Re: [PATCH 1/6] statx: Add a system call to make enhanced file info available

[J. Bruce Fields]

On Thu, May 05, 2016 at 03:48:16PM -0400, Jeff Layton wrote:
> On Thu, 2016-05-05 at 10:09 +1000, NeilBrown wrote:
> > On Thu, May 05 2016, Dave Chinner wrote:
> > 
> > > 
> > > On Fri, Apr 29, 2016 at 01:57:43PM +0100, David Howells wrote:
> > > > 
> > > > Â(4) File creation time (st_btime*), data version (st_version), inode
> > > > ÂÂÂÂÂgeneration number (st_gen).
> > > > 
> > > > ÂÂÂÂÂThese will be returned if available whether the caller asked for them or
> > > > ÂÂÂÂÂnot.ÂÂThe corresponding bits in st_mask will be set or cleared as
> > > > ÂÂÂÂÂappropriate to indicate a valid value.
> > > IMO, exposing the inode generation number to anyone is a potential
> > > security problem because they are used in file handles.
> > "security through obscurity".ÂÂWe have Kerberos working really nicely
> > for NFS these days.ÂÂDo we still care?
> > 
> > What if the generation number were only made available to "root"?ÂÂWould
> > that allay your concerns?
> > Would that still be useful?
> > We already have name_to_handle_at().ÂÂExposing the generation number
> > could/should follow the same rules at that.ÂÂOr maybe the exposure of
> > each field should be guided by the filesystem, depending on (for
> > example) whether it is used to provide uniqueness to the filehandle.
> > 
> > > 
> > > 
> > > > 
> > > > ÂÂÂÂÂIf the caller didn't ask for them, then they may be approximated.ÂÂFor
> > > > ÂÂÂÂÂexample, NFS won't waste any time updating them from the server, unless
> > > > ÂÂÂÂÂas a byproduct of updating something requested.
> > > I would suggest that exposing them from the NFS server is something
> > > we most definitely don't want to do because they are the only thing
> > > that keeps remote users from guessing filehandles with ease....
> > Given that the NFS protocol does not define a "generation number"
> > attribute, I think there is no risk for them being exposed from the NFS
> > server ... except implicitly within the filehandle of course.
> > 
> > NeilBrown
> 
> 
> 
> I don't see a real attack vector here either, but OTOH is there a
> potential user of this at the moment? An earlier chunk of the patch
> description says:
> 
> (7) Inode generation number: Useful for FUSE and userspace NFS servers
> ÂÂÂÂÂ[Bernd Schubert].ÂÂThis was asked for but later deemed unnecessary
> ÂÂÂÂÂwith the open-by-handle capability available
> 
> ...the last bit seems to indicate that we don't really need this
> anyway, as most userland servers now work with filehandles from the
> kernel.
> 
> Maybe leave it out for now? It can always be added later.

Sounds like a good compromise to me!

That said, filehandles can never be changed, and generally have to be
exposed on the network, so I don't think it's worth going to great
lengths to try keep them secret.

--b.