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

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Message-ID: <CANq1E4Sz2U2D7hvWLEknbrKURG_ZbMiqAsVnd5yRF1Qesm3nEg@mail.gmail.com>
Date:	Wed, 3 Feb 2016 12:56:39 +0100
From:	David Herrmann <dh.herrmann@...il.com>
To:	Simon McVittie <simon.mcvittie@...labora.co.uk>
Cc:	Willy Tarreau <w@....eu>, "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>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	socketpair@...il.com,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: [PATCH v2] unix: properly account for FDs passed over unix sockets

Hi

On Wed, Feb 3, 2016 at 12:36 PM, Simon McVittie
<simon.mcvittie@...labora.co.uk> wrote:
> 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?)

With the revised patch from Hannes, this should no longer be needed.
My original concern was only about accounting inflight-fds on the
file-owner, rather than the sender.

However, with Hannes' revised patch, a different DoS attack against
dbus-daemon is possible. Imagine a peer that receives batches of FDs,
but never dequeues them. They will be accounted on the inflight-limit
of dbus-daemon, as such causing messages of independent peers to be
rejected in case they carry FDs.
Preferably, dbus-daemon would avoid queuing more than 16 FDs on a
single destination (total). But that would require POLLOUT to be
capped by the number of queued fds. A possible workaround is to add
CAP_SYS_RESOURCE to dbus-daemon.

Thanks
David