ns: add active reference count

The namespace tree is, among other things, currently used to support
file handles for namespaces. When a namespace is created it is placed on
the namespace trees and when it is destroyed it is removed from the
namespace trees.

While a namespace is on the namespace trees with a valid reference count
it is possible to reopen it through a namespace file handle. This is all
fine but has some issues that should be addressed.

On current kernels a namespace is visible to userspace in the
following cases:

(1) The namespace is in use by a task.
(2) The namespace is persisted through a VFS object (namespace file
descriptor or bind-mount).
Note that (2) only cares about direct persistence of the namespace
itself not indirectly via e.g., file->f_cred file references or
similar.
(3) The namespace is a hierarchical namespace type and is the parent of
a single or multiple child namespaces.

Case (3) is interesting because it is possible that a parent namespace
might not fulfill any of (1) or (2), i.e., is invisible to userspace but
it may still be resurrected through the NS_GET_PARENT ioctl().

Currently namespace file handles allow much broader access to namespaces
than what is currently possible via (1)-(3). The reason is that
namespaces may remain pinned for completely internal reasons yet are
inaccessible to userspace.

For example, a user namespace my remain pinned by get_cred() calls to
stash the opener's credentials into file->f_cred. As it stands file
handles allow to resurrect such a users namespace even though this
should not be possible via (1)-(3). This is a fundamental uapi change
that we shouldn't do if we don't have to.

Consider the following insane case: Various architectures support the
CONFIG_MMU_LAZY_TLB_REFCOUNT option which uses lazy TLB destruction.
When this option is set a userspace task's struct mm_struct may be used
for kernel threads such as the idle task and will only be destroyed once
the cpu's runqueue switches back to another task. But because of ptrace()
permission checks struct mm_struct stashes the user namespace of the
task that struct mm_struct originally belonged to. The kernel thread
will take a reference on the struct mm_struct and thus pin it.

So on an idle system user namespaces can be persisted for arbitrary
amounts of time which also means that they can be resurrected using
namespace file handles. That makes no sense whatsoever. The problem is
of course excarabted on large systems with a huge number of cpus.

To handle this nicely we introduce an active reference count which
tracks (1)-(3). This is easy to do as all of these things are already
managed centrally. Only (1)-(3) will count towards the active reference
count and only namespaces which are active may be opened via namespace
file handles.

The problem is that namespaces may be resurrected. Which means that they
can become temporarily inactive and will be reactived some time later.
Currently the only example of this is the SIOGCSKNS socket ioctl. The
SIOCGSKNS ioctl allows to open a network namespace file descriptor based
on a socket file descriptor.

If a socket is tied to a network namespace that subsequently becomes
inactive but that socket is persisted by another process in another
network namespace (e.g., via SCM_RIGHTS of pidfd_getfd()) then the
SIOCGSKNS ioctl will resurrect this network namespace.

So calls to open_related_ns() and open_namespace() will end up
resurrecting the corresponding namespace tree.

Note that the active reference count does not regulate the lifetime of
the namespace itself. This is still done by the normal reference count.
The active reference count can only be elevated if the regular reference
count is elevated.

The active reference count also doesn't regulate the presence of a
namespace on the namespace trees. It only regulates its visiblity to
namespace file handles (and in later patches to listns()).

A namespace remains on the namespace trees from creation until its
actual destruction. This will allow the kernel to always reach any
namespace trivially and it will also enable subsystems like bpf to walk
the namespace lists on the system for tracing or general introspection
purposes.

Note that different namespaces have different visibility lifetimes on
current kernels. While most namespace are immediately released when the
last task using them exits, the user- and pid namespace are persisted
and thus both remain accessible via /proc/<pid>/ns/<ns_type>.

The user namespace lifetime is aliged with struct cred and is only
released through exit_creds(). However, it becomes inaccessible to
userspace once the last task using it is reaped, i.e., when
release_task() is called and all proc entries are flushed. Similarly,
the pid namespace is also visible until the last task using it has been
reaped and the associated pid numbers are freed.

The active reference counts of the user- and pid namespace are
decremented once the task is reaped.

Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

fs/nsfs.c

···
 static void nsfs_evict(struct inode *inode)
 {
 	struct ns_common *ns = inode->i_private;
+
+	__ns_ref_active_put(ns);
 	clear_inode(inode);
 	ns->ops->put(ns);
 }
···
 	inode->i_mode |= S_IRUGO;
 	inode->i_fop = &ns_file_operations;
 	inode->i_ino = ns->inum;
+
+	/*
+	 * Bring the namespace subtree back to life if we have to. This
+	 * can happen when e.g., all processes using a network namespace
+	 * and all namespace files or namespace file bind-mounts have
+	 * died but there are still sockets pinning it. The SIOCGSKNS
+	 * ioctl on such a socket will resurrect the relevant namespace
+	 * subtree.
+	 */
+	__ns_ref_active_resurrect(ns);
 	return 0;
 }
 
···
 		if (ns->inum != fid->ns_inum)
 			return NULL;
 
-		if (!__ns_ref_get(ns))
+		/*
+		 * This is racy because we're not actually taking an
+		 * active reference. IOW, it could happen that the
+		 * namespace becomes inactive after this check.
+		 * We don't care because nsfs_init_inode() will just
+		 * resurrect the relevant namespace tree for us. If it
+		 * has been active here we just allow it's resurrection.
+		 * We could try to take an active reference here and
+		 * then drop it again. But really, why bother.
+		 */
+		if (!ns_get_unless_inactive(ns))
 			return NULL;
 	}
 
···
 	nsfs_mnt->mnt_sb->s_flags &= ~SB_NOUSER;
 	nsfs_root_path.mnt = nsfs_mnt;
 	nsfs_root_path.dentry = nsfs_mnt->mnt_root;
+}
+
+void nsproxy_ns_active_get(struct nsproxy *ns)
+{
+	ns_ref_active_get(ns->mnt_ns);
+	ns_ref_active_get(ns->uts_ns);
+	ns_ref_active_get(ns->ipc_ns);
+	ns_ref_active_get(ns->pid_ns_for_children);
+	ns_ref_active_get(ns->cgroup_ns);
+	ns_ref_active_get(ns->net_ns);
+	ns_ref_active_get(ns->time_ns);
+	ns_ref_active_get(ns->time_ns_for_children);
+}
+
+void nsproxy_ns_active_put(struct nsproxy *ns)
+{
+	ns_ref_active_put(ns->mnt_ns);
+	ns_ref_active_put(ns->uts_ns);
+	ns_ref_active_put(ns->ipc_ns);
+	ns_ref_active_put(ns->pid_ns_for_children);
+	ns_ref_active_put(ns->cgroup_ns);
+	ns_ref_active_put(ns->net_ns);
+	ns_ref_active_put(ns->time_ns);
+	ns_ref_active_put(ns->time_ns_for_children);
 }

include/linux/ns_common.h

···
 
 #include <linux/refcount.h>
 #include <linux/rbtree.h>
+#include <linux/vfsdebug.h>
 #include <uapi/linux/sched.h>
+#include <uapi/linux/nsfs.h>
 
 struct proc_ns_operations;
 
···
 extern const struct proc_ns_operations timens_operations;
 extern const struct proc_ns_operations timens_for_children_operations;
 
+/*
+ * Namespace lifetimes are managed via a two-tier reference counting model:
+ *
+ * (1) __ns_ref (refcount_t): Main reference count tracking memory
+ *     lifetime. Controls when the namespace structure itself is freed.
+ *     It also pins the namespace on the namespace trees whereas (2)
+ *     only regulates their visibility to userspace.
+ *
+ * (2) __ns_ref_active (atomic_t): Reference count tracking active users.
+ *     Controls visibility of the namespace in the namespace trees.
+ *     Any live task that uses the namespace (via nsproxy or cred) holds
+ *     an active reference. Any open file descriptor or bind-mount of
+ *     the namespace holds an active reference. Once all tasks have
+ *     called exited their namespaces and all file descriptors and
+ *     bind-mounts have been released the active reference count drops
+ *     to zero and the namespace becomes inactive. IOW, the namespace
+ *     cannot be listed or opened via file handles anymore.
+ *
+ *     Note that it is valid to transition from active to inactive and
+ *     back from inactive to active e.g., when resurrecting an inactive
+ *     namespace tree via the SIOCGSKNS ioctl().
+ *
+ * Relationship and lifecycle states:
+ *
+ * - Active (__ns_ref_active > 0):
+ *   Namespace is actively used and visible to userspace. The namespace
+ *   can be reopened via /proc/<pid>/ns/<ns_type>, via namespace file
+ *   handles, or discovered via listns().
+ *
+ * - Inactive (__ns_ref_active == 0, __ns_ref > 0):
+ *   No tasks are actively using the namespace and it isn't pinned by
+ *   any bind-mounts or open file descriptors anymore. But the namespace
+ *   is still kept alive by internal references. For example, the user
+ *   namespace could be pinned by an open file through file->f_cred
+ *   references when one of the now defunct tasks had opened a file and
+ *   handed the file descriptor off to another process via a UNIX
+ *   sockets. Such references keep the namespace structure alive through
+ *   __ns_ref but will not hold an active reference.
+ *
+ * - Destroyed (__ns_ref == 0):
+ *   No references remain. The namespace is removed from the tree and freed.
+ *
+ * State transitions:
+ *
+ * Active -> Inactive:
+ *   When the last task using the namespace exits it drops its active
+ *   references to all namespaces. However, user and pid namespaces
+ *   remain accessible until the task has been reaped.
+ *
+ * Inactive -> Active:
+ *   An inactive namespace tree might be resurrected due to e.g., the
+ *   SIOCGSKNS ioctl() on a socket.
+ *
+ * Inactive -> Destroyed:
+ *   When __ns_ref drops to zero the namespace is removed from the
+ *   namespaces trees and the memory is freed (after RCU grace period).
+ *
+ * Initial namespaces:
+ *   Boot-time namespaces (init_net, init_pid_ns, etc.) start with
+ *   __ns_ref_active = 1 and remain active forever.
+ */
 struct ns_common {
 	u32 ns_type;
 	struct dentry *stashed;
···
 			u64 ns_id;
 			struct rb_node ns_tree_node;
 			struct list_head ns_list_node;
+			atomic_t __ns_ref_active; /* do not use directly */
 		};
 		struct rcu_head ns_rcu;
 	};
···
 
 int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_operations *ops, int inum);
 void __ns_common_free(struct ns_common *ns);
+
+static __always_inline bool is_initial_namespace(struct ns_common *ns)
+{
+	VFS_WARN_ON_ONCE(ns->inum == 0);
+	return unlikely(in_range(ns->inum, MNT_NS_INIT_INO,
+				 IPC_NS_INIT_INO - MNT_NS_INIT_INO + 1));
+}
 
 #define to_ns_common(__ns)                                    \
 	_Generic((__ns),                                      \
···
 	.ops			= to_ns_operations(&nsname),				\
 	.stashed		= NULL,							\
 	.__ns_ref		= REFCOUNT_INIT(refs),					\
+	.__ns_ref_active	= ATOMIC_INIT(1),					\
 	.ns_list_node		= LIST_HEAD_INIT(nsname.ns.ns_list_node),		\
 }
 
···
 
 #define ns_common_free(__ns) __ns_common_free(to_ns_common((__ns)))
 
+static __always_inline __must_check int __ns_ref_active_read(const struct ns_common *ns)
+{
+	return atomic_read(&ns->__ns_ref_active);
+}
+
 static __always_inline __must_check bool __ns_ref_put(struct ns_common *ns)
 {
-	return refcount_dec_and_test(&ns->__ns_ref);
+	if (refcount_dec_and_test(&ns->__ns_ref)) {
+		VFS_WARN_ON_ONCE(__ns_ref_active_read(ns));
+		return true;
+	}
+	return false;
 }
 
 static __always_inline __must_check bool __ns_ref_get(struct ns_common *ns)
 {
-	return refcount_inc_not_zero(&ns->__ns_ref);
+	if (refcount_inc_not_zero(&ns->__ns_ref))
+		return true;
+	VFS_WARN_ON_ONCE(__ns_ref_active_read(ns));
+	return false;
 }
 
 static __always_inline __must_check int __ns_ref_read(const struct ns_common *ns)
···
 #define ns_ref_put(__ns) __ns_ref_put(to_ns_common((__ns)))
 #define ns_ref_put_and_lock(__ns, __lock) \
 	refcount_dec_and_lock(&to_ns_common((__ns))->__ns_ref, (__lock))
+
+#define ns_ref_active_read(__ns) \
+	((__ns) ? __ns_ref_active_read(to_ns_common(__ns)) : 0)
+
+void __ns_ref_active_get_owner(struct ns_common *ns);
+
+static __always_inline void __ns_ref_active_get(struct ns_common *ns)
+{
+	WARN_ON_ONCE(atomic_add_negative(1, &ns->__ns_ref_active));
+	VFS_WARN_ON_ONCE(is_initial_namespace(ns) && __ns_ref_active_read(ns) <= 0);
+}
+#define ns_ref_active_get(__ns) \
+	do { if (__ns) __ns_ref_active_get(to_ns_common(__ns)); } while (0)
+
+static __always_inline bool __ns_ref_active_get_not_zero(struct ns_common *ns)
+{
+	if (atomic_inc_not_zero(&ns->__ns_ref_active)) {
+		VFS_WARN_ON_ONCE(!__ns_ref_read(ns));
+		return true;
+	}
+	return false;
+}
+
+#define ns_ref_active_get_owner(__ns) \
+	do { if (__ns) __ns_ref_active_get_owner(to_ns_common(__ns)); } while (0)
+
+void __ns_ref_active_put_owner(struct ns_common *ns);
+
+static __always_inline void __ns_ref_active_put(struct ns_common *ns)
+{
+	if (atomic_dec_and_test(&ns->__ns_ref_active)) {
+		VFS_WARN_ON_ONCE(is_initial_namespace(ns));
+		VFS_WARN_ON_ONCE(!__ns_ref_read(ns));
+		__ns_ref_active_put_owner(ns);
+	}
+}
+#define ns_ref_active_put(__ns) \
+	do { if (__ns) __ns_ref_active_put(to_ns_common(__ns)); } while (0)
+
+static __always_inline struct ns_common *__must_check ns_get_unless_inactive(struct ns_common *ns)
+{
+	VFS_WARN_ON_ONCE(__ns_ref_active_read(ns) && !__ns_ref_read(ns));
+	if (!__ns_ref_active_read(ns))
+		return NULL;
+	if (!__ns_ref_get(ns))
+		return NULL;
+	return ns;
+}
+
+void __ns_ref_active_resurrect(struct ns_common *ns);
+
+#define ns_ref_active_resurrect(__ns) \
+	do { if (__ns) __ns_ref_active_resurrect(to_ns_common(__ns)); } while (0)
 
 #endif

include/linux/nsfs.h

···
 
 #define current_in_namespace(__ns) (__current_namespace_from_type(__ns) == __ns)
 
+void nsproxy_ns_active_get(struct nsproxy *ns);
+void nsproxy_ns_active_put(struct nsproxy *ns);
+
 #endif /* _LINUX_NSFS_H */

include/linux/nsproxy.h

···
  */
 
 int copy_namespaces(u64 flags, struct task_struct *tsk);
+void switch_cred_namespaces(const struct cred *old, const struct cred *new);
 void exit_nsproxy_namespaces(struct task_struct *tsk);
+void get_cred_namespaces(struct task_struct *tsk);
+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);

kernel/cred.c

···
 		kdebug("share_creds(%p{%ld})",
 		       p->cred, atomic_long_read(&p->cred->usage));
 		inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
+		get_cred_namespaces(p);
 		return 0;
 	}
 
···
 
 	p->cred = p->real_cred = get_cred(new);
 	inc_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
+	get_cred_namespaces(p);
+
 	return 0;
 
 error_put:
···
 	 */
 	if (new->user != old->user || new->user_ns != old->user_ns)
 		inc_rlimit_ucounts(new->ucounts, UCOUNT_RLIMIT_NPROC, 1);
+
 	rcu_assign_pointer(task->real_cred, new);
 	rcu_assign_pointer(task->cred, new);
 	if (new->user != old->user || new->user_ns != old->user_ns)
 		dec_rlimit_ucounts(old->ucounts, UCOUNT_RLIMIT_NPROC, 1);
+	if (new->user_ns != old->user_ns)
+		switch_cred_namespaces(old, new);
 
 	/* send notifications */
 	if (!uid_eq(new->uid,   old->uid)  ||

kernel/exit.c

···
 	write_unlock_irq(&tasklist_lock);
 	/* @thread_pid can't go away until free_pids() below */
 	proc_flush_pid(thread_pid);
+	exit_cred_namespaces(p);
 	add_device_randomness(&p->se.sum_exec_runtime,
 			      sizeof(p->se.sum_exec_runtime));
 	free_pids(post.pids);

kernel/fork.c

···
 	delayacct_tsk_free(p);
 bad_fork_cleanup_count:
 	dec_rlimit_ucounts(task_ucounts(p), UCOUNT_RLIMIT_NPROC, 1);
+	exit_cred_namespaces(p);
 	exit_creds(p);
 bad_fork_free:
 	WRITE_ONCE(p->__state, TASK_DEAD);

kernel/nscommon.c

···
 
 #include <linux/ns_common.h>
 #include <linux/proc_ns.h>
+#include <linux/user_namespace.h>
 #include <linux/vfsdebug.h>
 
 #ifdef CONFIG_DEBUG_VFS
···
 
 int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_operations *ops, int inum)
 {
+	int ret;
+
 	refcount_set(&ns->__ns_ref, 1);
 	ns->stashed = NULL;
 	ns->ops = ops;
···
 		ns->inum = inum;
 		return 0;
 	}
-	return proc_alloc_inum(&ns->inum);
+	ret = proc_alloc_inum(&ns->inum);
+	if (ret)
+		return ret;
+	/*
+	 * Tree ref starts at 0. It's incremented when namespace enters
+	 * active use (installed in nsproxy) and decremented when all
+	 * active uses are gone. Initial namespaces are always active.
+	 */
+	if (is_initial_namespace(ns))
+		atomic_set(&ns->__ns_ref_active, 1);
+	else
+		atomic_set(&ns->__ns_ref_active, 0);
+	return 0;
 }
 
 void __ns_common_free(struct ns_common *ns)
 {
 	proc_free_inum(ns->inum);
+}
+
+static struct ns_common *ns_owner(struct ns_common *ns)
+{
+	struct user_namespace *owner;
+
+	if (unlikely(!ns->ops))
+		return NULL;
+	VFS_WARN_ON_ONCE(!ns->ops->owner);
+	owner = ns->ops->owner(ns);
+	VFS_WARN_ON_ONCE(!owner && ns != to_ns_common(&init_user_ns));
+	if (!owner)
+		return NULL;
+	/* Skip init_user_ns as it's always active */
+	if (owner == &init_user_ns)
+		return NULL;
+	return to_ns_common(owner);
+}
+
+void __ns_ref_active_get_owner(struct ns_common *ns)
+{
+	ns = ns_owner(ns);
+	if (ns)
+		WARN_ON_ONCE(atomic_add_negative(1, &ns->__ns_ref_active));
+}
+
+/*
+ * The active reference count works by having each namespace that gets
+ * created take a single active reference on its owning user namespace.
+ * That single reference is only released once the child namespace's
+ * active count itself goes down.
+ *
+ * A regular namespace tree might look as follow:
+ * Legend:
+ * + : adding active reference
+ * - : dropping active reference
+ * x : always active (initial namespace)
+ *
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        +      +
+ *                        user_ns1 (2)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        +   +   +
+ *                        user_ns2 (3)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * If both net_ns and pid_ns put their last active reference on
+ * themselves it will cascade to user_ns1 dropping its own active
+ * reference and dropping one active reference on user_ns2:
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        -      -
+ *                        user_ns1 (0)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        +   -   +
+ *                        user_ns2 (2)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * The iteration stops once we reach a namespace that still has active
+ * references.
+ */
+void __ns_ref_active_put_owner(struct ns_common *ns)
+{
+	for (;;) {
+		ns = ns_owner(ns);
+		if (!ns)
+			return;
+		if (!atomic_dec_and_test(&ns->__ns_ref_active))
+			return;
+	}
+}
+
+/*
+ * The active reference count works by having each namespace that gets
+ * created take a single active reference on its owning user namespace.
+ * That single reference is only released once the child namespace's
+ * active count itself goes down. This makes it possible to efficiently
+ * resurrect a namespace tree:
+ *
+ * A regular namespace tree might look as follow:
+ * Legend:
+ * + : adding active reference
+ * - : dropping active reference
+ * x : always active (initial namespace)
+ *
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        +      +
+ *                        user_ns1 (2)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        +   +   +
+ *                        user_ns2 (3)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * If both net_ns and pid_ns put their last active reference on
+ * themselves it will cascade to user_ns1 dropping its own active
+ * reference and dropping one active reference on user_ns2:
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        -      -
+ *                        user_ns1 (0)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        +   -   +
+ *                        user_ns2 (2)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * Assume the whole tree is dead but all namespaces are still active:
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        -      -
+ *                        user_ns1 (0)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        -   -   -
+ *                        user_ns2 (0)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * Now assume the net_ns gets resurrected (.e.g., via the SIOCGSKNS ioctl()):
+ *
+ *                 net_ns          pid_ns
+ *                       \        /
+ *                        +      -
+ *                        user_ns1 (0)
+ *                            |
+ *                 ipc_ns     |     uts_ns
+ *                       \    |    /
+ *                        -   +   -
+ *                        user_ns2 (0)
+ *                            |
+ *            cgroup_ns       |       mnt_ns
+ *                     \      |      /
+ *                      x     x     x
+ *                      init_user_ns (1)
+ *
+ * If net_ns had a zero reference count and we bumped it we also need to
+ * take another reference on its owning user namespace. Similarly, if
+ * pid_ns had a zero reference count it also needs to take another
+ * reference on its owning user namespace. So both net_ns and pid_ns
+ * will each have their own reference on the owning user namespace.
+ *
+ * If the owning user namespace user_ns1 had a zero reference count then
+ * it also needs to take another reference on its owning user namespace
+ * and so on.
+ */
+void __ns_ref_active_resurrect(struct ns_common *ns)
+{
+	/* If we didn't resurrect the namespace we're done. */
+	if (atomic_fetch_add(1, &ns->__ns_ref_active))
+		return;
+
+	/*
+	 * We did resurrect it. Walk the ownership hierarchy upwards
+	 * until we found an owning user namespace that is active.
+	 */
+	for (;;) {
+		ns = ns_owner(ns);
+		if (!ns)
+			return;
+
+		if (atomic_fetch_add(1, &ns->__ns_ref_active))
+			return;
+	}
 }

kernel/nsproxy.c

···
 #include <linux/syscalls.h>
 #include <linux/cgroup.h>
 #include <linux/perf_event.h>
+#include <linux/nstree.h>
 
 static struct kmem_cache *nsproxy_cachep;
 
···
 	if ((flags & CLONE_VM) == 0)
 		timens_on_fork(new_ns, tsk);
 
+	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);
···
 
 	might_sleep();
 
+	if (new)
+		nsproxy_ns_active_get(new);
+
 	task_lock(p);
 	ns = p->nsproxy;
 	p->nsproxy = new;
···
 void exit_nsproxy_namespaces(struct task_struct *p)
 {
 	switch_task_namespaces(p, NULL);
+}
+
+void switch_cred_namespaces(const struct cred *old, const struct cred *new)
+{
+	ns_ref_active_get(new->user_ns);
+	ns_ref_active_put(old->user_ns);
+}
+
+void get_cred_namespaces(struct task_struct *tsk)
+{
+	ns_ref_active_get(tsk->real_cred->user_ns);
+}
+
+void exit_cred_namespaces(struct task_struct *tsk)
+{
+	ns_ref_active_put(tsk->real_cred->user_ns);
 }
 
 int exec_task_namespaces(void)

kernel/nstree.c

···
 	write_sequnlock(&ns_tree->ns_tree_lock);
 
 	VFS_WARN_ON_ONCE(node);
+
+	/*
+	 * Take an active reference on the owner namespace. This ensures
+	 * that the owner remains visible while any of its child namespaces
+	 * are active. For init namespaces this is a no-op as ns_owner()
+	 * returns NULL for namespaces owned by init_user_ns.
+	 */
+	__ns_ref_active_get_owner(ns);
 }
 
 void __ns_tree_remove(struct ns_common *ns, struct ns_tree *ns_tree)

kernel/pid.c

···
 void free_pid(struct pid *pid)
 {
 	int i;
+	struct pid_namespace *active_ns;
 
 	lockdep_assert_not_held(&tasklist_lock);
+
+	active_ns = pid->numbers[pid->level].ns;
+	ns_ref_active_put(active_ns);
 
 	spin_lock(&pidmap_lock);
 	for (i = 0; i <= pid->level; i++) {
···
 	}
 	spin_unlock(&pidmap_lock);
 	idr_preload_end();
+	ns_ref_active_get(ns);
 
 	return pid;