Re: nfsd vurlerability submit

["bfields@fieldses.org" <bfields@fieldses.org>]

On Thu, Jan 21, 2021 at 02:01:13PM -0600, Patrick Goetz wrote:
> I didn't respond to this message immediately, but it's been
> bothering me ever since. When I do a bind mount like this in
> /etc/fstab:
> 
>   /data2/xray      /srv/nfs/xray        none    defaults,bind    0
> 
> it's my understanding that the kernel keeps track of the resulting
> /srv/nfs/xray filesystem in it's vfs somehow.  Even when directly on
> the server I can't "break out" of /srv/nfs/xray to get to the other
> directories in /data.  Then how on earth would an NFS client do
> this?

As I said, NFS allows you to look up objects by filehandle (so,
basically by inode number), not just by path.

Also, note, mounting something over a directory doesn't hide what's
under the mountpoint.  And it's unwise to depend on directory
permissions alone to hide contents of anything underneath that
directory.

> I thought the whole point of doing a bind mount like this is to
> solve the problem of exporting leaves of a directory hierarchy. In
> particular,
> 
>   "So in your example, if /data2/xray is on the same filesystem as
>   /data2, then the server will happily allow operations on
>   filehandles anywhere in /data2."
> 
> Yes, sure; but I'm not exporting /data2/xray; I'm exporting
> /srv/nfs/xray, a bind mount to the preceding.  Am I missing
> something, or is NFS too insecure to use in any context requiring
> differentiated security settings on different folders in the same
> directory structure?

Definitely do *not* depend on NFS to enforce different export options on
different subdirectories of the same filesystem.

> It's not practical to making everything you export its own partition;
> although I suppose one could do this with ZFS datasets.

I'd be happy to hear about any use cases where that's not practical.

As Christophe pointed out, xfs/ext4 project ids are another option.

--b.