Re: [PATCH 0/10] af_unix: add multicast and filtering features to AF_UNIX

[Javier Martinez Canillas <javier.martinez@collabora.co.uk>]

On 02/28/2012 04:24 PM, Javier Martinez Canillas wrote:
> On 02/28/2012 03:28 PM, David Lamparter wrote:
>> On Tue, Feb 28, 2012 at 11:47:39AM +0100, Rodrigo Moya wrote:
>>> > - slow readers: dropping packets vs blocking the sender. Although
>>> >   datagrams are not reliable on IP, datagrams on Unix sockets are
>>> never
>>> >   lost. So if one receiver has its buffer full the sender is blocked
>>> > instead of dropping packets. That way we guarantee a reliable
>>> > communication channel.
>> 
>> This sounds like a terribly nice way to f*ck the entire D-Bus system by
>> having one broken (or malicious) desktop application. What's the
>> intended way of coping with users that block the socket by not reading?
>> 
>> 
>> -David L.
> 
> The problem is that D-bus expects a reliable transport method (TCP or
> SOCK_STREAM Unix socks) but this is not the case with multicast Unix
> sockets. Since our implementation is for SOCK_SEQPACKET and SOCK_DGRAM
> socket types.
> 
> So, you have to either add another layer to the D-bus protocol to make
> it reliable (acks, retransmissions, flow control, etc) or avoid losing
> D-bus messages (by blocking the sender if one of the receivers has its
> buffer full).
> 

Also, this problem exists with current D-bus implementation. If a
malicious desktop application doesn't read its socket then the messages
sent to it will be buffered in the daemon:
https://bugs.freedesktop.org/show_bug.cgi?id=33606

dbus-daemon memory usage will ballooning until
max_incoming_bytes/max_outgoing_bytes limit is reached (1GB for session
bus in default configuration)

<limit name="max_incoming_bytes">1000000000</limit>
<limit name="max_outgoing_bytes">1000000000</limit>

It only works because not many applications are broken and user-space
memory is virtualized. But if you bypass the daemon and use a multicast
transport layer (as in our multicast Unix socket implementation) you
don't have that much memory to buffer the packets.

So you have to either block the senders or:

- drop the slow reader
- kill the spammer
- have an infinite amount of memory

Regards,
Javier