Re: NFSv3 and xprtsec policies

[Scott Mayhew <smayhew@redhat.com>]

On Thu, 02 May 2024, Chuck Lever III wrote:

> 
> 
> > On May 2, 2024, at 1:37 PM, Scott Mayhew <smayhew@redhat.com> wrote:
> > 
> > On Thu, 02 May 2024, Chuck Lever III wrote:
> > 
> >>> On May 2, 2024, at 11:54 AM, Scott Mayhew <smayhew@redhat.com> wrote:
> >>> 
> >>> Red Hat QE identified an "interesting" issue with NFSv3 and TLS, in that an
> >>> NFSv3 client can mount with "xprtsec=none" a filesystem exported with
> >>> "xprtsec=tls:mtls" (in the sense that the client gets the filehandle and adds a
> >>> mount to its mount table - it can't actually access the mount).
> >>> 
> >>> Here's an example using machines from the recent Bakeathon.
> >>> 
> >>> Mounting a server with TLS enabled:
> >>> 
> >>> # mount -o v4.2,sec=sys,xprtsec=tls oracle-102.chuck.lever.oracle.com.nfsv4.dev:/export/tls /mnt
> >>> # umount /mnt
> >>> 
> >>> Trying to mount without "xprtsec=tls" shows that the filesystem is not exported with "xprtsec=none":
> >>> 
> >>> # mount -o v4.2,sec=sys oracle-102.chuck.lever.oracle.com.nfsv4.dev:/export/tls /mnt
> >>> mount.nfs: Operation not permitted for oracle-102.chuck.lever.oracle.com.nfsv4.dev:/export/tls on /mnt
> >>> 
> >>> Yet a v3 mount without "xprtsec=tls" works:
> >>> 
> >>> # mount -o v3,sec=sys oracle-102.chuck.lever.oracle.com.nfsv4.dev:/export/tls /mnt
> >>> # umount /mnt
> >>> 
> >>> and a mount with no explicit version and without "xprtsec=tls" falls back to
> >>> v3 and also "works":
> >>> 
> >>> # mount -o sec=sys oracle-102.chuck.lever.oracle.com.nfsv4.dev:/export/tls /mnt
> >>> # grep ora /proc/mounts
> >>> oracle-102.chuck.lever.oracle.com.nfsv4.dev:/export/tls /mnt nfs
> >>> +rw,relatime,vers=3,rsize=524288,wsize=524288,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=100.64.0.49,mountvers=3,mountport=20048,mountproto=udp,local_lock=none,addr=100.64.0.49 0 0
> >>> 
> >>> Even though the filesystem is mounted, the client can't do anything with it:
> >>> 
> >>> # ls /mnt
> >>> ls: cannot open directory '/mnt': Permission denied
> >>> 
> >>> When krb5 is used with NFSv3, the server returns a list of pseudoflavors in
> >>> mountres3_ok (https://datatracker.ietf.org/doc/html/rfc1813#section-5.2.1).
> >>> The client compares that list with its own list of auth flavors parsed from the
> >>> mount request and returns -EACCES if no match is found (see
> >>> nfs_verify_authflavors()).
> >>> 
> >>> Perhaps we should be doing something similar with xprtsec policies?
> >> 
> >> The problem might be in how you've set up the exports. With NFSv3,
> >> the parent export needs the "crossmnt" export option in order for
> >> NFSv3 to behave like NFSv4 in this regard, although I could have
> >> missed something.
> > 
> > I was mounting your server though :)
> 
> OK, then not the same bug that Olga found last year.
> 
> We should find out what FreeBSD does in this case.

I thought about that.  Rick's servers from the BAT are offline, and I
don't think he was exporting v3 anyway.

> 
> 
> >>> Should
> >>> there be an errata to RFC 9289 and a request from IANA for assigned numbers for
> >>> pseudo-flavors corresponding to xprtsec policies?
> >> 
> >> No. Transport-layer security is not an RPC security flavor or
> >> pseudo-flavor. These two things are not related.
> >> 
> >> (And in fact, I proposed something like this for NFSv4 SECINFO,
> >> but it was rejected).
> > 
> > I thought it might be a stretch to try to use mountres3.auth_flavors for
> > this, but since RFC 9289 does refer to AUTH_TLS as an authentication
> > flavor and https://www.iana.org/assignments/rpc-authentication-numbers/rpc-authentication-numbers.xhtml
> > also lists TLS under the Flavor Name column I thought it might make
> > sense to treat xprtsec policies as if they were pseudo-flavors even
> > though they're not, if only to give the client a way to determine that
> > the mount should fail.
> 
> RPC_AUTH_TLS is used only when a client probes a server to see if
> it supports RPC-with-TLS. At all other times, the client uses one
> of the normal, legitimate flavors. It does not represent a security
> flavor that can be used during regular operation.
> 
> NFSv3 mount failover logic is still open for discussion (ie, incomplete).
> 
> Would it help if rpc.mountd stuck RPC_AUTH_TLS in the auth_flavors
> list? I think clients that don't recognize it should ignore it,
> but I'm not sure. What should a client do if it sees that flavor in
> the list? It's not one that can be used for any other procedure than
> a NULL RPC.

Maybe?  After the client gets the filehandle it's calling FSINFO and
PATHCONF.  The latter get NFS3ERR_ACCES, but nfs_probe_fsinfo() isn't
checking for a negative return code from the PATHCONF operation.  If it
did, it could maybe use the -EACCES coupled with the knowledge that the
server had RPC_AUTH_TLS enabled to emit an error message saying to check
the xprtsec policies (but I don't think that would be as definitive as
what I had in mind) and to fail the mount.

-Scott
> 
> 
> >>> If not, this behavior should at least be documented in the man pages.
> >> 
> >> "crossmnt", and it's kin "nohide", are explained in exports(5).
> > 
> > rpc.mountd doesn't do any access checking based on xprtsec policies on
> > the export (or krb5 pseudo-flavors, for that matter), so I don't see how
> > "crossmount" or "nohide" would have any effect here.
> 
> No, they don't, you are correct.
> 
> 
> --
> Chuck Lever
> 
>