linux-kernel - Re: [PATCH 03/08] allow sethostname in a container

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Date:	Fri, 4 Feb 2011 15:56:15 +0000
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	LSM <linux-security-module@...r.kernel.org>,
	James Morris <jmorris@...ei.org>,
	Kees Cook <kees.cook@...onical.com>,
	containers@...ts.linux-foundation.org,
	kernel list <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Bastian Blank <bastian@...di.eu.org>
Subject: Re: [PATCH 03/08] allow sethostname in a container

Quoting Serge E. Hallyn (serge@...lyn.com):
> Quoting Serge E. Hallyn (serge@...lyn.com):
> > Signed-off-by: Serge E. Hallyn <serge.hallyn@...onical.com>
> > ---
> >  kernel/sys.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index 2745dcd..9b9b03b 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -1171,7 +1171,7 @@ SYSCALL_DEFINE2(sethostname, char __user *, name, int, len)
> >  	int errno;
> >  	char tmp[__NEW_UTS_LEN];
> >  
> > -	if (!capable(CAP_SYS_ADMIN))
> > +	if (!ns_capable(current->nsproxy->uts_ns->user_ns, CAP_SYS_ADMIN))
> >  		return -EPERM;
> >  	if (len < 0 || len > __NEW_UTS_LEN)
> >  		return -EINVAL;
> > -- 
> > 1.7.0.4
> 
> An interesting note here is that since the task doing ns_exec (and
> therefore in the init_user_ns) requires CAP_SYS_ADMIN to unshare,
> this check will actually always be true if uts_ns was not unshared.

Noone ever called me on this, so for the sake of posterity reading the
m-l archives:  what I said above is not true.  If uts_ns was not
unshared, then current->nsproxy->uts_ns->user_ns != current_user_ns(),
so current should not have ns_capable(current->nsproxy->uts_ns->user_ns,
CAP_SYS_ADMIN).  So the check will always return false.

> If uts is unshared, then regular capabilities semantics in the
> child user_ns apply (that is, root can do sethostname, unpriv user
> cannot)  The intent is that user namespaces will eventually allow
> unprivileged users to unshare, after which this will make much more
> sense.
> 
> -serge
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