linux-kernel - Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

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Date:	Wed, 30 Apr 2014 13:33:48 +0000
From:	Serge Hallyn <serge.hallyn@...ntu.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Theodore Ts'o <tytso@....edu>,
	Andy Lutomirski <luto@...capital.net>,
	Marian Marinov <mm@...com>,
	containers@...ts.linux-foundation.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	lxc-devel <lxc-devel@...ts.sourceforge.net>
Subject: Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> Theodore Ts'o <tytso@....edu> writes:
> 
> > On Wed, Apr 30, 2014 at 12:16:41AM +0000, Serge Hallyn wrote:
> >> I forget the details, but there was another case where I wanted to
> >> have the userns which 'owns' the whole fs available.  I guess we'd
> >> have to check against that instead of using inode_capable.
> >
> > Yes, that sounds right.
> >
> > And *please* tell me that that under no circumstances can anyone other
> > than root@...t_user_ns is allowed to use mknod....
> 
> Nope.  mknod not allowed.  capable(CAP_MKNOD) is required is required
> and I can't see any reason to change that.
> 
> As a rule of thumb, the only additional actions allowed in a user
> namespace above and beyond what an ordinary unpriviliged user would be
> allowed to do are those things which we only don't allow because they
> could confuse a setuid root executable.
> 
> 
> If we ever allow the creation of immutable files by unprivileged users
> those files would at least have to be kept completely separate from the
> files the global root encounters (aka a disjoint mount namespace).
> 
> I do not currently see a path to safely using immutable files with just
> user namespace root permission.

It's very far off, but I think the path is:

1. at first mount of a blockdev, note the cred (or just userns) which
mounted it
2. work on auditing superblock readers so we can start allowing some
blockdev mounts in user namespaces :)
3. check for privilege against the userns owning a superblock

-serge
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