Re: [PATCH] nfsd: Fix NFSD_MAY_BYPASS_GSS and NFSD_MAY_BYPASS_GSS_ON_ROOT

From: Chuck Lever
Date: Tue Oct 08 2024 - 14:58:23 EST

On Mon, Oct 07, 2024 at 09:13:17AM +1100, 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. 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.

May I add "Reviewed-by: NeilBrown <neilb@xxxxxxx>" ?


> NeilBrown
>
>
> >
> >
> > > I think that what you want to do makes sense. Higher security can be
> > > downgraded to AUTH_UNIX, but AUTH_NULL mustn't be upgraded to to
> > > AUTH_UNIX.
> > >
> > > Maybe that needs to be explicit in the code. The bypass is ONLY allowed
> > > for AUTH_UNIX and only if something other than AUTH_NULL is allowed.
> > >
> > > Thanks,
> > > NeilBrown
> > >
> > >
> > >
> > > >
> > > > Signed-off-by: Pali Rohár <pali@xxxxxxxxxx>
> > > > ---
> > > > fs/nfsd/export.c | 19 ++++++++++++++++++-
> > > > fs/nfsd/export.h | 2 +-
> > > > fs/nfsd/nfs4proc.c | 2 +-
> > > > fs/nfsd/nfs4xdr.c | 2 +-
> > > > fs/nfsd/nfsfh.c | 12 +++++++++---
> > > > fs/nfsd/vfs.c | 2 +-
> > > > 6 files changed, 31 insertions(+), 8 deletions(-)
> > > >
> > > > diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
> > > > index 50b3135d07ac..eb11d3fdffe1 100644
> > > > --- a/fs/nfsd/export.c
> > > > +++ b/fs/nfsd/export.c
> > > > @@ -1074,7 +1074,7 @@ static struct svc_export *exp_find(struct cache_detail *cd,
> > > > return exp;
> > > > }
> > > >
> > > > -__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp)
> > > > +__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp, bool may_bypass_gss)
> > > > {
> > > > struct exp_flavor_info *f, *end = exp->ex_flavors + exp->ex_nflavors;
> > > > struct svc_xprt *xprt = rqstp->rq_xprt;
> > > > @@ -1120,6 +1120,23 @@ __be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp)
> > > > if (nfsd4_spo_must_allow(rqstp))
> > > > return 0;
> > > >
> > > > + /* Some calls may be processed without authentication
> > > > + * on GSS exports. For example NFS2/3 calls on root
> > > > + * directory, see section 2.3.2 of rfc 2623.
> > > > + * For "may_bypass_gss" check that export has really
> > > > + * enabled some GSS flavor and also check that the
> > > > + * used auth flavor is without auth (none or sys).
> > > > + */
> > > > + if (may_bypass_gss && (
> > > > + rqstp->rq_cred.cr_flavor == RPC_AUTH_NULL ||
> > > > + rqstp->rq_cred.cr_flavor == RPC_AUTH_UNIX)) {
> > > > + for (f = exp->ex_flavors; f < end; f++) {
> > > > + if (f->pseudoflavor == RPC_AUTH_GSS ||
> > > > + f->pseudoflavor >= RPC_AUTH_GSS_KRB5)
> > > > + return 0;
> > > > + }
> > > > + }
> > > > +
> > > > denied:
> > > > return rqstp->rq_vers < 4 ? nfserr_acces : nfserr_wrongsec;
> > > > }
> > > > diff --git a/fs/nfsd/export.h b/fs/nfsd/export.h
> > > > index ca9dc230ae3d..dc7cf4f6ac53 100644
> > > > --- a/fs/nfsd/export.h
> > > > +++ b/fs/nfsd/export.h
> > > > @@ -100,7 +100,7 @@ struct svc_expkey {
> > > > #define EX_WGATHER(exp) ((exp)->ex_flags & NFSEXP_GATHERED_WRITES)
> > > >
> > > > int nfsexp_flags(struct svc_rqst *rqstp, struct svc_export *exp);
> > > > -__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp);
> > > > +__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp, bool may_bypass_gss);
> > > >
> > > > /*
> > > > * Function declarations
> > > > diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> > > > index 2e39cf2e502a..0f67f4a7b8b2 100644
> > > > --- a/fs/nfsd/nfs4proc.c
> > > > +++ b/fs/nfsd/nfs4proc.c
> > > > @@ -2791,7 +2791,7 @@ nfsd4_proc_compound(struct svc_rqst *rqstp)
> > > >
> > > > if (current_fh->fh_export &&
> > > > need_wrongsec_check(rqstp))
> > > > - op->status = check_nfsd_access(current_fh->fh_export, rqstp);
> > > > + op->status = check_nfsd_access(current_fh->fh_export, rqstp, false);
> > > > }
> > > > encode_op:
> > > > if (op->status == nfserr_replay_me) {
> > > > diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
> > > > index 97f583777972..b45ea5757652 100644
> > > > --- a/fs/nfsd/nfs4xdr.c
> > > > +++ b/fs/nfsd/nfs4xdr.c
> > > > @@ -3775,7 +3775,7 @@ nfsd4_encode_entry4_fattr(struct nfsd4_readdir *cd, const char *name,
> > > > nfserr = nfserrno(err);
> > > > goto out_put;
> > > > }
> > > > - nfserr = check_nfsd_access(exp, cd->rd_rqstp);
> > > > + nfserr = check_nfsd_access(exp, cd->rd_rqstp, false);
> > > > if (nfserr)
> > > > goto out_put;
> > > >
> > > > diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
> > > > index dd4e11a703aa..ed0eabfa3cb0 100644
> > > > --- a/fs/nfsd/nfsfh.c
> > > > +++ b/fs/nfsd/nfsfh.c
> > > > @@ -329,6 +329,7 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type, int access)
> > > > {
> > > > struct nfsd_net *nn = net_generic(SVC_NET(rqstp), nfsd_net_id);
> > > > struct svc_export *exp = NULL;
> > > > + bool may_bypass_gss = false;
> > > > struct dentry *dentry;
> > > > __be32 error;
> > > >
> > > > @@ -375,8 +376,13 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type, int access)
> > > > * which clients virtually always use auth_sys for,
> > > > * even while using RPCSEC_GSS for NFS.
> > > > */
> > > > - if (access & NFSD_MAY_LOCK || access & NFSD_MAY_BYPASS_GSS)
> > > > + if (access & NFSD_MAY_LOCK)
> > > > goto skip_pseudoflavor_check;
> > > > + /*
> > > > + * NFS4 PUTFH may bypass GSS (see nfsd4_putfh() in nfs4proc.c).
> > > > + */
> > > > + if (access & NFSD_MAY_BYPASS_GSS)
> > > > + may_bypass_gss = true;
> > > > /*
> > > > * Clients may expect to be able to use auth_sys during mount,
> > > > * even if they use gss for everything else; see section 2.3.2
> > > > @@ -384,9 +390,9 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type, int access)
> > > > */
> > > > if (access & NFSD_MAY_BYPASS_GSS_ON_ROOT
> > > > && exp->ex_path.dentry == dentry)
> > > > - goto skip_pseudoflavor_check;
> > > > + may_bypass_gss = true;
> > > >
> > > > - error = check_nfsd_access(exp, rqstp);
> > > > + error = check_nfsd_access(exp, rqstp, may_bypass_gss);
> > > > if (error)
> > > > goto out;
> > > >
> > > > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> > > > index 29b1f3613800..b2f5ea7c2187 100644
> > > > --- a/fs/nfsd/vfs.c
> > > > +++ b/fs/nfsd/vfs.c
> > > > @@ -320,7 +320,7 @@ nfsd_lookup(struct svc_rqst *rqstp, struct svc_fh *fhp, const char *name,
> > > > err = nfsd_lookup_dentry(rqstp, fhp, name, len, &exp, &dentry);
> > > > if (err)
> > > > return err;
> > > > - err = check_nfsd_access(exp, rqstp);
> > > > + err = check_nfsd_access(exp, rqstp, false);
> > > > if (err)
> > > > goto out;
> > > > /*
> > > > --
> > > > 2.20.1
> > > >
> > > >
> > >
> >
> > --
> > Chuck Lever
> >
>

--
Chuck Lever