linux-kernel - Re: CAP_SYS_ADMIN on restart(2)

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Message-ID: <20090415211609.GA6704@us.ibm.com>
Date:	Wed, 15 Apr 2009 16:16:09 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Oren Laadan <orenl@...columbia.edu>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>, xemul@...allels.com,
	containers@...ts.linux-foundation.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: CAP_SYS_ADMIN on restart(2)

Quoting Oren Laadan (orenl@...columbia.edu):
> 
> 
> Serge E. Hallyn wrote:
> > Quoting Dave Hansen (dave@...ux.vnet.ibm.com):
> >> On Wed, 2009-04-15 at 23:21 +0400, Alexey Dobriyan wrote:
> >>> Is sysctl to control CAP_SYS_ADMIN on restart(2) OK?
> >> If the point is not to let users even *try* restarting things if it
> >> *might* not work, then I guess this might be reasonable.  
> >>
> >> If the goal is to increase security by always requiring CAP_SYS_ADMIN
> >> for "dangerous" operations, I fear it will be harmful.  We may have
> >> people adding features that are not considering the security impact of
> >> what they're doing just because the cases they care about all require
> >> privilege.
> > 
> > Nah, I disagree.  (Or put another way, that wouldn't be the goal)
> > There are two administrators we want to satisfy:
> > 
> > 1. the one who wants his users to do partial checkpoints, but doesn't
> > want to risk giving away any privilege at all in the process.  He'll
> > be satisified by setting restart(2) to not require cap_sys_admin,
> > and his users just won't be able to do a whole container.  A lot of
> > users will be happy with that (though no SYSVIPC support, then).
> 
> There is also a middle way: use setuid program to allow creation
> of a new namespace (under your favorite policy), then drop the
> privileges and continue as unprivileged inside that container.
> 
> IOW, don't make the initial container-creation a barrier for the
> entire operation.

That is still possible here.  But I don't think it's relevant.

What Alexey wants, I believe, is for users to be able to not have
to worry about there being exploitable bugs in restart(2) which
unprivileged users can play with.  And for the usual distro-kernel
reasons, saying use 'CONFIG_CHECKPOINT=n' is not an option.

-serge
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