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

From: Patrick McHardy <[email-masked]>
Date: Wed, 18 Apr 2007 10:50:42 +0200

Evgeniy Polyakov wrote:
> On Wed, Apr 18, 2007 at 10:26:31AM +0200, Patrick McHardy ([email-masked]) wrote:
> 
>>>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.
> 
> 
> I thought that with releasing a socket, which will have a callback
> attached only results in a leak of the callback? In that case we can
> just free it in dump() just like it is done in no-error path already.
> Or do I miss something additional?

That would only work if there is nothing to dump (cb->dump returns 0).
Otherwise it is not freed.

>>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.
> 
> 
> Doesn't it called from ->sk_data_ready() which is synchronous with
> respect to sendmsg, not sure about conntrack though, but it looks so?

Yes, but for kernel sockets we end up calling the input function,
which when mutex_trylock is used returns immediately when some
other process is already processing the queue, so the requesting
process might close the socket before the request is processed.