linux-kernel - Re: RFC: fsyscall

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Message-ID: <20150910134350.GC27602@mail.hallyn.com>
Date:	Thu, 10 Sep 2015 08:43:50 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	David Drysdale <drysdale@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: RFC: fsyscall

On Tue, Sep 08, 2015 at 07:25:17PM -0500, Eric W. Biederman wrote:
> Andy Lutomirski <luto@...capital.net> writes:
> 
> > On Tue, Sep 8, 2015 at 4:07 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> 
> >> Perhaps I had missed it but I don't recall capsicum being able to wrap
> >> things like reboot(2).
> >>
> >
> > Ah, so you want to be able to grant BPF-defined capabilities :)
> 
> Pretty much.
> 
> Where I am focusing is turning Posix capabilities into real
> capabilities.  I would not mind if the functionality was a bit more
> general.  Say to be able to handle things like security labels, or
> anywhere else you might reasonably be asked can you do X?
> 
> But I would be happy if we just managed to wrap the Posix capabilities
> and turned them into real capablilities.

If there were a clever way to exec an open fd, then you could do this
by passing an fd to a copy of /bin/reboot which has fP=CAP_SYS_BOOT,
or prefereably fI=CAP_SYS_BOOT,fE=1 and leave pI=CAP_SYS_BOOT in the
task.

A cleaner way to do this is to have a service which can reboot, which
looks at unix socket peercreds to determine whether the granter may
reboot, then passes it an fd which the granter may pass to a grantee.
Then the grantee passes the fd to the service, which recognizes it and
reboots.

-serge
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