Re: [PATCH] nfsd: Fix NFSD_MAY_BYPASS_GSS and NFSD_MAY_BYPASS_GSS_ON_ROOT

[NeilBrown]

On Mon, 07 Oct 2024, Chuck Lever III wrote:
> 
> 
> > On Oct 6, 2024, at 6:29 PM, Pali Rohár <pali@xxxxxxxxxx> wrote:
> > 
> > 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?
> 
> SECINFO reports user authentication flavors and pseudoflavors.
> 
> RPC_AUTH_TLS is not a user authentication flavor, it is merely
> a query to see if the server peer supports RPC-with-TLS.
> 
> So far the nfsv4 WG has not been able to come to consensus
> about how a server's transport layer security policies should
> be reported to clients. There does not seem to be a clean way
> to do that with existing NFSv4 protocol elements, so a
> protocol extension might be needed.

Interesting...

The distinction between RPC_AUTH_GSS_KRB5I and RPC_AUTH_GSS_KRB5P is not
about user authentication, it is about transport privacy.

And the distinction between xprtsec=tls and xprtsec=mtls seems to be
precisely about user authentication.

I would describe the current pseudo flavours as not "a clean way" to
advise the client of security requirements, but they are at least
established practice.

RPC_AUTH_SYS_TLS  seems to me to be an obvious sort of pseudo flavour.

But I suspect all these arguments and more have already been discussed
within the working group and people can sensibly have different
opinions.

Thanks for helping me understand NFS/TLS a bit better.

NeilBrown



> 
> 
> >> 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
> 
> 
> --
> Chuck Lever
> 
> 
>