Re: kernel panic happens when disconnecting Bluetooth headset

[Marcel Holtmann <marcel@holtmann.org>]

Hi Nick,

> >> Processing a RFCOMM UA frame when the socket is closed and we were not
> >> the
> >> RFCOMM initiator would cause rfcomm_session_put() to be called twice
> >> during
> >> rfcomm_process_rx(). This would cause a kernel panic in
> >> rfcomm_session_close.
> >>
> >> This could be easily reproduced during disconnect with devices such as
> >> Motorola H270 that send RFCOMM UA followed quickly by L2CAP disconnect
> >> request.
> >> This hcidump for this looks like:
> >>
> >> 2009-09-21 17:22:37.788895 < ACL data: handle 1 flags 0x02 dlen 8
> >>    L2CAP(d): cid 0x0041 len 4 [psm 3]
> >>      RFCOMM(s): DISC: cr 0 dlci 20 pf 1 ilen 0 fcs 0x7d
> >> 2009-09-21 17:22:37.906204 > HCI Event: Number of Completed Packets
> >> (0x13)
> >> plen 5
> >>    handle 1 packets 1
> >> 2009-09-21 17:22:37.933090 > ACL data: handle 1 flags 0x02 dlen 8
> >>    L2CAP(d): cid 0x0040 len 4 [psm 3]
> >>      RFCOMM(s): UA: cr 0 dlci 20 pf 1 ilen 0 fcs 0x57
> >> 2009-09-21 17:22:38.636764 < ACL data: handle 1 flags 0x02 dlen 8
> >>    L2CAP(d): cid 0x0041 len 4 [psm 3]
> >>      RFCOMM(s): DISC: cr 0 dlci 0 pf 1 ilen 0 fcs 0x9c
> >> 2009-09-21 17:22:38.744125 > HCI Event: Number of Completed Packets
> >> (0x13)
> >> plen 5
> >>    handle 1 packets 1
> >> 2009-09-21 17:22:38.763687 > ACL data: handle 1 flags 0x02 dlen 8
> >>    L2CAP(d): cid 0x0040 len 4 [psm 3]
> >>      RFCOMM(s): UA: cr 0 dlci 0 pf 1 ilen 0 fcs 0xb6
> >> 2009-09-21 17:22:38.783554 > ACL data: handle 1 flags 0x02 dlen 12
> >>    L2CAP(s): Disconn req: dcid 0x0040 scid 0x0041
> >>
> >> Avoid calling rfcomm_session_put() twice by skipping this call
> >> in rfcomm_recv_ua() if the socket is closed.
> >>
> >> Picked from:
> >> http://android.git.kernel.org/?p=kernel/common.git;a=commit;h=1048e007842da2d6440679e1ca80f45438a6369d
> >>
> >> Signed-off-by: Nick Pelly <npelly@google.com>
> >> Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@nokia.com>
> >> ---
> >>  net/bluetooth/rfcomm/core.c |    3 ++-
> >>  1 files changed, 2 insertions(+), 1 deletions(-)
> >>
> >> diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c
> >> index 0313e88..56ffcb8 100644
> >> --- a/net/bluetooth/rfcomm/core.c
> >> +++ b/net/bluetooth/rfcomm/core.c
> >> @@ -1148,7 +1148,8 @@ static int rfcomm_recv_ua(struct rfcomm_session
> >> *s, u8 dlci)
> >>                         break;
> >>
> >>                 case BT_DISCONN:
> >> -                       rfcomm_session_put(s);
> >> +                       if (s->sock->sk->sk_state != BT_CLOSED)
> >> +                               rfcomm_session_put(s);
> >>                         break;
> >>                 }
> >>         }
> >
> > I am not a big fan of conditionally decreasing reference counts. I do
> > think it would be better to fix this by holding an extra pair of
> > reference counts or actually fixing the imbalance. What about the other
> > patches I proposed?
> 
> Your proposed patch was to add an extra hold() / put() reference count
> around the offending put(). I did test this patch, and found it does
> not fix the underlying imbalance, it just moves the kernel panic
> somewhere else.
> 
> As best I can tell, my patch does address the underlying imbalance. It
> is in production on Android phones and seems to work well. As best I
> can tell, there is not a cleaner solution that does not involve
> significant refactoring of rfcomm refcounting.

the RFCOMM reference counting is something nasty and it does need to be
re-written. One thing that needs to happen that we stop using the L2CAP
sockets directly. We have to put a proper L2CAP in-kernel specific API
in between that ensures we are not mixing things. That is the one issues
that we always had in this area.

Before applying this patch, I like to have additionally a comment in
front of this conditional put call that explains a little bit the
problem area here. The long explanation with logs etc. should be in the
commit message. I have to make sure that we fully understand what is
going on here and why we did it.

Regards

Marcel