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Message-ID: <20240912230035.2u6ymgrp326u4rid@pali>
Date: Fri, 13 Sep 2024 01:00:35 +0200
From: Pali Rohár <pali@...nel.org>
To: NeilBrown <neilb@...e.de>
Cc: Chuck Lever <chuck.lever@...cle.com>, Jeff Layton <jlayton@...nel.org>,
Olga Kornievskaia <okorniev@...hat.com>,
Dai Ngo <Dai.Ngo@...cle.com>, Tom Talpey <tom@...pey.com>,
linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nfsd: Fix NFSD_MAY_BYPASS_GSS and
NFSD_MAY_BYPASS_GSS_ON_ROOT
On Friday 13 September 2024 08:52:20 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.
Ok. But this is not obvious really obvious ass the option has GSS in its name.
> 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.
Ok, this sound good.
> Thanks,
> NeilBrown
>
>
>
> >
> > Signed-off-by: Pali Rohár <pali@...nel.org>
> > ---
> > 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
> >
> >
>
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