Re: [PATCH 6.6.y] nfsd: decouple the xprtsec policy check from check_nfsd_access()

Chuck Lever
2025-10-20T20:51:37Z

On 10/20/25 4:50 PM, Scott Mayhew wrote:
> [ Upstream commit e4f574ca9c6dfa66695bb054ff5df43ecea873ec ]
>
> This is a backport of e4f574ca9c6d specifically for the 6.6-stable
> kernel. It differs from the upstream version mainly in that it's
> working around the absence of some 6.12-era commits:
> - 1459ad57673b nfsd: Move error code mapping to per-version proc code.
> - 0a183f24a7ae NFSD: Handle @rqstp == NULL in check_nfsd_access()
> - 5e66d2d92a1c nfsd: factor out __fh_verify to allow NULL rqstp to be
> passed
>
> A while back I had reported that an NFSv3 client could successfully
> mount using '-o xprtsec=none' an export that had been exported with
> 'xprtsec=tls:mtls'. By "successfully" I mean that the mount command
> would succeed and the mount would show up in /proc/mount. Attempting
> to do anything futher with the mount would be met with NFS3ERR_ACCES.
>
> Transport Layer Security isn't an RPC security flavor or pseudo-flavor,
> so we shouldn't be conflating them when determining whether the access
> checks can be bypassed. Split check_nfsd_access() into two helpers, and
> have fh_verify() call the helpers directly since fh_verify() has
> logic that allows one or both of the checks to be skipped. All other
> sites will continue to call check_nfsd_access().
>
> Link: https://lore.kernel.org/linux-nfs/ZjO3Qwf_G87yNXb2@aion/
> Fixes: 9280c5774314 ("NFSD: Handle new xprtsec= export option")
> Signed-off-by: Scott Mayhew <smayhew@redhat.com>
> ---
> fs/nfsd/export.c | 60 +++++++++++++++++++++++++++++++++++++++++-------
> fs/nfsd/export.h | 2 ++
> fs/nfsd/nfsfh.c | 12 +++++++++-
> 3 files changed, 65 insertions(+), 9 deletions(-)
>
> diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
> index 4b5d998cbc2f..f4e77859aa85 100644
> --- a/fs/nfsd/export.c
> +++ b/fs/nfsd/export.c
> @@ -1071,28 +1071,62 @@ static struct svc_export *exp_find(struct cache_detail *cd,
> return exp;
> }
>
> -__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp)
> +/**
> + * check_xprtsec_policy - check if access to export is allowed by the
> + * xprtsec policy
> + * @exp: svc_export that is being accessed.
> + * @rqstp: svc_rqst attempting to access @exp.
> + *
> + * Helper function for check_nfsd_access(). Note that callers should be
> + * using check_nfsd_access() instead of calling this function directly. The
> + * one exception is fh_verify() since it has logic that may result in one
> + * or both of the helpers being skipped.
> + *
> + * Return values:
> + * %nfs_ok if access is granted, or
> + * %nfserr_acces or %nfserr_wrongsec if access is denied
> + */
> +__be32 check_xprtsec_policy(struct svc_export *exp, struct svc_rqst *rqstp)
> {
> - struct exp_flavor_info *f, *end = exp->ex_flavors + exp->ex_nflavors;
> struct svc_xprt *xprt = rqstp->rq_xprt;
>
> if (exp->ex_xprtsec_modes & NFSEXP_XPRTSEC_NONE) {
> if (!test_bit(XPT_TLS_SESSION, &xprt->xpt_flags))
> - goto ok;
> + return nfs_ok;
> }
> if (exp->ex_xprtsec_modes & NFSEXP_XPRTSEC_TLS) {
> if (test_bit(XPT_TLS_SESSION, &xprt->xpt_flags) &&
> !test_bit(XPT_PEER_AUTH, &xprt->xpt_flags))
> - goto ok;
> + return nfs_ok;
> }
> if (exp->ex_xprtsec_modes & NFSEXP_XPRTSEC_MTLS) {
> if (test_bit(XPT_TLS_SESSION, &xprt->xpt_flags) &&
> test_bit(XPT_PEER_AUTH, &xprt->xpt_flags))
> - goto ok;
> + return nfs_ok;
> }
> - goto denied;
>
> -ok:
> + return rqstp->rq_vers < 4 ? nfserr_acces : nfserr_wrongsec;
> +}
> +
> +/**
> + * check_security_flavor - check if access to export is allowed by the
> + * xprtsec policy
> + * @exp: svc_export that is being accessed.
> + * @rqstp: svc_rqst attempting to access @exp.
> + *
> + * Helper function for check_nfsd_access(). Note that callers should be
> + * using check_nfsd_access() instead of calling this function directly. The
> + * one exception is fh_verify() since it has logic that may result in one
> + * or both of the helpers being skipped.
> + *
> + * Return values:
> + * %nfs_ok if access is granted, or
> + * %nfserr_acces or %nfserr_wrongsec if access is denied
> + */
> +__be32 check_security_flavor(struct svc_export *exp, struct svc_rqst *rqstp)
> +{
> + struct exp_flavor_info *f, *end = exp->ex_flavors + exp->ex_nflavors;
> +
> /* legacy gss-only clients are always OK: */
> if (exp->ex_client == rqstp->rq_gssclient)
> return 0;
> @@ -1117,10 +1151,20 @@ __be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp)
> if (nfsd4_spo_must_allow(rqstp))
> return 0;
>
> -denied:
> return rqstp->rq_vers < 4 ? nfserr_acces : nfserr_wrongsec;
> }
>
> +__be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp)
> +{
> + __be32 status;
> +
> + status = check_xprtsec_policy(exp, rqstp);
> + if (status != nfs_ok)
> + return status;
> +
> + return check_security_flavor(exp, rqstp);
> +}
> +
> /*
> * Uses rq_client and rq_gssclient to find an export; uses rq_client (an
> * auth_unix client) if it's available and has secinfo information;
> diff --git a/fs/nfsd/export.h b/fs/nfsd/export.h
> index ca9dc230ae3d..4a48b2ad5606 100644
> --- a/fs/nfsd/export.h
> +++ b/fs/nfsd/export.h
> @@ -100,6 +100,8 @@ 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_xprtsec_policy(struct svc_export *exp, struct svc_rqst *rqstp);
> +__be32 check_security_flavor(struct svc_export *exp, struct svc_rqst *rqstp);
> __be32 check_nfsd_access(struct svc_export *exp, struct svc_rqst *rqstp);
>
> /*
> diff --git a/fs/nfsd/nfsfh.c b/fs/nfsd/nfsfh.c
> index c2495d98c189..283c1a60c846 100644
> --- a/fs/nfsd/nfsfh.c
> +++ b/fs/nfsd/nfsfh.c
> @@ -370,6 +370,16 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type, int access)
> if (error)
> goto out;
>
> + /*
> + * NLM is allowed to bypass the xprtsec policy check because lockd
> + * doesn't support xprtsec.
> + */
> + if (!(access & NFSD_MAY_LOCK)) {
> + error = check_xprtsec_policy(exp, rqstp);
> + if (error)
> + goto out;
> + }
> +
> /*
> * pseudoflavor restrictions are not enforced on NLM,
> * which clients virtually always use auth_sys for,
> @@ -386,7 +396,7 @@ fh_verify(struct svc_rqst *rqstp, struct svc_fh *fhp, umode_t type, int access)
> && exp->ex_path.dentry == dentry)
> goto skip_pseudoflavor_check;
>
> - error = check_nfsd_access(exp, rqstp);
> + error = check_security_flavor(exp, rqstp);
> if (error)
> goto out;
>

Acked-by: Chuck Lever <chuck.lever@oracle.com>

--
Chuck Lever