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

[Andreas Gruenbacher]

On Thu, 2 May 2019 at 05:57, NeilBrown <neilb@xxxxxxxx> wrote:
> On Wed, May 01 2019, Amir Goldstein wrote:
> > On Wed, May 1, 2019 at 10:03 PM NeilBrown <neilb@xxxxxxxx> 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.

I still think the nfs client could make this problem mostly go away by
not exposing "system.nfs4_acl" xattrs when the acl is equivalent to
the file mode. The richacl patches contain a workable abgorithm for
that. The problem would remain for files that have an actual NFS4 ACL,
which just cannot be mapped to a file mode or to POSIX ACLs in the
general case, as well as for files that have a POSIX ACL. Mapping NFS4
ACL that used to be a POSIX ACL back to POSIX ACLs could be achieved
in many cases as well, but the code would be quite messy. A better way
seems to be to using a filesystem that doesn't support POSIX ACLs in
the first place. Unfortunately, xfs doesn't allow turning off POSIX
ACLs, for example.

Andreas

> >> >> > 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.
> >
> > For example? permissions given away to do what?
> > Note that ovl_permission() only check permissions of *mounter*
> > to read the lower NFS file and ovl_open()/ovl_read_iter() access
> > the lower file with *mounter* credentials.
> >
> > I might be wrong, but seems to me that once admin mounted
> > overlayfs with lower NFS, NFS ACLs are not being enforced at all
> > even before copy up.
>
> I guess it is just as well that copy-up fails then - if the lower-level
> permission check is being ignored.
>
> >
> >> So if the
> >>  sysadmin wants this (and some clearly do), they need a way to
> >>  explicitly say "I accept the risk".  If only standard Unix permissions
> >>  are used, there is no risk, so this seems reasonable.
> >>
> >>  So I would like to propose a new option for overlayfs
> >>     nocopyupacl:   when overlayfs is copying a file (or directory etc)
> >>         from the lower filesystem to the upper filesystem, it does not
> >>         copy extended attributes with the "system." prefix.  These are
> >>         used for storing ACL information and this is sometimes not
> >>         compatible between different filesystem types (e.g. ext4 and
> >>         NFSv4).  Standard Unix ownership permission flags (rwx) *are*
> >>         copied so this option does not risk giving away inappropriate
> >>         permissions unless the lowerfs uses unusual ACLs.
> >>
> >>
> >
> > I am wondering if it would make more sense for nfs to register a
> > security_inode_copy_up_xattr() hook.
> > That is the mechanism that prevents copying up other security.*
> > xattrs?
>
> No, I don't think that would make sense.
> Support some day support for nfs4 acls were added to ext4 (not a totally
> ridiculous suggestion).  We would then want NFS to allow it's ACLs to be
> copied up.
>
> Thanks,
> NeilBrown
>
>
> >
> > Thanks,
> > Amir.