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

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Date:   Thu, 02 May 2019 13:57:03 +1000
From:   NeilBrown <neilb@...e.com>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Andreas Grünbacher 
        <andreas.gruenbacher@...il.com>,
        Patrick Plagwitz <Patrick_Plagwitz@....de>,
        "linux-unionfs\@vger.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 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.
>> >> >
>> >> > 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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