linux-kernel - Re: unprivileged mounts git tree

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Message-ID: <20080904174851.GA13693@hallyn.com>
Date:	Thu, 4 Sep 2008 12:48:51 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	serue@...ibm.com, ebiederm@...ssion.com, akpm@...ux-foundation.org,
	hch@...radead.org, viro@...IV.linux.org.uk,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: unprivileged mounts git tree

Quoting Miklos Szeredi (miklos@...redi.hu):
> On Thu, 4 Sep 2008, Serge E. Hallyn wrote:
> > Quoting Miklos Szeredi (miklos@...redi.hu):
> > > On Thu, 04 Sep 2008, Miklos Szeredi wrote:
> > > > On Thu, 4 Sep 2008, Serge E. Hallyn wrote:
> > > > > Are you going to revert the change forcing CL_SLAVE for
> > > > > !capable(CAP_SYS_ADMIN)?  I don't think we want that - I think that
> > > > > *within* a set of user mounts, propagation should be safe, right?
> > > > > 
> > > > > Will you be able to do this soon?  If not, should we just do the part
> > > > > returning -EPERM when turning a shared mount into a user mount? 
> > > > 
> > > > OK, let's do that first and the tricky part (propagation vs. user
> > > > mounts) later.  Will push after I've tested it.
> > > 
> > > Here it is:
> > > 
> > > git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs.git unprivileged-mounts
> > 
> > but you're still doing
> > 
> > 	if (IS_MNT_SHARED(old_nd.path.mnt) && !capable(CAP_SYS_ADMIN))
> > 		goto out;
> > 
> > shouldn't it be something like
> > 
> > 	if (IS_MNT_SHARED(old_nd.path.mnt) && (old_nd.path.mnt & MNT_USER))
> > 		goto out;
> > 
> > ?
> 
> Why would that be an error?  There's no real security gain to be had
> from restricting a privileged user, but could cause a lot of
> annoyance.  If we think this is dangerous, then protection should be
> built into mount(8) with an option to override.  But not into the
> kernel, IMO.

We disagree on that.  But can we agree that the check you added is wrong?
There is no reason why a user mount should not be able to do shared
mounts, is there?  So should the check above just go away then?

-serge
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