linux-kernel - Re: capabilities question(s)

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Date:	Thu, 10 Nov 2011 15:59:47 +0000
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Gábor Lénárt <lgb@....hu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: capabilities question(s)

Quoting Gábor Lénárt (lgb@....hu):
> Hi All,
> 
> I have the problem that I can't find too much informations about
> capabilities implemented by the Linux kernel, and its usage. 
> I even don't have header file sys/capability.h suggested by man
> page of capget (on ubuntu, if it does count). However it's not the major
> problem, as I've found example to use syscall() from sys/syscall.h and
> stuffs in linux/capability.h. Well, ignore it, I've just found out
> that I need libcap-dev to install. However the problem remains:
> 
> What I want to do: I need user to be able to do chroot() once, then I want
> to drop that capability (to be safe?).  My program (a server) runs as root,
> but after initialization it changes uid/gid (in a forked process).  However
> I want to keep chroot capability even after the uid/gid change so I can do a
> chroot() even as non-root.  It it possible at all? I need this so I feel

What you want is

	prctl(PR_SET_KEEPCAPS, 1);

That will let you keep the capabilities across the uid change.  Note that
this will get reset, and your privilege dropped, after a subsequent execve().
If that doesn't work for you, then you may want to look into running that
task with different securebits settings (using prctl(PR_SET_SECUREBITS, XYZ)).
See 'man 7 capabilities' for more information on that.

HTH,
-serge

> more secure not to run things as root, since I need to do many things before
> I can call chroot(). So I guessed, it's more secure to use non-root user
> to do these, before I can do chroot().
> 
> As far as I can see, I can set capability as root, but after uid/gid change,
> I lost that capability, and - of course - I cannot set it as a user then.
> 
> Or should I use root user anyway, just drop all of the capabilities I don't
> need just keep uid/gid change and chroot? Anyway, then I have the problem
> that I am still root for the filesystem permission checks, and it's
> something I don't want to do, because the things my server do before chroot
> is not so secure ...
> 
> Any ideas are welcome.
> 
> Best regards,
> 
> Gábor
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