Re: [RFC] Bluetooth: hci_sock: Fix calling lock_sock when handling HCI_DEV_UNREG

Hi Tetsuo,

On Sat, Jul 17, 2021 at 7:22 PM Tetsuo Handa
[off-list ref] wrote:

Luiz Augusto von Dentz wrote:

quoted

This removes the reference of hci_dev to hci_pinfo since the reference
cannot prevent hdev to be freed hci_pinfo only keeps the index so in
case the device is unregistered and freed hci_dev_get will return NULL
thus prevent UAF.

I'm not convinced that this change is safe.

vhci_release() (which will call hci_unregister_dev(hdev)) is called when
refcount to /dev/vchi dropped to 0. That is, vhci_release() might be called
while e.g. hci_sock_bound_ioctl() is in progress.

Since hci_unregister_dev(hdev) calls list_del(&hdev->list) with hci_dev_list_lock
held for write, hci_dev_get(hci_pi(sk)->dev) which scans hci_dev_list with
hci_dev_list_lock held for read will start returning NULL. But I think that
this change introduces two race windows.

Since hci_unregister_dev(hdev) then calls hci_sock_dev_event(hdev, HCI_DEV_UNREG)
and finally calls ida_simple_remove(&hci_index_ida, id), subsequent
hci_register_dev(struct hci_dev *hdev) call can re-assign the id which
hci_pi(sk)->dev is tracking, by calling ida_simple_get() and finally calling
list_add(&hdev->list, &hci_dev_list) with hci_dev_list_lock held for write.

We can perform a pointer comparison if that makes you happy. Anyway I
don't know if you guys have realized already but this is probably
impossible to reproduce with real hardware since the
registration/unregistration comes for other buses no memfault would
hold the thread that long for unplugging and replugging a device on
the bus, vhci is usually only used for emulation/testing/CI, not sure
who got the idea that finding vulnerabilities on our emulator would be
a great feat.

Therefore, I think that first race window is that

+       /* Commands may use copy_from_user which is unsafe while holding hdev as
+        * it could be unregistered in the meantime.
+        */
+       hci_dev_put(hdev);
+       hdev = NULL;

causes hci_sock_bound_ioctl() to check flags on an intended hdev and
e.g. hci_sock_reject_list_add() to operate on an unintended hdev.

Also, even if hci_sock_bound_ioctl() and hci_sock_reject_list_add() reached
the same hdev, I think that there still is second race window that

  hci_unregister_dev() {                       hci_sock_reject_list_add() {
                                                 hdev = hci_dev_get(hci_pi(sk)->dev);
    write_lock(&hci_dev_list_lock);
    list_del(&hdev->list);
    write_unlock(&hci_dev_list_lock);

    hci_sock_dev_event(hdev, HCI_DEV_UNREG);

    hci_dev_lock(hdev);
    hci_bdaddr_list_clear(&hdev->reject_list);
    hci_dev_unlock(hdev);
                                                 hci_dev_lock(hdev);
                                                 err = hci_bdaddr_list_add(&hdev->reject_list, &bdaddr, BDADDR_BREDR); // <= Adding after clear all; at least memory leak.
                                                 hci_dev_unlock(hdev);
                                                 hci_dev_put(hdev);
  }

. That is, an attempt to replace pointer reference with index number is racy.

After all, hci_sock_dev_event(hdev, HCI_DEV_UNREG) is responsible for
waiting for already started e.g. hci_sock_reject_list_add().

Both blocks do require hci_dev_lock, so if the second block had
acquired the lock isn't it obvious that the first won't execute until
hci_dev_unlock is complete? Anyway after all these discussion Im even
more convinced that the real problem lies in hci_dev_get/hold, after
all references are usually used to prevent the objects to be freed but
in this case it doesn't and no locking will gonna fix that.

-- 
Luiz Augusto von Dentz