Re: [PATCH 1/1] simplified security.nscapability xattr

Serge E. Hallyn
2016-05-03T14:25:32Z

Quoting Eric W. Biederman (ebiederm@xmission.com):
> "Serge E. Hallyn" <serge@hallyn.com> writes:
>
> > Quoting Andrew G. Morgan (morgan@kernel.org):
> >>
> >> I guess I'm confused how we have strayed so far that this isn't an obvious
> >> requirement. Uid=0 as being the root of privilege was the basic problem
> >> that capabilities were designed to change.
> >
> > The task executing the file can be any uid mapped into the namespace. The
> > file only has to be owned by the root of the user_ns. Which I agree is
> > unfortunate. We can work around it by putting the root uid into the xattr
> > itself (which still isn't orthogonal but allows the file to at least by
> > owned by non-root), but the problem then is that a task needs to know its
> > global root k_uid just to write the xattr.
>
> The root kuid is just make_kuids(user_ns, 0) so it is easy to find.
>
> It might be a hair better to use the userns->owner instead of the root
> uid. That would allow user namespaces without a mapped root to still
> use file capabilities.

That's all fine if the kernel does it for us magically. Which is what we're
talking about below. Above I was talking about userspace putting it into
the xattr.

> >> Uid is an acl concept. Capabilities are supposed to be independent of that.
> >>
> >> If we want to support NS file capabilities I would look at replacing the
> >> xattr syscall with a dedicated file capabilities modification syscall. Then
> >
> > That was one ofthe possibilities I'd mentioned in my earlier proposal,
> > fwiw. The problem is if we want tar to still work unmodified then
> > simple xattr operations still have to work.
> >
> > Maybe there's workable semantics there though. Worth thinking about.
>
> If the problem is compatibilty please look at
> posix_acl_fix_xattr_from_user. With something similar for the

All right. Excellent. I simply didn't think something like that would
be acceptable. I tend to think of xattrs as just out of band file contents,
but generally under user control. I guess that's not right.

> security.capability attribute we can perform whatever transformation
> makes sense. I admit adding 4 bytes is a bit of a pain in that context
> but not a big one.

If we can do all the magic in the kernel behind the scenes, then I
absolutely do not mind adding a new security.capability version with 4
more bytes. Userspace can just write the old xattr format with the new
version number, kernel fills in the userns owner kuid. It's what I
originally wanted to do, but didn't think was acceptable.

Sounds great!

-serge