Re: kernel panic happens when disconnecting Bluetooth headset

Hi Nick,

quoted

quoted

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 <redacted>
Signed-off-by: Andrei Emeltchenko <redacted>
---
 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