Re: [PATCH v4] Introduce v3 namespaced file capabilities

[Stefan Berger]

On 06/13/2017 01:18 PM, Serge E. Hallyn wrote:
> 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?

In the case of IMA appraisal the extended attribute security.ima would 
be a signature. For c1 and c2 to use that file they would all have to 
have the same key on their (isolated IMA namespace ) keyring. I think 
this type of setup could be arranged.
Following your attack description in the introduction I would say that 
we would want to prevent malicious modification of a security.ima 
extended attribute:

"Root in a non-initial user ns cannot be trusted to write a traditional security.ima 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 signature in the security.ima xattr, and prevent the file from being accessible on the host."



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

I am not sure about that. I suppose any extended attribute modifications 
would have to be designed for the case where a shared filesystem is used 
that also shares the extended attributes, not assuming an overlay 
filesystem that automatically isolates the extend attributes. With the 
shared filesystem I'd like to prevent any type of setting of extended 
attributes by a child container or more generally anyone mounting it as 
a '2nd consumer', which would make it a shared filesystem. Only the 
process that mounts a filesystem as the '1st consumer' would be able to 
set the extended attributes. I am assuming that using an overlay fs 
would always make you the '1st consumer' -- I would hope that these 
conditions could be detected. And probably the process should also write 
along its host uid as part of writing out the xattr. If all extended 
attributes were to support this model, maybe the 'uid' could be 
associated with the 'name' of the xattr rather than its 'value' (not 
sure whether that's possible).

   Stefan

> -serge