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Date:	Wed, 3 Feb 2016 12:56:39 +0100
From:	David Herrmann <>
To:	Simon McVittie <>
Cc:	Willy Tarreau <>, "David S. Miller" <>,
	netdev <>,
	linux-kernel <>,
	Linus Torvalds <>,
	Eric Dumazet <>,
	Hannes Frederic Sowa <>,,
	Tetsuo Handa <>
Subject: Re: [PATCH v2] unix: properly account for FDs passed over unix sockets


On Wed, Feb 3, 2016 at 12:36 PM, Simon McVittie
<> 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.


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