On Tue, Apr 08, 2014 at 10:41:45AM -0400, Jeff Layton wrote: > On Tue, 8 Apr 2014 10:22:00 -0400 > Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > On Tue, 8 Apr 2014 10:03:33 -0400 > > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote: > > > > > On Tue, Apr 08, 2014 at 09:49:03AM -0400, Jeff Layton wrote: > > > > On Tue, 8 Apr 2014 08:35:01 -0400 > > > > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote: > > > > > > > > > On Tue, Apr 08, 2014 at 08:21:40AM -0400, Jeff Layton wrote: > > > > > > I've recently been hunting down some problems with delegation handling > > > > > > and have run across a problem with the client authenticates CB_COMPOUND > > > > > > requests. I could use some advice on how best to fix it. > > > > > > > > > > > > Specifically, check_gss_callback_principal() tries to look up the > > > > > > callback client and then tries to compare the ticket in it against the > > > > > > clp->cl_hostname: > > > > > > > > > > > > /* Expect a GSS_C_NT_HOSTBASED_NAME like "nfs@serverhostname" */ > > > > > > > > > > > > if (memcmp(p, "nfs@", 4) != 0) > > > > > > return 0; > > > > > > p += 4; > > > > > > if (strcmp(p, clp->cl_hostname) != 0) > > > > > > return 0; > > > > > > return 1; > > > > > > > > > > > > The problem is that there is no guarantee that those hostnames will be > > > > > > the same. If, for instance, I mount "foo:/" and the SPN is > > > > > > "nfs/foo.bar.baz" that strcmp will return true, and the CB_COMPOUND > > > > > > request will get tossed out [1]. Ditto if I happen to mount a CNAME of the > > > > > > server. > > > > > > > > > > It sounds like a bug to me that the mount is succeeding without the name > > > > > matching. > > > > > > > > > > The security provided by krb5 is much weaker if we don't check that the > > > > > name provided on the commandline matches what the server authenticates > > > > > as. > > > > > > > > > > > > > The logic in gssd for this is pretty awful. > > > > > > > > It will basically trust DNS if there is no '.' in the hostname that was > > > > used at mount time. That'll make it take the address and > > > > reverse-resolve it. > > > > > > Argh, OK, I guess this is the compromise Simo made in "Avoid DNS reverse > > > resolution for server names (take 3)". > > > > > > > We could add yet another band-aid and make it so that DNS is never > > > > trusted. I'll note that for cifs, we took that route. You have to mount > > > > the canonical name of the server in order to use krb5. > > > > > > I wish we could do that, but I suppose it's too harsh to break > > > already-working fstabs. Maybe we could phase it in somehow. > > > > > > > > > Now that we try to use krb5 on the callback channel even when sec=sys > > > > > > is specified, this is very problematic. > > > > > > > > > > And similarly I think the attempt to opportunistically use krb5 for > > > > > state management should fail and fall back on auth_sys if the server's > > > > > name doesn't match. > > > > > > > > > > > > > Like Trond pointed out, the problem is that gssd doesn't give us that > > > > info currently. We could change it to do that of course, but that > > > > basically means revving the downcall. > > > > > > It might be easier to rev the upcall so that the kernel could ask gssd > > > to do strict checking? Since it's just a bunch of name=value pairs it > > > shouldn't be a huge pain to revise. > > > > > > > Yeah, that might work, but it will definitely break anyone who's not > > mounting the canonical server name today. > > > > OTOH, if we're going to do that, then we don't really need to rev the > > upcall. Just fix gssd to do this strict checking by default (and maybe > > add a command-line option to allow it to trust DNS like it does today). > > > > > > > > I think that the ideal thing would be to stash the SPN that we use to > > > > > > do the SETCLIENTID call and use that in the comparison above. > > > > > > Unfortunately, the rpc_cred doesn't really seem to carry this info and > > > > > > I don't see where we get enough information in the rpc.gssd downcall to > > > > > > figure out what that SPN should be. > > > > > > > > > > > > Anyone have thoughts or should we just remove the above check until we > > > > > > come up with a better way to do this? > > > > > > > > > > > > [1]: there's another bug that can cause the client to send a bogus > > > > > > reply instead of dropping the request as intended, but that's > > > > > > relatively simple to fix. > > > > > > > > > > So I believe the matching really is a requirement and that it would be > > > > > wrong to weaken it. > > > > > > > > > > It sounds like there's also a server bug here if it's giving out > > > > > delegations to a client that isn't responding to callbacks. > > > > > > > > > > > > > The server uses CB_NULL requests to probe the callback port, and those > > > > aren't affected by this problem. Worse, since CB_NULL requests don't > > > > even contain the callback_ident, we can't even use them to hook up the > > > > nfs_client with the SPN used in them. > > > > > > Ah, got it. > > > > > > The server should still stop delegations as soon as a CB_RECALL times > > > out, though, so at least the problem should clear up after that? > > > > > > --b. > > > > Yes, that seems to be what happens eventually. What I generally see is > > that we get a set of read delegations from the server, eventually the > > server sends a bunch of CB_RECALL requests, which are "dropped" (sort > > of -- I have a patch to really make those be dropped). Eventually ~60s > > later, the client returns the delegations. > > > > I'm a little unclear on what eventually triggers the DELEGRETURNs -- > > maybe the server takes down the callback channel? I need to look a > > little closer at that piece... > > > > Ahh and FWIW... > > What happens is that the RENEW gets a CB_PATH_DOWN error, and the > client then sends back all of the delegations. OK, great, so that part is all working as it should. --b. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
