ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

[Serge Hallyn]

Quoting Eric W. Biederman (ebiederm at xmission.com):
> Theodore Ts'o <tytso at mit.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 at init_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