Re: [PATCH] overlayfs: ignore empty NFSv4 ACLs in ext4 upperdir

[Amir Goldstein]

On Fri, May 3, 2019 at 1:31 PM J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
>
> On Fri, May 03, 2019 at 01:26:01PM -0400, Amir Goldstein wrote:
> > On Fri, May 3, 2019 at 12:03 PM J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
> > >
> > > On Thu, May 02, 2019 at 12:02:33PM +1000, NeilBrown wrote:
> > > > On Tue, Dec 06 2016, J. Bruce Fields wrote:
> > > >
> > > > > On Tue, Dec 06, 2016 at 02:18:31PM +0100, Andreas Gruenbacher wrote:
> > > > >> On Tue, Dec 6, 2016 at 11:08 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> > > > >> > On Tue, Dec 6, 2016 at 12:24 AM, Andreas GrÃnbacher
> > > > >> > <andreas.gruenbacher@xxxxxxxxx> wrote:
> > > > >> >> 2016-12-06 0:19 GMT+01:00 Andreas GrÃnbacher <andreas.gruenbacher@xxxxxxxxx>:
> > > > >> >
> > > > >> >>> It's not hard to come up with a heuristic that determines if a
> > > > >> >>> system.nfs4_acl value is equivalent to a file mode, and to ignore the
> > > > >> >>> attribute in that case. (The file mode is transmitted in its own
> > > > >> >>> attribute already, so actually converting .) That way, overlayfs could
> > > > >> >>> still fail copying up files that have an actual ACL. It's still an
> > > > >> >>> ugly hack ...
> > > > >> >>
> > > > >> >> Actually, that kind of heuristic would make sense in the NFS client
> > > > >> >> which could then hide the "system.nfs4_acl" attribute.
> > > > >> >
> > > > >> > Even simpler would be if knfsd didn't send the attribute if not
> > > > >> > necessary.  Looks like there's code actively creating the nfs4_acl on
> > > > >> > the wire even if the filesystem had none:
> > > > >> >
> > > > >> >     pacl = get_acl(inode, ACL_TYPE_ACCESS);
> > > > >> >     if (!pacl)
> > > > >> >         pacl = posix_acl_from_mode(inode->i_mode, GFP_KERNEL);
> > > > >> >
> > > > >> > What's the point?
> > > > >>
> > > > >> That's how the protocol is specified.
> > > > >
> > > > > Yep, even if we could make that change to nfsd it wouldn't help the
> > > > > client with the large number of other servers that are out there
> > > > > (including older knfsd's).
> > > > >
> > > > > --b.
> > > > >
> > > > >> (I'm not saying that that's very helpful.)
> > > > >>
> > > > >> Andreas
> > > >
> > > > Hi everyone.....
> > > >  I have a customer facing this problem, and so stumbled onto the email
> > > >  thread.
> > > >  Unfortunately it didn't resolve anything.  Maybe I can help kick things
> > > >  along???
> > > >
> > > >  The core problem here is that NFSv4 and ext4 use different and largely
> > > >  incompatible ACL implementations.  There is no way to accurately
> > > >  translate from one to the other in general (common specific examples
> > > >  can be converted).
> > > >
> > > >  This means that either:
> > > >    1/ overlayfs cannot use ext4 for upper and NFS for lower (or vice
> > > >       versa) or
> > > >    2/ overlayfs need to accept that sometimes it cannot copy ACLs, and
> > > >       that is OK.
> > > >
> > > >  Silently not copying the ACLs is probably not a good idea as it might
> > > >  result in inappropriate permissions being given away.  So if the
> > > >  sysadmin wants this (and some clearly do), they need a way to
> > > >  explicitly say "I accept the risk".
> > >
> > > So, I feel like silently copying ACLs up *also* carries a risk, if that
> > > means switching from server-enforcement to client-enforcement of those
> > > permissions.
> > >
> > > Sorry, I know we had another thread recently about permissions in this
> > > situation, and I've forgotten the conclusion.
> > >
> > > Out of curiosity, what's done with selinux labels?
> > >
> >
> > overlayfs calls security_inode_copy_up_xattr(name) which
> > can fail (<0) allow (0) or skip(1).
> >
> > selinux_inode_copy_up_xattr() as well as smack_inode_copy_up_xattr()
> > skip their own xattr on copy up and fail any other xattr copy up.
>
> If it's OK to silently skip copying up security labels, maybe it's OK to
> silently skip NFSv4 ACLs too?
>

I think overlayfs inode security context is taken from overlayfs
mount parameters (i.e. per container context) and therefore
the lower security. xattr are ignored (CC Vivek).

Thanks,
Amir.