Re: [PATCH] nfsd: Fix NFSD_MAY_BYPASS_GSS and NFSD_MAY_BYPASS_GSS_ON_ROOT

["Pali Rohár" <pali@kernel.org>]

On Monday 07 October 2024 09:13:17 NeilBrown wrote:
> On Mon, 07 Oct 2024, Chuck Lever wrote:
> > On Fri, Sep 13, 2024 at 08:52:20AM +1000, NeilBrown wrote:
> > > On Fri, 13 Sep 2024, Pali Rohár wrote:
> > > > Currently NFSD_MAY_BYPASS_GSS and NFSD_MAY_BYPASS_GSS_ON_ROOT do not bypass
> > > > only GSS, but bypass any authentication method. This is problem specially
> > > > for NFS3 AUTH_NULL-only exports.
> > > > 
> > > > The purpose of NFSD_MAY_BYPASS_GSS_ON_ROOT is described in RFC 2623,
> > > > section 2.3.2, to allow mounting NFS2/3 GSS-only export without
> > > > authentication. So few procedures which do not expose security risk used
> > > > during mount time can be called also with AUTH_NONE or AUTH_SYS, to allow
> > > > client mount operation to finish successfully.
> > > > 
> > > > The problem with current implementation is that for AUTH_NULL-only exports,
> > > > the NFSD_MAY_BYPASS_GSS_ON_ROOT is active also for NFS3 AUTH_UNIX mount
> > > > attempts which confuse NFS3 clients, and make them think that AUTH_UNIX is
> > > > enabled and is working. Linux NFS3 client never switches from AUTH_UNIX to
> > > > AUTH_NONE on active mount, which makes the mount inaccessible.
> > > > 
> > > > Fix the NFSD_MAY_BYPASS_GSS and NFSD_MAY_BYPASS_GSS_ON_ROOT implementation
> > > > and really allow to bypass only exports which have some GSS auth flavor
> > > > enabled.
> > > > 
> > > > The result would be: For AUTH_NULL-only export if client attempts to do
> > > > mount with AUTH_UNIX flavor then it will receive access errors, which
> > > > instruct client that AUTH_UNIX flavor is not usable and will either try
> > > > other auth flavor (AUTH_NULL if enabled) or fails mount procedure.
> > > > 
> > > > This should fix problems with AUTH_NULL-only or AUTH_UNIX-only exports if
> > > > client attempts to mount it with other auth flavor (e.g. with AUTH_NULL for
> > > > AUTH_UNIX-only export, or with AUTH_UNIX for AUTH_NULL-only export).
> > > 
> > > The MAY_BYPASS_GSS flag currently also bypasses TLS restrictions.  With
> > > your change it doesn't.  I don't think we want to make that change.
> > 
> > Neil, I'm not seeing this, I must be missing something.
> > 
> > RPC_AUTH_TLS is used only on NULL procedures.
> > 
> > The export's xprtsec= setting determines whether a TLS session must
> > be present to access the files on the export. If the TLS session
> > meets the xprtsec= policy, then the normal user authentication
> > settings apply. In other words, I don't think execution gets close
> > to check_nfsd_access() unless the xprtsec policy setting is met.
> 
> check_nfsd_access() is literally the ONLY place that ->ex_xprtsec_modes
> is tested and that seems to be where xprtsec= export settings are stored.
> 
> > 
> > I'm not convinced check_nfsd_access() needs to care about
> > RPC_AUTH_TLS. Can you expand a little on your concern?
> 
> Probably it doesn't care about RPC_AUTH_TLS which as you say is only
> used on NULL procedures when setting up the TLS connection.
> 
> But it *does* care about NFS_XPRTSEC_MTLS etc.
> 
> But I now see that RPC_AUTH_TLS is never reported by OP_SECINFO as an
> acceptable flavour, so the client cannot dynamically determine that TLS
> is required.

Why is not RPC_AUTH_TLS announced in NFS4 OP_SECINFO? Should not NFS4
OP_SECINFO report all possible auth methods for particular filehandle?

> So there is no value in giving non-tls clients access to
> xprtsec=mtls exports so they can discover that for themselves.  The
> client needs to explicitly mount with tls, or possibly the client can
> opportunistically try TLS in every case, and call back.
> 
> So the original patch is OK.
> 
> NeilBrown