From: bfields@fieldses.org (J. Bruce Fields) Subject: Re: [PATCH 1/6] statx: Add a system call to make enhanced file info available Date: Fri, 6 May 2016 14:07:42 -0400 Message-ID: <20160506180742.GA13350@fieldses.org> References: <20160429125736.23636.47874.stgit@warthog.procyon.org.uk> <20160429125743.23636.85219.stgit@warthog.procyon.org.uk> <20160504225601.GZ26977@dastard> <87shxxbc1e.fsf@notabene.neil.brown.name> <1462477696.12332.17.camel@poochiereds.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: NeilBrown , Dave Chinner , David Howells , linux-fsdevel@vger.kernel.org, linux-afs@vger.kernel.org, linux-nfs@vger.kernel.org, samba-technical@lists.samba.org, linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org To: Jeff Layton Return-path: Content-Disposition: inline In-Reply-To: <1462477696.12332.17.camel@poochiereds.net> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 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: > >=20 > > >=20 > > > On Fri, Apr 29, 2016 at 01:57:43PM +0100, David Howells wrote: > > > >=20 > > > > =C2=A0(4) File creation time (st_btime*), data version (st_vers= ion), inode > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0generation number (st_gen). > > > >=20 > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0These will be returned if availab= le whether the caller asked for them or > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0not.=C2=A0=C2=A0The corresponding= bits in st_mask will be set or cleared as > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0appropriate to indicate a valid v= alue. > > > IMO, exposing the inode generation number to anyone is a potentia= l > > > security problem because they are used in file handles. > > "security through obscurity".=C2=A0=C2=A0We have Kerberos working r= eally nicely > > for NFS these days.=C2=A0=C2=A0Do we still care? > >=20 > > What if the generation number were only made available to "root"?=C2= =A0=C2=A0Would > > that allay your concerns? > > Would that still be useful? > > We already have name_to_handle_at().=C2=A0=C2=A0Exposing the genera= tion number > > could/should follow the same rules at that.=C2=A0=C2=A0Or 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= =2E > >=20 > > >=20 > > >=20 > > > >=20 > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0If the caller didn't ask for them= , then they may be approximated.=C2=A0=C2=A0For > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0example, NFS won't waste any time= updating them from the server, unless > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0as a byproduct of updating someth= ing requested. > > > I would suggest that exposing them from the NFS server is somethi= ng > > > we most definitely don't want to do because they are the only thi= ng > > > 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. > >=20 > > NeilBrown >=20 >=20 >=20 > 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: >=20 > (7) Inode generation number: Useful for FUSE and userspace NFS server= s > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0[Bernd Schubert].=C2=A0=C2=A0This was a= sked for but later deemed unnecessary > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0with the open-by-handle capability avai= lable >=20 > ...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. >=20 > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel= " in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html