Re: Better interop for NFS/SMB file share mode/reservation

Amir Goldstein
2019-04-30T00:31:51Z

...
> > > No. I'm saying that whether you intended to or not, you _are_
> > > implementing a mandatory lock over NFS. No talk about O_SHARE flags and
> > > it being an opt-in process for local applications changes the fact that
> > > non-local applications (i.e. the ones that count ) are being subjected
> > > to a mandatory lock with all the potential for denial of service that
> > > implies.
> > > So we need a mechanism beyond O_SHARE in order to ensure this system
> > > cannot be used on sensitive files that need to be accessible to all. It
> > > could be an export option, or a mount option, or it could be a more
> > > specific mechanism (e.g. the setgid with no execute mode bit as using
> > > in POSIX mandatory locks).
> > >
> >
> > That's a great point.
> >
> > I was focused on the local fs piece in order to support NFS/SMB serving,
> > but we also have to consider that people using nfs or cifs filesystems
> > would want to use this interface to have their clients set deny bits as
> > well.
> >
> > So, I think you're right that we can't really do this without involving
> > non-cooperating processes in some way.
>
> It's been 5+ years since I touched that code but I still like the idea of having a separate mount option for mountpoints used by Samba and NFS servers and clients to avoid security attacks on the sensitive files. For some sensitive files on such mountpoints a more selective mechanism may be used to prevent deny flags to be set (like mentioned above). Or we may think about adding another flag e.g. O_DENYFORCE available to root only that tells the kernel to not take into account deny flags already set on a file - might be useful for recovery tools.
>
> About O_DENYDELETE: I don't understand how we may reach a good interop story without a proper implementation of this flag. Windows apps may set it and Samba needs to respect it. If an NFS client removes such an opened file, what will Samba tell the Windows client?
>

Samba will tell the Windows client:
"Sorry, my administrator has decided to trade off interop with nfs on
share modes,
with DENY_DELETE functionality, so I cannot grant you DENY_DELETE that you
requested."
Not sure if that is workable. Samba developers need to chime in.

Thanks,
Amir.