[PATCH] nfsd: properly tear down server when write_ports fails

[Jeff Layton <jlayton@kernel.org>]

When the initial write to the "portlist" file fails, we'll currently put
the reference to the nn->nfsd_serv, but leave the pointer intact. This
leads to a UAF if someone tries to write to "portlist" again.

Simple reproducer, from a host with nfsd shut down:

    # echo "foo 2049" > /proc/fs/nfsd/portlist
    # echo "foo 2049" > /proc/fs/nfsd/portlist

The kernel will oops on the second one when it trips over the dangling
nn->nfsd_serv pointer. There is a similar bug in __write_ports_addfd.

This patch fixes it by adding some extra logic to nfsd_put to ensure
that nfsd_last_thread is called prior to putting the reference when the
conditions are right.

Fixes: 9f28a971ee9f ("nfsd: separate nfsd_last_thread() from nfsd_put()")
Closes: https://issues.redhat.com/browse/RHEL-19081
Reported-by: Zhi Li <yieli@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
This should probably go to stable, but we'll need to backport for v6.6
since older kernels don't have nfsd_nl_rpc_status_get_done. We should
just be able to drop that hunk though.
---
 fs/nfsd/nfsctl.c | 32 ++++++++++++++++++++++++++++----
 fs/nfsd/nfsd.h   |  8 +-------
 fs/nfsd/nfssvc.c |  2 +-
 3 files changed, 30 insertions(+), 12 deletions(-)

diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
index 3e15b72f421d..1ceccf804e44 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c
@@ -61,6 +61,30 @@ enum {
 	NFSD_MaxReserved
 };
 
+/**
+ * nfsd_put - put the reference to the nfsd_serv for given net
+ * @net: the net namespace for the serv
+ * @err: current error for the op
+ *
+ * When putting a reference to the nfsd_serv from a control operation
+ * we must first call nfsd_last_thread if all of these are true:
+ *
+ * - the configuration operation is going fail
+ * - there are no running threads
+ * - there are no successfully configured ports
+ *
+ * Otherwise, just put the serv reference.
+ */
+static inline void nfsd_put(struct net *net, int err)
+{
+	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
+	struct svc_serv *serv = nn->nfsd_serv;
+
+	if (err < 0 && !nn->nfsd_serv->sv_nrthreads && !nn->keep_active)
+		nfsd_last_thread(net);
+	svc_put(serv);
+}
+
 /*
  * write() for these nodes.
  */
@@ -709,7 +733,7 @@ static ssize_t __write_ports_addfd(char *buf, struct net *net, const struct cred
 	    !nn->nfsd_serv->sv_nrthreads && !xchg(&nn->keep_active, 1))
 		svc_get(nn->nfsd_serv);
 
-	nfsd_put(net);
+	nfsd_put(net, err);
 	return err;
 }
 
@@ -748,7 +772,7 @@ static ssize_t __write_ports_addxprt(char *buf, struct net *net, const struct cr
 	if (!nn->nfsd_serv->sv_nrthreads && !xchg(&nn->keep_active, 1))
 		svc_get(nn->nfsd_serv);
 
-	nfsd_put(net);
+	nfsd_put(net, 0);
 	return 0;
 out_close:
 	xprt = svc_find_xprt(nn->nfsd_serv, transport, net, PF_INET, port);
@@ -757,7 +781,7 @@ static ssize_t __write_ports_addxprt(char *buf, struct net *net, const struct cr
 		svc_xprt_put(xprt);
 	}
 out_err:
-	nfsd_put(net);
+	nfsd_put(net, err);
 	return err;
 }
 
@@ -1687,7 +1711,7 @@ int nfsd_nl_rpc_status_get_dumpit(struct sk_buff *skb,
 int nfsd_nl_rpc_status_get_done(struct netlink_callback *cb)
 {
 	mutex_lock(&nfsd_mutex);
-	nfsd_put(sock_net(cb->skb->sk));
+	nfsd_put(sock_net(cb->skb->sk), 0);
 	mutex_unlock(&nfsd_mutex);
 
 	return 0;
diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
index f5ff42f41ee7..3aa8cd2c19ac 100644
--- a/fs/nfsd/nfsd.h
+++ b/fs/nfsd/nfsd.h
@@ -113,13 +113,6 @@ int		nfsd_pool_stats_open(struct inode *, struct file *);
 int		nfsd_pool_stats_release(struct inode *, struct file *);
 void		nfsd_shutdown_threads(struct net *net);
 
-static inline void nfsd_put(struct net *net)
-{
-	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
-
-	svc_put(nn->nfsd_serv);
-}
-
 bool		i_am_nfsd(void);
 
 struct nfsdfs_client {
@@ -153,6 +146,7 @@ struct nfsd_net;
 enum vers_op {NFSD_SET, NFSD_CLEAR, NFSD_TEST, NFSD_AVAIL };
 int nfsd_vers(struct nfsd_net *nn, int vers, enum vers_op change);
 int nfsd_minorversion(struct nfsd_net *nn, u32 minorversion, enum vers_op change);
+void nfsd_last_thread(struct net *net);
 void nfsd_reset_versions(struct nfsd_net *nn);
 int nfsd_create_serv(struct net *net);
 
diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index fe61d9bbcc1f..d6939e23ffcf 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -542,7 +542,7 @@ static struct notifier_block nfsd_inet6addr_notifier = {
 /* Only used under nfsd_mutex, so this atomic may be overkill: */
 static atomic_t nfsd_notifier_refcount = ATOMIC_INIT(0);
 
-static void nfsd_last_thread(struct net *net)
+void nfsd_last_thread(struct net *net)
 {
 	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
 	struct svc_serv *serv = nn->nfsd_serv;

---
base-commit: a39b6ac3781d46ba18193c9dbb2110f31e9bffe9
change-id: 20231211-nfsd-fixes-d9f21d5c12d7

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>