Re: [PATCH v4] Introduce v3 namespaced file capabilities

[Serge E. Hallyn]

Quoting Stefan Berger (stefanb@xxxxxxxxxxxxxxxxxx):
> On 05/08/2017 02:11 PM, Serge E. Hallyn wrote:
> >Root in a non-initial user ns cannot be trusted to write a traditional
> >security.capability xattr.  If it were allowed to do so, then any
> >unprivileged user on the host could map his own uid to root in a private
> >namespace, write the xattr, and execute the file with privilege on the
> >host.
> >
> >However supporting file capabilities in a user namespace is very
> >desirable.  Not doing so means that any programs designed to run with
> >limited privilege must continue to support other methods of gaining and
> >dropping privilege.  For instance a program installer must detect
> >whether file capabilities can be assigned, and assign them if so but set
> >setuid-root otherwise.  The program in turn must know how to drop
> >partial capabilities, and do so only if setuid-root.
> 
> Hi Serge,
> 
> 
>   I have been looking at patch below primarily to learn how we could
> apply a similar technique to security.ima and security.evm for a
> namespaced IMA. From the paragraphs above I thought that you solved
> the problem of a shared filesystem where one now can write different
> security.capability xattrs by effectively supporting for example
> security.capability[uid=1000] and security.capability[uid=2000]
> written into the filesystem. Each would then become visible as
> security.capability if the userns mapping is set appropriately.
> However, this doesn't seem to be how it is implemented. There seems
> to be only a single such entry with uid appended to it and, if it
> was a shared filesystem, the first one to set this attribute blocks
> everyone else from writing the xattr. Is that how it works? Would

Yes, that's how this works here.  I'd considered allowing multiple
entries, but I didn't feel that was needed for this case.  In a previous
implementation (which is probably in the lkml archives somewhere) I
supported variable length xattr so that multiple containers could
each write a value tagged with their own userns.rootid.  Instead,
in the final version, if root in any parent container writes an
xattr, it will take effect in child user namespaces.  Which is
sensible - the parent presumbly laid out the filesystem to create
the child container.

> that work differently with an overlay filesystem ? I think a similar

Certainly an overlay filesystem should be an easy case as the container
can have its own copy of the inode with its own xattr.  Btrfs/zfs
would be nicer as the whole file wouldn't need to be copied.

> model could also work for IMA, but maybe you have some thoughts. The
> only thing I would be concerned about is blocking the parent
> container's root user from setting an xattr.

So if you have container c1 creating child container c2 on host h1,
then if c1 creates an xattr, can c2 not use that?  And if h1 writes it,
can c1 and c2 use it?

If they can't, then I guess for IMA multiple xattrs would need to be
supported.

-serge