linux-kernel - Re: RFC: fsyscall

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Message-ID: <20150910140120.GA28071@mail.hallyn.com>
Date:	Thu, 10 Sep 2015 09:01:20 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	David Drysdale <drysdale@...gle.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: RFC: fsyscall

On Thu, Sep 10, 2015 at 02:51:28PM +0100, David Drysdale wrote:
> On Thu, Sep 10, 2015 at 2:43 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> > 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
> 
> execveat(fd, "", argv, envp, AT_EMPTY_PATH) ?

???  I looked for it but I don't have a manpage for it.  I see it at
man7.org though.  Thanks :)

> > 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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