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

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Date:   Fri, 3 May 2019 13:26:01 -0400
From:   Amir Goldstein <amir73il@...il.com>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     NeilBrown <neilb@...e.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Andreas Grünbacher <andreas.gruenbacher@...il.com>,
        Patrick Plagwitz <Patrick_Plagwitz@....de>,
        "linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>,
        Linux NFS list <linux-nfs@...r.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] overlayfs: ignore empty NFSv4 ACLs in ext4 upperdir

On Fri, May 3, 2019 at 12:03 PM J. Bruce Fields <bfields@...ldses.org> 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@...redi.hu> wrote:
> > >> > On Tue, Dec 6, 2016 at 12:24 AM, Andreas Grünbacher
> > >> > <andreas.gruenbacher@...il.com> wrote:
> > >> >> 2016-12-06 0:19 GMT+01:00 Andreas Grünbacher <andreas.gruenbacher@...il.com>:
> > >> >
> > >> >>> 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.

Thanks,
Amir.