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

[NeilBrown <neilb@suse.com>]

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On Thu, May 02 2019, Andreas Gruenbacher wrote:

> On Thu, 2 May 2019 at 05:57, NeilBrown <neilb@suse.com> wrote:
>> On Wed, May 01 2019, Amir Goldstein wrote:
>> > On Wed, May 1, 2019 at 10:03 PM NeilBrown <neilb@suse.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@szeredi.hu> wrote:
>> >> >> > On Tue, Dec 6, 2016 at 12:24 AM, Andreas Grünbacher
>> >> >> > <andreas.gruenbacher@gmail.com> wrote:
>> >> >> >> 2016-12-06 0:19 GMT+01:00 Andreas Grünbacher <andreas.gruenbacher@gmail.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.

Maybe ... but this feels a bit like "sweeping it under the carpet".
What happens if some file on the lower layer does have a more complex
ACL?
Do we just fail any attempt to modify that object?  Doesn't that violate
the law of least surprise?

Maybe if the lower-layer has an i_op->permission method, then overlayfs
should *always* call that for permission checking - unless a
chmod/chown/etc has happened on the file.  That way, we wouldn't need to
copy-up the ACL, but would still get correct ACL testing.

Thanks,
NeilBrown


> 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.

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