont_work+0x5e0/0x5e0

[ 472.075308] kthread+0x2b7/0x350

[ 472.075308] ? pr_cont_work+0x5e0/0x5e0

[ 472.075308] ? kthread_blkcg+0xd0/0xd0

[ 472.075308] ret_from_fork+0x4d/0x80

[ 472.075308] ? kthread_blkcg+0xd0/0xd0

[ 472.075308] ret_from_fork_asm+0x11/0x20

[ 472.075308] </TASK>

[ 472.075308] ==================================================================

[ 472.094860] Disabling lock debugging due to kernel taint

[ 472.096136] BUG: kernel NULL pointer dereference, address: 0000000000000158

[ 472.096136] #PF: supervisor write access in kernel mode

[ 472.096136] #PF: error_code(0x0002) - not-present page

[ 472.096136] PGD 0 P4D 0

[ 472.096136] Oops: 0002 [#1] PREEMPT SMP KASAN NOPTI

[ 472.096136] CPU: 0 PID: 7 Comm: kworker/0:0 Tainted: G B 6.9.0-rc5-00356-g78c0094a146b #36

[ 472.096136] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu4

[ 472.096136] Workqueue: events l2cap_chan_timeout

[ 472.096136] RIP: 0010:mutex_lock+0x88/0xc0

[ 472.096136] Code: be 08 00 00 00 e8 f8 23 1f fd 4c 89 f7 be 08 00 00 00 e8 eb 23 1f fd 42 80 3c 23 00 74 08 48 88

[ 472.096136] RSP: 0018:ffff88800744fc78 EFLAGS: 00000246

[ 472.096136] RAX: 0000000000000000 RBX: 1ffff11000e89f8f RCX: ffffffff8457c865

[ 472.096136] RDX: 0000000000000001 RSI: 0000000000000008 RDI: ffff88800744fc78

[ 472.096136] RBP: 0000000000000158 R08: ffff88800744fc7f R09: 1ffff11000e89f8f

[ 472.096136] R10: dffffc0000000000 R11: ffffed1000e89f90 R12: dffffc0000000000

[ 472.096136] R13: 0000000000000158 R14: ffff88800744fc78 R15: ffff888007405a00

[ 472.096136] FS: 0000000000000000(0000) GS:ffff88806d200000(0000) knlGS:0000000000000000

[ 472.096136] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

[ 472.096136] CR2: 0000000000000158 CR3: 000000000da32000 CR4: 00000000000006f0

[ 472.096136] Call Trace:

[ 472.096136] <TASK>

[ 472.096136] ? __die_body+0x8d/0xe0

[ 472.096136] ? page_fault_oops+0x6b8/0x9a0

[ 472.096136] ? kernelmode_fixup_or_oops+0x20c/0x2a0

[ 472.096136] ? do_user_addr_fault+0x1027/0x1340

[ 472.096136] ? _printk+0x7a/0xa0

[ 472.096136] ? mutex_lock+0x68/0xc0

[ 472.096136] ? add_taint+0x42/0xd0

[ 472.096136] ? exc_page_fault+0x6a/0x1b0

[ 472.096136] ? asm_exc_page_fault+0x26/0x30

[ 472.096136] ? mutex_lock+0x75/0xc0

[ 472.096136] ? mutex_lock+0x88/0xc0

[ 472.096136] ? mutex_lock+0x75/0xc0

[ 472.096136] l2cap_chan_timeo

---truncated---