Re: [PATCH 0/4] exportfs: add flag to allow marking export operations as only supporting file handles

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2024-12-09 at 11:35 -0500, Chuck Lever wrote:
> On 12/9/24 11:30 AM, Amir Goldstein wrote:
> > On Mon, Dec 9, 2024 at 2:46 PM Christoph Hellwig <hch@infradead.org> wrote:
> > > 
> > > On Mon, Dec 09, 2024 at 09:58:58AM +0100, Amir Goldstein wrote:
> > > > To be clear, exporting pidfs or internal shmem via an anonymous fd is
> > > > probably not possible with existing userspace tools, but with all the new
> > > > mount_fd and magic link apis, I can never be sure what can be made possible
> > > > to achieve when the user holds an anonymous fd.
> > > > 
> > > > The thinking behind adding the EXPORT_OP_LOCAL_FILE_HANDLE flag
> > > > was that when kernfs/cgroups was added exportfs support with commit
> > > > aa8188253474 ("kernfs: add exportfs operations"), there was no intention
> > > > to export cgroupfs over nfs, only local to uses, but that was never enforced,
> > > > so we thought it would be good to add this restriction and backport it to
> > > > stable kernels.
> > > 
> > > Can you please explain what the problem with exporting these file
> > > systems over NFS is?  Yes, it's not going to be very useful.  But what
> > > is actually problematic about it?  Any why is it not problematic with
> > > a userland nfs server?  We really need to settle that argumet before
> > > deciding a flag name or polarity.
> > > 
> > 
> > I agree that it is not the end of the world and users do have to explicitly
> > use fsid= argument to be able to export cgroupfs via nfsd.
> > 
> > The idea for this patch started from the claim that Jeff wrote that cgroups
> > is not allowed for nfsd export, but I couldn't find where it is not allowed.
> > 

I think that must have been a wrong assumption on my part. I don't see
anything that specifically prevents that either. If cgroupfs is mounted
and you tell mountd to export it, I don't see what would prevent that.

To be clear, I don't see how you would trick bog-standard mountd into
exporting a filesystem that isn't mounted into its namespace, however.
Writing a replacement for mountd is always a possibilty.

> > I have no issue personally with leaving cgroupfs exportable via nfsd
> > and changing restricting only SB_NOUSER and SB_KERNMOUNT fs.
> > 
> > Jeff, Chuck, what is your opinion w.r.t exportability of cgroupfs via nfsd?
> 
> We all seem to be hard-pressed to find a usage scenario where exporting
> pseudo-filesystems via NFS is valuable. But maybe someone has done it
> and has a good reason for it.
> 
> The issue is whether such export should be consistently and actively
> prevented.
> 
> I'm not aware of any specific security issues with it.
> 
> 

I'm not either, but we are in new territory here. nfsd is a network
service, so it does present more of an attack surface vs. local access.

In general, you do have to take active steps to export a filesystem,
but if someone exports / with "crossmnt", everything mounted is
potentially accessible. That's obviously a dumb thing to do, but people
make mistakes, and it's possible that doing this could be part of a
wider exploit.

I tend to think it safest to make exporting via nfsd an opt-in thing on
a per-fs basis (along the lines of this patchset). If someone wants to
allow access to more "exotic" filesystems, let them argue their use-
case on the list first.
-- 
Jeff Layton <jlayton@kernel.org>