Is commit 4d94f0555827 safe?

[Takashi Iwai <tiwai@suse.de>]

[ I posted without the subject line mistakenly, so resent again;
  sorry if you have seen already read it ]

Hi Luiz,

due to the CVE assignment, I stumbled on the recent fix for BT
hci_core, the commit 4d94f0555827 ("Bluetooth: hci_core: Fix sleeping
function called from invalid context"), and wonder whether it's really
safe.

As already asked question at the patch review:
  https://patchwork.kernel.org/comment/26147087/
the code allows the callbacks to be called even after
hci_unregister_cb() returns.

Your assumption was that it's never called without the module removal,
but isn't hci_unregister_cb() also called from iso_exit() which can be
triggered via set_iso_socket_func() in mgmt.c?  Also, any 3rd party
module could call hci_unregister_cb() in a wild way, too -- even if
the function still remains, it doesn't mean that you can call it
safely if the caller already assumes it being unregistered.

In addition to that, I feel what the patch does as a bit too
heavy-lifting: it does kmalloc() and copy the whole hci_cb object,
which isn't quite small for each.  If the callback is still safe to
call after RCU protection, you may just keep the hci_cb pointer
instead of copying the whole content, too?

I couldn't find v1 patch in the patchwork, so not sure whether this
has been already discussed.  If so, let me know.


Thanks!

Takashi