linux-kernel - Re: [systemd-devel] [PATCH v5 12/14] autoconf: xen: enable explicit preference option for xenstored preference

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Date:	Wed, 4 Jun 2014 19:52:56 -0700
From:	Cameron Norman <camerontnorman@...il.com>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	Lennart Poettering <mzxreary@...inter.de>, luto@....edu,
	Keir Fraser <keir@....org>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Tim Deegan <tim@....org>,
	Ian Jackson <ian.jackson@...citrix.com>,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, ebiederm@...ssion.com,
	Jan Beulich <jbeulich@...e.com>,
	xen-devel@...ts.xenproject.org,
	systemd-devel@...ts.freedesktop.org, morgan@...nel.org
Subject: Re: [systemd-devel] [PATCH v5 12/14] autoconf: xen: enable explicit
 preference option for xenstored preference

On Wed, Jun 4, 2014 at 5:31 PM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
> On Sun, Jun 01, 2014 at 08:15:47AM +0200, Lennart Poettering wrote:
>> On Fri, 30.05.14 01:29, Luis R. Rodriguez (mcgrof@...e.com) wrote:
>>
>> > I'm cc'ing a few security folks as I'd appreciate review on the ideas here,
>> > in particular that of a launcher idea on system to replace alternatives on the
>> > ExecStart= line of a systemd service unit file, alternative ideas are of
>> > course welcomed. I'm also Cc'ing systemd-devel as this subject was reviewed
>> > a little while ago with nothing concrete being recommended but instead a few
>> > options being now archived as possibilities. I'm looking for a bit wider
>> > review of the approaches and recomendations.
>> >
>> > Some general background for non xen folks: old xen requires the launch of
>> > a daemon which implements supports of the xenstore, which is the database
>> > that xen uses for information about guests / dom0. There are two supported
>> > daemons, xenstored (C version) and oxenstored (Ocaml version) but they do the
>> > same thing. Right now old init lets you override which one you pick through
>> > an environment variable on /etc/{sysconfig,default}/xencommons, the script
>> > will use the appropriate on there. Systemd doesn't let you use variables on
>> > the ExecStart line of a service unit file so alternatives are required.
>> >
>> > The reason I'm being very careful here this could set a precedent and at
>> > least for the launcher idea it'd require the usage of getenv() and execve(),
>> > and secure alternatives for these (secure_getenv(), execve_nosecurity())
>> > have either been merged or suggested before for Linux. The systemd discussion
>> > is only specific to Linux but if we have a launcher we could consider it for
>> > other supported OSes. All that said I'd like proper review of the security
>> > implications of *all* strategies but obviously in particular the launcher
>> > idea. I want to tread carefuly before setting precedents.
>>
>> You can also just invoke a shell script from ExecStart=. I mean, we try
>> to deemphesize them in the boot process, but there's nothing wrong with
>> using shell, if you need to parse shell configuraiton fragments and just
>> want to execute on ot another program...
>
> I tried this and it didn't work given that systemd expects sd_notify()
> to be called from the parent process, in this case the shell script.

Just use exec before the daemon command. I am pretty certain it can be
just like this:

    ExecStart=/bin/sh -c "exec $XENSTORED"

xenstored then has the same PID as the sh process, and $NOTIFY_SOCKET
works just fine.

Best,
--
Cameron
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