Date:   Thu, 2 May 2019 17:08:14 +0200
From:   Andreas Grünbacher <andreas.gruenbacher@...il.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        NeilBrown <neilb@...e.com>, Amir Goldstein <amir73il@...il.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        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

Am Do., 2. Mai 2019 um 16:28 Uhr schrieb Miklos Szeredi <miklos@...redi.hu>:
> On Thu, May 2, 2019 at 10:05 AM Andreas Gruenbacher <agruenba@...hat.com> wrote:
> > On Thu, 2 May 2019 at 05:57, NeilBrown <neilb@...e.com> wrote:
> > > On Wed, May 01 2019, Amir Goldstein wrote:
> > > > On Wed, May 1, 2019 at 10:03 PM NeilBrown <neilb@...e.com> 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.
> >
> > 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.
>
> How about mounting NFSv4 with noacl?  That should fix this issue, right?

You'll still see permissions that differ from what the filesystem
enforces, and copy-up would change that behavior.

Andreas

> Thanks,
> Miklos
>
>
>
> >
> > 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.

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