This v2 patchset contains:
a) minor layout fixes for patches 2 and 3 based on comments from Gustavo
b) as per Gustavo's comment, Marcel's Ack has been added to get patch
This patchset consists of the following 6 patches that reworks the RFCOMM
session refcnt by adding handling for a NULL RFCOMM session pointer when the
session has been deleted followed by removing the now redundant refcnt
mechanism:
----------------------------------------------------------------
Dean Jenkins (6):
Bluetooth: Avoid rfcomm_session_timeout using freed session
Bluetooth: Check rfcomm session and DLC exists on socket close
Bluetooth: Return RFCOMM session ptrs to avoid freed session
Bluetooth: Remove RFCOMM session refcnt
Bluetooth: Remove redundant call to rfcomm_send_disc
Bluetooth: Remove redundant RFCOMM BT_CLOSED settings
include/net/bluetooth/rfcomm.h | 6 ----
net/bluetooth/rfcomm/core.c | 163 ++++++++++++++++++++++++++++++++++++++++++++++++++----------------------------------------------------------
2 files changed, 76 insertions(+), 93 deletions(-)
For completeness, the original v1 patchset information is retained below:
The motivation for the change was RFCOMM kernel crashes in a 2.6.34 based ARM
SMP kernel. It is noted that RFCOMM in kernel 3.7 still contains the RFCOMM
session refcnt so the patches have been forward ported. Crashes were due to
malfunctions of the RFCOMM session refcnt such as premature deletion of the
session and double deletion of the session. Some crashes were caused by
the interaction of 2 kernel threads under very high processor loading.
Note that the refcnt is incorrectly initialised, it should be set to 1 before
allowing the session structure to be used, the old code used a value of 0 and
this contributes to a weak implementation of the refcnt.
In kernels later than 2.6.34, the threading model for RFCOMM was modified to
use work queues and this reduced the probability of a RFCOMM crash. This
helps to explain why the reports of RFCOMM crashes went away. However, the
implementation of the RFCOMM session refcnt is weak and is in fact logically
incorrect by using the RFCOMM data channel "initiator" flag. The control
channel disconnection procedures do not care about how the control channel
was established eg. by host or remote device request.
The patches also make it clear when the RFCOMM session has been deleted by
using a NULL pointer. The old code has a possibility to access freed RFCOMM
session structures because there are multiple local copies of the pointer. The
patches ensure that local copies of the pointer are set to NULL when the
session has been deleted so avoiding any possibility of accessing freed memory.
Therefore, the patchset cleans up the disconnection of the RFCOMM session
control channel and now the code is deterministic and is understandable by
code review.
These patches were lightly tested against kernel 3.7.3 on a x86 PC. The design
of the patches were proven on a commercial project using a modified embedded ARM
SMP kernel 2.6.34.13. All previously observed RFCOMM control channel
disconnection kernel crashes on ARM 2.6.34.13 went away with the patches
applied.
Patch summaries are shown below:
0001: Bluetooth: Avoid rfcomm_session_timeout using freed session
Use del_timer_sync() instead of del_timer() as this ensures
that rfcomm_session_timeout() is not running on a different
CPU when rfcomm_session_put() is called. This avoids a race
condition on SMP systems because potentially
rfcomm_session_timeout() could reuse the freed RFCOMM session
structure caused by the execution of rfcomm_session_put().
Note that this modification makes the reason for the RFCOMM
session refcnt mechanism redundant.
0002: Bluetooth: Check rfcomm session and DLC exists on socket close
A race condition exists between near simultaneous asynchronous
DLC data channel disconnection requests from the host and remote device.
This causes the socket layer to request a socket shutdown at the same
time the rfcomm core is processing the disconnect request from the remote
device.
The socket layer retains a copy of a struct rfcomm_dlc d pointer.
The d pointer refers to a copy of a struct rfcomm_session.
When the socket layer thread performs a socket shutdown, the thread
may wait on a rfcomm lock in rfcomm_dlc_close(). This means that
whilst the thread waits, the rfcomm_session and/or rfcomm_dlc structures
pointed to by d maybe freed due to rfcomm core handling. Consequently,
when the rfcomm lock becomes available and the thread runs, a
malfunction could occur as a freed rfcomm_session structure and/or a
freed rfcomm_dlc structure will be erroneously accessed.
Therefore, after the rfcomm lock is acquired, check that the struct
rfcomm_session is still valid by searching the rfcomm session list.
If the session is valid then validate the d pointer by searching the
rfcomm session list of active DLCs for the rfcomm_dlc structure
pointed by d.
0003: Bluetooth: Return RFCOMM session ptrs to avoid freed session
Unfortunately, the design retains local copies of the s RFCOMM
session pointer in various code blocks and this invites the erroneous
access to a freed RFCOMM session structure.
Therefore, return the RFCOMM session pointer back up the call stack
to avoid accessing a freed RFCOMM session structure. When the RFCOMM
session is deleted, NULL is passed up the call stack.
If active DLCs exist when the rfcomm session is terminating,
avoid a memory leak of rfcomm_dlc structures by ensuring that
rfcomm_session_close() is used instead of rfcomm_session_del().
0004: Bluetooth: Remove RFCOMM session refcnt
Previous commits have improved the handling of the RFCOMM session
timer and the RFCOMM session pointers such that freed RFCOMM
session structures should no longer be erroneously accessed. The
RFCOMM session refcnt now has no purpose and will be deleted by
this commit.
Note that the RFCOMM session is now deleted as soon as the
RFCOMM control channel link is no longer required. This makes the
lifetime of the RFCOMM session deterministic and absolute.
Previously with the refcnt, there was uncertainty about when
the session structure would be deleted because the relative
refcnt prevented the session structure from being deleted at will.
It was noted that the refcnt could malfunction under very heavy
real-time processor loading in embedded SMP environments. This
could cause premature RFCOMM session deletion or double session
deletion that could result in kernel crashes. Removal of the
refcnt prevents this issue.
There are 4 connection / disconnection RFCOMM session scenarios:
host initiated control link ---> host disconnected control link
host initiated ctrl link ---> remote device disconnected ctrl link
remote device initiated ctrl link ---> host disconnected ctrl link
remote device initiated ctrl link ---> remote device disc'ed ctrl link
The control channel connection procedures are independent of the
disconnection procedures. Strangely, the RFCOMM session refcnt was
applying special treatment so erroneously combining connection and
disconnection events. This commit fixes this issue by removing
some session code that used the "initiator" member of the session
structure that was intended for use with the data channels.
0005: Bluetooth: Remove redundant call to rfcomm_send_disc
In rfcomm_session_del() remove the redundant call to
rfcomm_send_disc() because it is not possible for the
session to be in BT_CONNECTED state during deletion
of the session.
0006: Bluetooth: Remove redundant RFCOMM BT_CLOSED settings
rfcomm_session_close() sets the RFCOMM session state to BT_CLOSED.
However, in multiple places immediately before the function is
called, the RFCOMM session is set to BT_CLOSED. Therefore,
remove these unnecessary state settings.
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