Re: [PATCH] Bluetooth/l2cap: Fix uaf in l2cap_connect

[Luiz Augusto von Dentz]

Hi Edward,

On Sun, Sep 8, 2024 at 3:22 AM Edward Adam Davis <eadavis@xxxxxx> wrote:
>
> [Syzbot reported]
> BUG: KASAN: slab-use-after-free in l2cap_connect.constprop.0+0x10d8/0x1270 net/bluetooth/l2cap_core.c:3949
> Read of size 8 at addr ffff8880241e9800 by task kworker/u9:0/54
>
> CPU: 0 UID: 0 PID: 54 Comm: kworker/u9:0 Not tainted 6.11.0-rc6-syzkaller-00268-g788220eee30d #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/06/2024
> Workqueue: hci2 hci_rx_work
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:93 [inline]
>  dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:119
>  print_address_description mm/kasan/report.c:377 [inline]
>  print_report+0xc3/0x620 mm/kasan/report.c:488
>  kasan_report+0xd9/0x110 mm/kasan/report.c:601
>  l2cap_connect.constprop.0+0x10d8/0x1270 net/bluetooth/l2cap_core.c:3949
>  l2cap_connect_req net/bluetooth/l2cap_core.c:4080 [inline]
>  l2cap_bredr_sig_cmd net/bluetooth/l2cap_core.c:4772 [inline]
>  l2cap_sig_channel net/bluetooth/l2cap_core.c:5543 [inline]
>  l2cap_recv_frame+0xf0b/0x8eb0 net/bluetooth/l2cap_core.c:6825
>  l2cap_recv_acldata+0x9b4/0xb70 net/bluetooth/l2cap_core.c:7514
>  hci_acldata_packet net/bluetooth/hci_core.c:3791 [inline]
>  hci_rx_work+0xaab/0x1610 net/bluetooth/hci_core.c:4028
>  process_one_work+0x9c5/0x1b40 kernel/workqueue.c:3231
>  process_scheduled_works kernel/workqueue.c:3312 [inline]
>  worker_thread+0x6c8/0xed0 kernel/workqueue.c:3389
>  kthread+0x2c1/0x3a0 kernel/kthread.c:389
>  ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
> ...
>
> Freed by task 5245:
>  kasan_save_stack+0x33/0x60 mm/kasan/common.c:47
>  kasan_save_track+0x14/0x30 mm/kasan/common.c:68
>  kasan_save_free_info+0x3b/0x60 mm/kasan/generic.c:579
>  poison_slab_object+0xf7/0x160 mm/kasan/common.c:240
>  __kasan_slab_free+0x32/0x50 mm/kasan/common.c:256
>  kasan_slab_free include/linux/kasan.h:184 [inline]
>  slab_free_hook mm/slub.c:2256 [inline]
>  slab_free mm/slub.c:4477 [inline]
>  kfree+0x12a/0x3b0 mm/slub.c:4598
>  l2cap_conn_free net/bluetooth/l2cap_core.c:1810 [inline]
>  kref_put include/linux/kref.h:65 [inline]
>  l2cap_conn_put net/bluetooth/l2cap_core.c:1822 [inline]
>  l2cap_conn_del+0x59d/0x730 net/bluetooth/l2cap_core.c:1802
>  l2cap_connect_cfm+0x9e6/0xf80 net/bluetooth/l2cap_core.c:7241
>  hci_connect_cfm include/net/bluetooth/hci_core.h:1960 [inline]
>  hci_conn_failed+0x1c3/0x370 net/bluetooth/hci_conn.c:1265
>  hci_abort_conn_sync+0x75a/0xb50 net/bluetooth/hci_sync.c:5583
>  abort_conn_sync+0x197/0x360 net/bluetooth/hci_conn.c:2917
>  hci_cmd_sync_work+0x1a4/0x410 net/bluetooth/hci_sync.c:328
>  process_one_work+0x9c5/0x1b40 kernel/workqueue.c:3231
>  process_scheduled_works kernel/workqueue.c:3312 [inline]
>  worker_thread+0x6c8/0xed0 kernel/workqueue.c:3389
>  kthread+0x2c1/0x3a0 kernel/kthread.c:389
>  ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
>
> [Analysis]
> There was a data race when accessing conn in hci_rx_work and hci_cmd_sync_work.
> This is because the hci dev lock was prematurely exited when executing
> hci_acldata_macket() in hci_rx_work, which resulted in it being released
> by hci_cmd_sync_work when accessing conn outside the lock.
>
> Reported-and-tested-by: syzbot+c12e2f941af1feb5632c@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=c12e2f941af1feb5632c
> Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
> ---
>  net/bluetooth/hci_core.c   | 3 ++-
>  net/bluetooth/l2cap_core.c | 2 --
>  2 files changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/net/bluetooth/hci_core.c b/net/bluetooth/hci_core.c
> index f25a21f532aa..4f7b45bb863f 100644
> --- a/net/bluetooth/hci_core.c
> +++ b/net/bluetooth/hci_core.c
> @@ -3776,18 +3776,19 @@ static void hci_acldata_packet(struct hci_dev *hdev, struct sk_buff *skb)
>
>         hci_dev_lock(hdev);
>         conn = hci_conn_hash_lookup_handle(hdev, handle);
> -       hci_dev_unlock(hdev);
>
>         if (conn) {
>                 hci_conn_enter_active_mode(conn, BT_POWER_FORCE_ACTIVE_OFF);
>
>                 /* Send to upper protocol */
>                 l2cap_recv_acldata(conn, skb, flags);
> +               hci_dev_unlock(hdev);
>                 return;
>         } else {
>                 bt_dev_err(hdev, "ACL packet for unknown connection handle %d",
>                            handle);
>         }
> +       hci_dev_unlock(hdev);

This is sort of risky, we shouldn't be calling this deep into the
stack with hci_dev_lock held.

>
>         kfree_skb(skb);
>  }
> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> index 9988ba382b68..b948b0a3b2f2 100644
> --- a/net/bluetooth/l2cap_core.c
> +++ b/net/bluetooth/l2cap_core.c
> @@ -4072,10 +4072,8 @@ static int l2cap_connect_req(struct l2cap_conn *conn,
>         if (cmd_len < sizeof(struct l2cap_conn_req))
>                 return -EPROTO;
>
> -       hci_dev_lock(hdev);
>         if (hci_dev_test_flag(hdev, HCI_MGMT))
>                 mgmt_device_connected(hdev, hcon, NULL, 0);
> -       hci_dev_unlock(hdev);

So this might explain why things gets freed while processing the
request, we are locking to call mgmt_device_connected which I suspect
is no longer needed ever since:

commit db11223571d489d1aab575a4ac4b7352d2d54e2f
Author: Luiz Augusto von Dentz <luiz.von.dentz@xxxxxxxxx>
Date:   Tue Oct 25 14:12:58 2022 -0700

    Bluetooth: btusb: Default CONFIG_BT_HCIBTUSB_POLL_SYNC=y

    poll_sync has been proven to fix races of USB data and event endpoints
    so this enables it by default.

    Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@xxxxxxxxx>
    Tested-by: Tedd Ho-Jeong An <tedd.an@xxxxxxxxx>

Anyway syzbot don't use btusb so I think this might be due some
command pending that the emulator is not responding and instead
sending data, and then there is the issue that 7b064edae38d
("Bluetooth: Fix authentication if acl data comes before remote
feature evt") attempted to fix which I think it actually made it worse
by moving the call to mgmt_device_connected into l2cap_core.c it sort
move the problem but didn't fix the actual problem.

Maybe something like the following would be a better approach:

https://gist.github.com/Vudentz/121a15fa4391b2b1f6c7e8d420a6846e

>
>         l2cap_connect(conn, cmd, data, L2CAP_CONN_RSP);
>         return 0;
> --
> 2.43.0
>


-- 
Luiz Augusto von Dentz