Re: [PATCH] Adding the nfs4_secure_mounts bool

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, Nov 12, 2013 at 05:29:46AM +0000, Myklebust, Trond wrote:
> 
> On Nov 12, 2013, at 0:11, NeilBrown <neilb@suse.de> wrote:
> 
> > On Mon, 11 Nov 2013 15:33:14 -0500 Chuck Lever <chuck.lever@oracle.com> wrote:
> > 
> >> 
> >> On Nov 11, 2013, at 1:59 PM, Steve Dickson <SteveD@redhat.com> wrote:
> >> 
> >>> On 11/11/13 13:30, Chuck Lever wrote:
> >>>> 
> >>>> On Nov 11, 2013, at 1:06 PM, Steve Dickson <SteveD@redhat.com> wrote:
> >>>> 
> >>>>> 
> >>>>> 
> >>>>> On 09/11/13 18:12, Myklebust, Trond wrote:
> >>>>>> One alternative to the above scheme, which I believe that I’ve 
> >>>>>> suggested before, is to have a permanent entry in rpc_pipefs 
> >>>>>> that rpc.gssd can open and that the kernel can use to detect 
> >>>>>> that it is running. If we make it /var/lib/nfs/rpc_pipefs/gssd/clnt00/gssd, 
> >>>>>> then AFAICS we don’t need to change nfs-utils at all, since all newer 
> >>>>>> versions of rpc.gssd will try to open for read anything of the form 
> >>>>>> /var/lib/nfs/rpc_pipefs/*/clntXX/gssd...
> >>>>> 
> >>>>> After further review I am going going have to disagree with you on this.
> >>>>> Since all the context is cached on the initial mount the kernel
> >>>>> should be using the call_usermodehelper() to call up to rpc.gssd 
> >>>>> to get the context, which means we could put this upcall noise 
> >>>>> to bed... forever! :-)
> >>>> 
> >>>> Ask Al Viro for his comments on whether the kernel should start 
> >>>> gssd (either a daemon or a script).  Hint: wear your kevlar underpants.
> >>> I was thinking gssd would become a the gssd-cmd command... Al does not
> >>> like the call_usermodehelper() interface?
> >> 
> >> He doesn't have a problem with call_usermodehelper() in general.  However, the kernel cannot guarantee security if it has to run a fixed command line.  Go ask him to explain.
> >> 
> >> 
> >>> 
> >>>> 
> >>>> Have you tried Trond's approach yet?
> >>> Looking into it... But nothing is trivial in that code... 
> >>> 
> >>>> 
> >>>>> I realize this is not going happen overnight, so I would still
> >>>>> like to propose my  nfs4_secure_mounts bool patch as bridge
> >>>>> to the new call_usermodehelper()  since its the cleanest 
> >>>>> solution so far... 
> >>>>> 
> >>>>> Thoughts?
> >>>> 
> >>>> We have workarounds already that work on every kernel since 3.8.
> >>>> 
> >>> The one that logs 5 to 20 lines (depending on thins are setup or not)
> >>> per mount? That does work in some environments but no all. ;-)
> >> 
> >> When does running rpc.gssd not work?
> > 
> > Oohh ooh.. Pick me.  Pick me!! I can answer that one.
> > 
> > Running rpc.gssd does not work if you are mounting a filesystem using the IP
> > address of the server and that IP address doesn't have a matching hostname
> > anywhere that can be found:
> > 
> > In a newly creating minimal kvm install without rpc.gssd running,
> >   mount 10.0.2.2:/home /mnt
> > 
> > sleeps for 15 seconds then succeeds.
> > If I start rpc.gssd, then the same command takes forever.
> > 
> > strace of rpc.gssd shows that it complains about not being able to resolve
> > the host name and "ERROR: failed to read service info".  Then it keeps the
> > pipes open but never sends any message on them, so the kernel just keeps on
> > waiting.
> > 
> > If I change "fail_keep_client" to "fail_destroy_client", then it closes the
> > pipe and we get the 15 second timeout back.
> > If I change  NI_NAMEREQD to 0, then the mount completes instantly.  (of course
> > that make serious compromise security so it was just for testing).
> > (Adding an entry to /etc/hosts also gives instant success).
> > 
> > I'm hoping that someone who understands this code will suggest something
> > clever so I don't have to dig through all of it ;-)
> 
> rpc.gssd is supposed to do a downcall with a zero-length window and an error message in any situation where it cannot establish a GSS context. Normally, I’d expect an EACCES for the above scenario.
> 
> IOW: that’s a blatant rpc.gssd bug. One that will also affect you when you're doing NFSv3 and add ‘sec=krb5’ to the mount options.

Also why is gssd trying to do a DNS lookup in this case?  This sounds
similar to what f9f5450f8f94 "Avoid DNS reverse resolution for server
names (take 3)" was trying to fix?

--b.