Re: [NETLINK] Don't attach callback to a going-away netlink socket

[Patrick McHardy <kaber@trash.net>]

Evgeniy Polyakov wrote:
> On Wed, Apr 18, 2007 at 12:16:18PM +0400, Pavel Emelianov (xemul@sw.ru) wrote:
> 
>>Sorry, I forgot to put netdev and David in Cc when I first sent it.
>>
>>There is a race between netlink_dump_start() and netlink_release()
>>that can lead to the situation when a netlink socket with non-zero
>>callback is freed.
> 
> 
> Out of curiosity, why not to fix a netlink_dump_start() to remove
> callback in error path, since in 'no-error' path it removes it in
> netlink_dump().


It already does (netlink_destroy_callback), but that doesn't help
with this race though since without this patch we don't enter the
error path.

> And, btw, can release method be called while socket is being used, I
> thought about proper reference counters should prevent this, but not
> 100% sure with RCU dereferencing of the descriptor.


The problem is asynchronous processing of the dump request in the
context of a different process. Process requests a dump, message
is queued and process returns from sendmsg since some other process
is already processing the queue. Then the process closes the socket,
resulting in netlink_release being called. When the dump request
is finally processed the race Pavel described might happen. This
can only happen for netlink families that use mutex_try_lock for
queue processing of course.