Re: [PATCH] nfsd: Fix NFSD_MAY_BYPASS_GSS and NFSD_MAY_BYPASS_GSS_ON_ROOT

From: NeilBrown
Date: Thu Sep 12 2024 - 18:52:45 EST

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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.

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
>
>

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