nsproxy: fix free_nsproxy() and simplify create_new_namespaces()

Make it possible to handle NULL being passed to the reference count
helpers instead of forcing the caller to handle this. Afterwards we can
nicely allow a cleanup guard to handle nsproxy freeing.

Active reference count handling is not done in nsproxy_free() but rather
in free_nsproxy() as nsproxy_free() is also called from setns() failure
paths where a new nsproxy has been prepared but has not been marked as
active via switch_task_namespaces().

Link: https://lore.kernel.org/[e-mail]
Link: https://patch.msgid.link/20251111-sakralbau-guthaben-7dcc277d337f@brauner
Fixes: 3c9820d5c64a ("ns: add active reference count")
Reported-by: [e-mail]
Reported-by: [e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

include/linux/ns_common.h

···
 }
 
 #define ns_ref_read(__ns) __ns_ref_read(to_ns_common((__ns)))
-#define ns_ref_inc(__ns) __ns_ref_inc(to_ns_common((__ns)))
-#define ns_ref_get(__ns) __ns_ref_get(to_ns_common((__ns)))
-#define ns_ref_put(__ns) __ns_ref_put(to_ns_common((__ns)))
+#define ns_ref_inc(__ns) \
+	do { if (__ns) __ns_ref_inc(to_ns_common((__ns))); } while (0)
+#define ns_ref_get(__ns) \
+	((__ns) ? __ns_ref_get(to_ns_common((__ns))) : false)
+#define ns_ref_put(__ns) \
+	((__ns) ? __ns_ref_put(to_ns_common((__ns))) : false)
 #define ns_ref_put_and_lock(__ns, __ns_lock) \
-	__ns_ref_dec_and_lock(to_ns_common((__ns)), __ns_lock)
+	((__ns) ? __ns_ref_dec_and_lock(to_ns_common((__ns)), __ns_lock) : false)
 
 #define ns_ref_active_read(__ns) \
 	((__ns) ? __ns_ref_active_read(to_ns_common(__ns)) : 0)

include/linux/nsproxy.h

···
 void exit_cred_namespaces(struct task_struct *tsk);
 void switch_task_namespaces(struct task_struct *tsk, struct nsproxy *new);
 int exec_task_namespaces(void);
-void free_nsproxy(struct nsproxy *ns);
+void deactivate_nsproxy(struct nsproxy *ns);
 int unshare_nsproxy_namespaces(unsigned long, struct nsproxy **,
 	struct cred *, struct fs_struct *);
 int __init nsproxy_cache_init(void);
···
 static inline void put_nsproxy(struct nsproxy *ns)
 {
 	if (refcount_dec_and_test(&ns->count))
-		free_nsproxy(ns);
+		deactivate_nsproxy(ns);
 }
 
 static inline void get_nsproxy(struct nsproxy *ns)

kernel/nsproxy.c

···
 	return nsproxy;
 }
 
+static inline void nsproxy_free(struct nsproxy *ns)
+{
+	put_mnt_ns(ns->mnt_ns);
+	put_uts_ns(ns->uts_ns);
+	put_ipc_ns(ns->ipc_ns);
+	put_pid_ns(ns->pid_ns_for_children);
+	put_time_ns(ns->time_ns);
+	put_time_ns(ns->time_ns_for_children);
+	put_cgroup_ns(ns->cgroup_ns);
+	put_net(ns->net_ns);
+	kmem_cache_free(nsproxy_cachep, ns);
+}
+
+void deactivate_nsproxy(struct nsproxy *ns)
+{
+	nsproxy_ns_active_put(ns);
+	nsproxy_free(ns);
+}
+
 /*
  * Create new nsproxy and all of its the associated namespaces.
  * Return the newly created nsproxy.  Do not attach this to the task,
···
 	nsproxy_ns_active_get(new_ns);
 	tsk->nsproxy = new_ns;
 	return 0;
-}
-
-void free_nsproxy(struct nsproxy *ns)
-{
-	nsproxy_ns_active_put(ns);
-
-	put_mnt_ns(ns->mnt_ns);
-	put_uts_ns(ns->uts_ns);
-	put_ipc_ns(ns->ipc_ns);
-	put_pid_ns(ns->pid_ns_for_children);
-	put_time_ns(ns->time_ns);
-	put_time_ns(ns->time_ns_for_children);
-	put_cgroup_ns(ns->cgroup_ns);
-	put_net(ns->net_ns);
-	kmem_cache_free(nsproxy_cachep, ns);
 }
 
 /*
···
 	if (nsset->fs && (flags & CLONE_NEWNS) && (flags & ~CLONE_NEWNS))
 		free_fs_struct(nsset->fs);
 	if (nsset->nsproxy)
-		free_nsproxy(nsset->nsproxy);
+		nsproxy_free(nsset->nsproxy);
 }
 
 static int prepare_nsset(unsigned flags, struct nsset *nsset)