[PATCH 1/1] simplified security.nscapability xattr

Serge E. Hallyn serge at hallyn.com
Tue May 3 14:25:26 UTC 2016


Quoting Eric W. Biederman (ebiederm at xmission.com):
> "Serge E. Hallyn" <serge at hallyn.com> writes:
> 
> > Quoting Andrew G. Morgan (morgan at 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


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