linux-kernel - Re: RFC: fsyscall

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Message-ID: <CAHse=S_tRFKEwQ=DR9N5r9nOv_uDkYJydoxRx_a2a+PgOwCWag@mail.gmail.com>
Date:	Thu, 10 Sep 2015 14:51:28 +0100
From:	David Drysdale <drysdale@...gle.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	"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 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) ?

> 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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