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

[Amir Goldstein]

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.
> >> >
> >> > 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.

> 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?

Thanks,
Amir.