linux-kernel - Re: [PATCH v2] unix: properly account for FDs passed over unix sockets

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Date:	Wed, 3 Feb 2016 12:56:22 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Simon McVittie <simon.mcvittie@...labora.co.uk>,
	David Herrmann <dh.herrmann@...il.com>,
	Willy Tarreau <w@....eu>
Cc:	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Eric Dumazet <edumazet@...gle.com>, socketpair@...il.com,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: [PATCH v2] unix: properly account for FDs passed over unix
 sockets

On 03.02.2016 12:36, Simon McVittie wrote:
> On 02/02/16 17:34, David Herrmann wrote:
>> Furthermore, with this patch in place, a process better not pass any
>> file-descriptors to an untrusted process.
> ...
>> Did anyone notify the dbus maintainers of this? They
>> might wanna document this, if not already done (CC: smcv).
>
> Sorry, I'm not clear from this message on what patterns I should be
> documenting as bad, and what the effect of non-compliance would be.
>
> dbus-daemon has a fd-passing feature, which uses AF_UNIX sockets'
> existing ability to pass fds to let users of D-Bus attach fds to their
> messages. The message is passed from the sending client to dbus-daemon,
> then from dbus-daemon to the recipient:
>
>               AF_UNIX             AF_UNIX
>                 |                    |
>      sender -------> dbus-daemon -------> recipient
>                 |                    |
>
> This has been API since before I was a D-Bus maintainer, so I have no
> influence over its existence; just like the kernel doesn't want to break
> user-space, dbus-daemon doesn't want to break its users.
>
> The system dbus-daemon (dbus-daemon --system) is a privilege boundary,
> and accepts senders and recipients with differing privileges. Without
> configuration, messages are denied by default. Recipients can open this
> up (by installing system-wide configuration) to allow arbitrary
> processes to send messages to them, so that they can carry out their own
> discretionary access control. Since 2014, the system dbus-daemon accepts
> up to 16 file descriptors per message by default.
>
> There is also a session or user dbus-daemon (dbus-daemon --session) per
> uid, but that isn't normally a privilege boundary, so any user trying to
> carry out a denial of service there is only hurting themselves.
>
> Am I right in saying that the advice I give to D-Bus users should be
> something like this?
>
> * system services should not send fds at all, unless they trust the
>    dbus-daemon
> * system services should not send fds via D-Bus that will be delivered
>    to recipients that they do not trust
> * sending fds to an untrusted recipient would enable that recipient to
>    carry out a denial-of-service attack (on what? the sender? the
>    dbus-daemon?)
>

The described behavior was simply a bug in the referenced patch. I 
already posted a follow-up to change this behavior so that only the 
current sending process is credited with the number of fds in flight:

<https://patchwork.ozlabs.org/patch/577653/>

Other processes (in this case the original opener of the file) isn't 
credited anymore if it does not send the fd itself.

That said, I don't think you need to change anything or give different 
advice because of this thread.

Thanks,
Hannes