[PATCH 0/4 v2] Add an interface to discover relationships between namespaces

From: Andrey Vagin <redacted>

Each namespace has an owning user namespace and now there is not way
to discover these relationships.

Pid and user namepaces are hierarchical. There is no way to discover
parent-child relationships too.

Why we may want to know relationships between namespaces?

One use would be visualization, in order to understand the running
system.  Another would be to answer the question: what capability does
process X have to perform operations on a resource governed by namespace
Y?

One more use-case (which usually called abnormal) is checkpoint/restart.
In CRIU we are going to dump and restore nested namespaces.

There [1] was a discussion about which interface to choose to determing
relationships between namespaces.

Eric suggested to add two ioctl-s [2]:

Grumble, Grumble.  I think this may actually a case for creating ioctls
for these two cases.  Now that random nsfs file descriptors are bind
mountable the original reason for using proc files is not as pressing.

One ioctl for the user namespace that owns a file descriptor.
One ioctl for the parent namespace of a namespace file descriptor.

Here is an implementaions of these ioctl-s.

$ man man7/namespaces.7
...
Since  Linux  4.X,  the  following  ioctl(2)  calls are supported for
namespace file descriptors.  The correct syntax is:

      fd = ioctl(ns_fd, ioctl_type);

where ioctl_type is one of the following:

NS_GET_USERNS
      Returns a file descriptor that refers to an owning user names‐
      pace.

NS_GET_PARENT
      Returns  a  file descriptor that refers to a parent namespace.
      This ioctl(2) can be used for pid  and  user  namespaces.  For
      user namespaces, NS_GET_PARENT and NS_GET_USERNS have the same
      meaning.

In addition to generic ioctl(2) errors, the following  specific  ones
can occur:

EINVAL NS_GET_PARENT was called for a nonhierarchical namespace.

EPERM  The  requested  namespace  is outside of the current namespace
      scope.

[1] https://lkml.org/lkml/2016/7/6/158
[2] https://lkml.org/lkml/2016/7/9/101

Changes for v2:
* don't return ENOENT for init_user_ns and init_pid_ns. There is nothing
  outside of the init namespace, so we can return EPERM in this case too.

The fewer special cases the easier the code is to get
correct, and the easier it is to read. // Eric

Cc: "Eric W. Biederman" <redacted>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: "Michael Kerrisk (man-pages)" <redacted>
Cc: "W. Trevor King" <redacted>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Serge Hallyn <redacted>

--
2.5.5

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From: Andrey Vagin <redacted>

Pid and user namepaces are hierarchical. There is no way to discover
parent-child relationships.

In a future we will use this interface to dump and restore nested
namespaces.

Signed-off-by: Andrei Vagin <redacted>
---
 fs/nsfs.c                 |  4 ++++
 include/linux/proc_ns.h   |  1 +
 include/uapi/linux/nsfs.h |  2 ++
 kernel/pid_namespace.c    | 19 +++++++++++++++++++
 kernel/user_namespace.c   |  1 +
 5 files changed, 27 insertions(+)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index be7d193..11a4b15 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c

@@ -170,6 +170,10 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
 	switch (ioctl) {
 	case NS_GET_USERNS:
 		return open_related_ns(ns, ns_get_owner);
+	case NS_GET_PARENT:
+		if (!ns->ops->get_parent)
+			return -EINVAL;
+		return open_related_ns(ns, ns->ops->get_parent);
 	default:
 		return -ENOTTY;
 	}

diff --git a/include/linux/proc_ns.h b/include/linux/proc_ns.h
index cfc8be7..e4a904b 100644
--- a/include/linux/proc_ns.h
+++ b/include/linux/proc_ns.h

@@ -19,6 +19,7 @@ struct proc_ns_operations {
 	void (*put)(struct ns_common *ns);
 	int (*install)(struct nsproxy *nsproxy, struct ns_common *ns);
 	struct user_namespace *(*get_owner)(struct ns_common *ns);
+	struct ns_common *(*get_parent)(struct ns_common *ns);
 };
 
 extern const struct proc_ns_operations netns_operations;

diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
index 5cacd5c..3af6172 100644
--- a/include/uapi/linux/nsfs.h
+++ b/include/uapi/linux/nsfs.h

@@ -7,5 +7,7 @@
 
 /* Returns a file descriptor that refers to an owning user namespace */
 #define NS_GET_USERNS	_IO(NSIO, 0x1)
+/* Returns a file descriptor that refers to a parent namespace */
+#define NS_GET_PARENT	_IO(NSIO, 0x2)
 
 #endif /* __LINUX_NSFS_H */

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 1860d9d..6cd473c 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c

@@ -405,6 +405,24 @@ static int pidns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 	return 0;
 }
 
+static struct ns_common *pidns_get_parent(struct ns_common *ns)
+{
+	struct pid_namespace *active = task_active_pid_ns(current);
+	struct pid_namespace *pid_ns, *p;
+
+	/* See if the parent is in the current namespace */
+	pid_ns = p = to_pid_ns(ns)->parent;
+	for (;;) {
+		if (!p)
+			return ERR_PTR(-EPERM);
+		if (p == active)
+			break;
+		p = p->parent;
+	}
+
+	return &get_pid_ns(pid_ns)->ns;
+}
+
 static struct user_namespace *pidns_get_owner(struct ns_common *ns)
 {
 	return to_pid_ns(ns)->user_ns;

@@ -417,6 +435,7 @@ const struct proc_ns_operations pidns_operations = {
 	.put		= pidns_put,
 	.install	= pidns_install,
 	.get_owner	= pidns_get_owner,
+	.get_parent	= pidns_get_parent,
 };
 
 static __init int pid_namespaces_init(void)

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index f0521e4..1516c47 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c

@@ -1080,6 +1080,7 @@ const struct proc_ns_operations userns_operations = {
 	.put		= userns_put,
 	.install	= userns_install,
 	.get_owner	= userns_get_owner,
+	.get_parent	= ns_get_owner,
 };
 
 static __init int user_namespaces_init(void)

-- 
2.5.5

From: Andrey Vagin <redacted>

Return -EPERM if an owning user namespace is outside of a process
current user namespace.

v2: In a first version ns_get_owner returned ENOENT for init_user_ns.
    This special cases was removed from this version. There is nothing
    outside of init_user_ns, so we can return EPERM.

Signed-off-by: Andrei Vagin <redacted>
---
 fs/namespace.c                 |  6 ++++++
 include/linux/proc_ns.h        |  1 +
 include/linux/user_namespace.h |  7 +++++++
 ipc/namespace.c                |  6 ++++++
 kernel/cgroup.c                |  6 ++++++
 kernel/pid_namespace.c         |  6 ++++++
 kernel/user_namespace.c        | 24 ++++++++++++++++++++++++
 kernel/utsname.c               |  6 ++++++
 net/core/net_namespace.c       |  6 ++++++
 9 files changed, 68 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 491b8f3..f985817 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c

@@ -3368,10 +3368,16 @@ static int mntns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 	return 0;
 }
 
+static struct user_namespace *mntns_get_owner(struct ns_common *ns)
+{
+	return to_mnt_ns(ns)->user_ns;
+}
+
 const struct proc_ns_operations mntns_operations = {
 	.name		= "mnt",
 	.type		= CLONE_NEWNS,
 	.get		= mntns_get,
 	.put		= mntns_put,
 	.install	= mntns_install,
+	.get_owner	= mntns_get_owner,
 };

diff --git a/include/linux/proc_ns.h b/include/linux/proc_ns.h
index de0e771..cfc8be7 100644
--- a/include/linux/proc_ns.h
+++ b/include/linux/proc_ns.h

@@ -18,6 +18,7 @@ struct proc_ns_operations {
 	struct ns_common *(*get)(struct task_struct *task);
 	void (*put)(struct ns_common *ns);
 	int (*install)(struct nsproxy *nsproxy, struct ns_common *ns);
+	struct user_namespace *(*get_owner)(struct ns_common *ns);
 };
 
 extern const struct proc_ns_operations netns_operations;

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 30ffe10..eb209d4 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h

@@ -106,6 +106,8 @@ extern ssize_t proc_setgroups_write(struct file *, const char __user *, size_t,
 extern int proc_setgroups_show(struct seq_file *m, void *v);
 extern bool userns_may_setgroups(const struct user_namespace *ns);
 extern bool current_in_userns(const struct user_namespace *target_ns);
+
+struct ns_common *ns_get_owner(struct ns_common *ns);
 #else
 
 static inline struct user_namespace *get_user_ns(struct user_namespace *ns)

@@ -139,6 +141,11 @@ static inline bool current_in_userns(const struct user_namespace *target_ns)
 {
 	return true;
 }
+
+static inline struct ns_common *ns_get_owner(struct ns_common *ns)
+{
+	return ERR_PTR(-EPERM);
+}
 #endif
 
 #endif /* _LINUX_USER_H */

diff --git a/ipc/namespace.c b/ipc/namespace.c
index 7309142..991a31e 100644
--- a/ipc/namespace.c
+++ b/ipc/namespace.c

@@ -188,10 +188,16 @@ static int ipcns_install(struct nsproxy *nsproxy, struct ns_common *new)
 	return 0;
 }
 
+static struct user_namespace *ipcns_get_owner(struct ns_common *ns)
+{
+	return to_ipc_ns(ns)->user_ns;
+}
+
 const struct proc_ns_operations ipcns_operations = {
 	.name		= "ipc",
 	.type		= CLONE_NEWIPC,
 	.get		= ipcns_get,
 	.put		= ipcns_put,
 	.install	= ipcns_install,
+	.get_owner	= ipcns_get_owner,
 };

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index e9e4427..4204693 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c

@@ -6421,12 +6421,18 @@ static void cgroupns_put(struct ns_common *ns)
 	put_cgroup_ns(to_cg_ns(ns));
 }
 
+static struct user_namespace *cgroupns_get_owner(struct ns_common *ns)
+{
+	return to_cg_ns(ns)->user_ns;
+}
+
 const struct proc_ns_operations cgroupns_operations = {
 	.name		= "cgroup",
 	.type		= CLONE_NEWCGROUP,
 	.get		= cgroupns_get,
 	.put		= cgroupns_put,
 	.install	= cgroupns_install,
+	.get_owner	= cgroupns_get_owner,
 };
 
 static __init int cgroup_namespaces_init(void)

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 30a7f33..1860d9d 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c

@@ -405,12 +405,18 @@ static int pidns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 	return 0;
 }
 
+static struct user_namespace *pidns_get_owner(struct ns_common *ns)
+{
+	return to_pid_ns(ns)->user_ns;
+}
+
 const struct proc_ns_operations pidns_operations = {
 	.name		= "pid",
 	.type		= CLONE_NEWPID,
 	.get		= pidns_get,
 	.put		= pidns_put,
 	.install	= pidns_install,
+	.get_owner	= pidns_get_owner,
 };
 
 static __init int pid_namespaces_init(void)

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 0edafe3..f0521e4 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c

@@ -1050,12 +1050,36 @@ static int userns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 	return commit_creds(cred);
 }
 
+struct ns_common *ns_get_owner(struct ns_common *ns)
+{
+	struct user_namespace *my_user_ns = current_user_ns();
+	struct user_namespace *owner, *p;
+
+	/* See if the owner is in the current user namespace */
+	owner = p = ns->ops->get_owner(ns);
+	for (;;) {
+		if (!p)
+			return ERR_PTR(-EPERM);
+		if (p == my_user_ns)
+			break;
+		p = p->parent;
+	}
+
+	return &get_user_ns(owner)->ns;
+}
+
+static struct user_namespace *userns_get_owner(struct ns_common *ns)
+{
+	return to_user_ns(ns)->parent;
+}
+
 const struct proc_ns_operations userns_operations = {
 	.name		= "user",
 	.type		= CLONE_NEWUSER,
 	.get		= userns_get,
 	.put		= userns_put,
 	.install	= userns_install,
+	.get_owner	= userns_get_owner,
 };
 
 static __init int user_namespaces_init(void)

diff --git a/kernel/utsname.c b/kernel/utsname.c
index f3b0bb4..3b3028f 100644
--- a/kernel/utsname.c
+++ b/kernel/utsname.c

@@ -154,10 +154,16 @@ static int utsns_install(struct nsproxy *nsproxy, struct ns_common *new)
 	return 0;
 }
 
+static struct user_namespace *utsns_get_owner(struct ns_common *ns)
+{
+	return to_uts_ns(ns)->user_ns;
+}
+
 const struct proc_ns_operations utsns_operations = {
 	.name		= "uts",
 	.type		= CLONE_NEWUTS,
 	.get		= utsns_get,
 	.put		= utsns_put,
 	.install	= utsns_install,
+	.get_owner	= utsns_get_owner,
 };

diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index 3e2812a..1ccba4b 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c

@@ -1016,11 +1016,17 @@ static int netns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 	return 0;
 }
 
+static struct user_namespace *netns_get_owner(struct ns_common *ns)
+{
+	return to_net_ns(ns)->user_ns;
+}
+
 const struct proc_ns_operations netns_operations = {
 	.name		= "net",
 	.type		= CLONE_NEWNET,
 	.get		= netns_get,
 	.put		= netns_put,
 	.install	= netns_install,
+	.get_owner	= netns_get_owner,
 };
 #endif

-- 
2.5.5

From: Andrey Vagin <redacted>

Each namespace has an owning user namespace and now there is not way
to discover these relationships.

Understending namespaces relationships allows to answer the question:
what capability does process X have to perform operations on a resource
governed by namespace Y?

After a long discussion, Eric W. Biederman proposed to use ioctl-s for
this purpose.

The NS_GET_USERNS ioctl returns a file descriptor to an owning user
namespace.
It returns EPERM if a target namespace is outside of a current user
namespace.

v2: rename parent to relative

Link: https://lkml.org/lkml/2016/7/6/158
Signed-off-by: Andrei Vagin <redacted>
---
 fs/nsfs.c                 | 95 ++++++++++++++++++++++++++++++++++++++++-------
 include/uapi/linux/nsfs.h | 11 ++++++
 2 files changed, 93 insertions(+), 13 deletions(-)
 create mode 100644 include/uapi/linux/nsfs.h

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 8f20d60..be7d193 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c

@@ -5,11 +5,16 @@
 #include <linux/magic.h>
 #include <linux/ktime.h>
 #include <linux/seq_file.h>
+#include <linux/user_namespace.h>
+#include <linux/nsfs.h>
 
 static struct vfsmount *nsfs_mnt;
 
+static long ns_ioctl(struct file *filp, unsigned int ioctl,
+			unsigned long arg);
 static const struct file_operations ns_file_operations = {
 	.llseek		= no_llseek,
+	.unlocked_ioctl = ns_ioctl,
 };
 
 static char *ns_dname(struct dentry *dentry, char *buffer, int buflen)

@@ -44,22 +49,14 @@ static void nsfs_evict(struct inode *inode)
 	ns->ops->put(ns);
 }
 
-void *ns_get_path(struct path *path, struct task_struct *task,
-			const struct proc_ns_operations *ns_ops)
+static void *__ns_get_path(struct path *path, struct ns_common *ns)
 {
 	struct vfsmount *mnt = mntget(nsfs_mnt);
 	struct qstr qname = { .name = "", };
 	struct dentry *dentry;
 	struct inode *inode;
-	struct ns_common *ns;
 	unsigned long d;
 
-again:
-	ns = ns_ops->get(task);
-	if (!ns) {
-		mntput(mnt);
-		return ERR_PTR(-ENOENT);
-	}
 	rcu_read_lock();
 	d = atomic_long_read(&ns->stashed);
 	if (!d)

@@ -68,7 +65,7 @@ again:
 	if (!lockref_get_not_dead(&dentry->d_lockref))
 		goto slow;
 	rcu_read_unlock();
-	ns_ops->put(ns);
+	ns->ops->put(ns);
 got_it:
 	path->mnt = mnt;
 	path->dentry = dentry;

@@ -77,7 +74,7 @@ slow:
 	rcu_read_unlock();
 	inode = new_inode_pseudo(mnt->mnt_sb);
 	if (!inode) {
-		ns_ops->put(ns);
+		ns->ops->put(ns);
 		mntput(mnt);
 		return ERR_PTR(-ENOMEM);
 	}

@@ -95,17 +92,89 @@ slow:
 		return ERR_PTR(-ENOMEM);
 	}
 	d_instantiate(dentry, inode);
-	dentry->d_fsdata = (void *)ns_ops;
+	dentry->d_fsdata = (void *)ns->ops;
 	d = atomic_long_cmpxchg(&ns->stashed, 0, (unsigned long)dentry);
 	if (d) {
 		d_delete(dentry);	/* make sure ->d_prune() does nothing */
 		dput(dentry);
 		cpu_relax();
-		goto again;
+		return ERR_PTR(-EAGAIN);
 	}
 	goto got_it;
 }
 
+void *ns_get_path(struct path *path, struct task_struct *task,
+			const struct proc_ns_operations *ns_ops)
+{
+	struct ns_common *ns;
+	void *ret;
+
+again:
+	ns = ns_ops->get(task);
+	if (!ns)
+		return ERR_PTR(-ENOENT);
+
+	ret = __ns_get_path(path, ns);
+	if (IS_ERR(ret) && PTR_ERR(ret) == -EAGAIN)
+		goto again;
+	return ret;
+}
+
+static int open_related_ns(struct ns_common *ns,
+		   struct ns_common *(*get_ns)(struct ns_common *ns))
+{
+	struct path path = {};
+	struct file *f;
+	void *err;
+	int fd;
+
+	fd = get_unused_fd_flags(O_CLOEXEC);
+	if (fd < 0)
+		return fd;
+
+	while (1) {
+		struct ns_common *relative;
+
+		relative = get_ns(ns);
+		if (IS_ERR(relative)) {
+			put_unused_fd(fd);
+			return PTR_ERR(relative);
+		}
+
+		err = __ns_get_path(&path, relative);
+		if (IS_ERR(err) && PTR_ERR(err) == -EAGAIN)
+			continue;
+		break;
+	}
+	if (IS_ERR(err)) {
+		put_unused_fd(fd);
+		return PTR_ERR(err);
+	}
+
+	f = dentry_open(&path, O_RDONLY, current_cred());
+	path_put(&path);
+	if (IS_ERR(f)) {
+		put_unused_fd(fd);
+		fd = PTR_ERR(f);
+	} else
+		fd_install(fd, f);
+
+	return fd;
+}
+
+static long ns_ioctl(struct file *filp, unsigned int ioctl,
+			unsigned long arg)
+{
+	struct ns_common *ns = get_proc_ns(file_inode(filp));
+
+	switch (ioctl) {
+	case NS_GET_USERNS:
+		return open_related_ns(ns, ns_get_owner);
+	default:
+		return -ENOTTY;
+	}
+}
+
 int ns_get_name(char *buf, size_t size, struct task_struct *task,
 			const struct proc_ns_operations *ns_ops)
 {

diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
new file mode 100644
index 0000000..5cacd5c
--- /dev/null
+++ b/include/uapi/linux/nsfs.h

@@ -0,0 +1,11 @@
+#ifndef __LINUX_NSFS_H
+#define __LINUX_NSFS_H
+
+#include <linux/ioctl.h>
+
+#define NSIO	0xb7
+
+/* Returns a file descriptor that refers to an owning user namespace */
+#define NS_GET_USERNS	_IO(NSIO, 0x1)
+
+#endif /* __LINUX_NSFS_H */

-- 
2.5.5

On Fri, Aug 26, 2016 at 04:08:08PM -0700, Andrei Vagin wrote:

Hi - sorry to be pedantic here, but *_get_owner makes me think
it will grab a reference too.  A bit unfortunate, maybe it doesn't
matter, but would mntns_owner(), netns_owner(), etc be better?

On Fri, Aug 26, 2016 at 04:08:08PM -0700, Andrei Vagin wrote:

get_user_ns() bumps the owner's refcount.  I don't see where
this is being dropped, especially when ns_ioctl() uses it in
the next patch.

On Tue, Aug 30, 2016 at 7:56 PM, Serge E. Hallyn [off-list ref] wrote:

On Fri, Aug 26, 2016 at 04:08:08PM -0700, Andrei Vagin wrote:

quoted

+struct ns_common *ns_get_owner(struct ns_common *ns)
+{
+     struct user_namespace *my_user_ns = current_user_ns();
+     struct user_namespace *owner, *p;
+
+     /* See if the owner is in the current user namespace */
+     owner = p = ns->ops->get_owner(ns);
+     for (;;) {
+             if (!p)
+                     return ERR_PTR(-EPERM);
+             if (p == my_user_ns)
+                     break;
+             p = p->parent;
+     }
+
+     return &get_user_ns(owner)->ns;

get_user_ns() bumps the owner's refcount.  I don't see where
this is being dropped, especially when ns_ioctl() uses it in
the next patch.

It is dropped in __ns_get_path if a namespace has a dentry, otherwise
it is dropped from nsfs_evict.

static void *__ns_get_path(struct path *path, struct ns_common *ns)
        |                return -EPERM;
...
        ns->ops->put(ns);                                                      |
got_it:
        |        /* See if the owner is in the current user namespace
*/
        path->mnt = mnt;
        |        owner = p = ns->ops->get_owner(ns);
        path->dentry = dentry;
        |        for (;;) {
        return NULL;
...

static void nsfs_evict(struct inode *inode)                                    |
{
        |        if (!ns_capable(user_ns, CAP_SYS_ADMIN))
        struct ns_common *ns = inode->i_private;
        |                return -EPERM;
        clear_inode(inode);                                                    |
        ns->ops->put(ns);
        |        cred = prepare_creds();
}

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On Tue, Aug 30, 2016 at 7:41 PM, Serge E. Hallyn [off-list ref] wrote:

On Fri, Aug 26, 2016 at 04:08:08PM -0700, Andrei Vagin wrote:

quoted

From: Andrey Vagin <redacted>

Return -EPERM if an owning user namespace is outside of a process
current user namespace.

v2: In a first version ns_get_owner returned ENOENT for init_user_ns.
    This special cases was removed from this version. There is nothing
    outside of init_user_ns, so we can return EPERM.

Signed-off-by: Andrei Vagin <redacted>
---
 fs/namespace.c                 |  6 ++++++
 include/linux/proc_ns.h        |  1 +
 include/linux/user_namespace.h |  7 +++++++
 ipc/namespace.c                |  6 ++++++
 kernel/cgroup.c                |  6 ++++++
 kernel/pid_namespace.c         |  6 ++++++
 kernel/user_namespace.c        | 24 ++++++++++++++++++++++++
 kernel/utsname.c               |  6 ++++++
 net/core/net_namespace.c       |  6 ++++++
 9 files changed, 68 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 491b8f3..f985817 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c

@@ -3368,10 +3368,16 @@ static int mntns_install(struct nsproxy *nsproxy, struct ns_common *ns)
      return 0;
 }

+static struct user_namespace *mntns_get_owner(struct ns_common *ns)
+{
+     return to_mnt_ns(ns)->user_ns;

Hi - sorry to be pedantic here, but *_get_owner makes me think
it will grab a reference too.  A bit unfortunate, maybe it doesn't
matter, but would mntns_owner(), netns_owner(), etc be better?

Actually it looks reasonable. Especially, when we have get_parent()
which grabs a reference.

Serge, could you look at the attached patch? I will need to rebase
other patches, if we accept this patch.

Thanks,
Andrei

Quoting Andrey Vagin (avagin@openvz.org):

On Tue, Aug 30, 2016 at 7:41 PM, Serge E. Hallyn [off-list ref] wrote:

quoted

On Fri, Aug 26, 2016 at 04:08:08PM -0700, Andrei Vagin wrote:

quoted

From: Andrey Vagin <redacted>

Return -EPERM if an owning user namespace is outside of a process
current user namespace.

v2: In a first version ns_get_owner returned ENOENT for init_user_ns.
    This special cases was removed from this version. There is nothing
    outside of init_user_ns, so we can return EPERM.

Signed-off-by: Andrei Vagin <redacted>
---
 fs/namespace.c                 |  6 ++++++
 include/linux/proc_ns.h        |  1 +
 include/linux/user_namespace.h |  7 +++++++
 ipc/namespace.c                |  6 ++++++
 kernel/cgroup.c                |  6 ++++++
 kernel/pid_namespace.c         |  6 ++++++
 kernel/user_namespace.c        | 24 ++++++++++++++++++++++++
 kernel/utsname.c               |  6 ++++++
 net/core/net_namespace.c       |  6 ++++++
 9 files changed, 68 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 491b8f3..f985817 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c

@@ -3368,10 +3368,16 @@ static int mntns_install(struct nsproxy *nsproxy, struct ns_common *ns)
      return 0;
 }

+static struct user_namespace *mntns_get_owner(struct ns_common *ns)
+{
+     return to_mnt_ns(ns)->user_ns;

Hi - sorry to be pedantic here, but *_get_owner makes me think
it will grab a reference too.  A bit unfortunate, maybe it doesn't
matter, but would mntns_owner(), netns_owner(), etc be better?

Actually it looks reasonable. Especially, when we have get_parent()
which grabs a reference.

Serge, could you look at the attached patch? I will need to rebase
other patches, if we accept this patch.

Thanks,
Andrei

From 423b2226dc9bcfefccbec21663e9287addd67e0f Mon Sep 17 00:00:00 2001
From: Andrey Vagin <redacted>
Date: Tue, 12 Jul 2016 15:32:36 -0700
Subject: [PATCH] kernel: add a helper to get an owning user namespace for a
 namespace

Return -EPERM if an owning user namespace is outside of a process
current user namespace.

v2: In a first version ns_get_owner returned ENOENT for init_user_ns.
    This special cases was removed from this version. There is nothing
    outside of init_user_ns, so we can return EPERM.
v3: rename ns->get_owner() to ns->owner(). get_* usually means that it
grabs a reference.

Thanks, that should make it easier for a simpleminded fool
like myself to follow the references :)

Signed-off-by: Andrei Vagin <redacted>

Acked-by: Serge Hallyn <serge@hallyn.com>

On Wed, Aug 31, 2016 at 01:38:35PM -0700, Andrey Vagin wrote:

On Tue, Aug 30, 2016 at 7:56 PM, Serge E. Hallyn [off-list ref] wrote:

quoted

On Fri, Aug 26, 2016 at 04:08:08PM -0700, Andrei Vagin wrote:

quoted

+struct ns_common *ns_get_owner(struct ns_common *ns)
+{
+     struct user_namespace *my_user_ns = current_user_ns();
+     struct user_namespace *owner, *p;
+
+     /* See if the owner is in the current user namespace */
+     owner = p = ns->ops->get_owner(ns);
+     for (;;) {
+             if (!p)
+                     return ERR_PTR(-EPERM);
+             if (p == my_user_ns)
+                     break;
+             p = p->parent;
+     }
+
+     return &get_user_ns(owner)->ns;

get_user_ns() bumps the owner's refcount.  I don't see where
this is being dropped, especially when ns_ioctl() uses it in
the next patch.

It is dropped in __ns_get_path if a namespace has a dentry, otherwise
it is dropped from nsfs_evict.

static void *__ns_get_path(struct path *path, struct ns_common *ns)
        |                return -EPERM;
...
        ns->ops->put(ns);                                                      |
got_it:
        |        /* See if the owner is in the current user namespace
*/
        path->mnt = mnt;
        |        owner = p = ns->ops->get_owner(ns);
        path->dentry = dentry;
        |        for (;;) {
        return NULL;
...

static void nsfs_evict(struct inode *inode)                                    |
{
        |        if (!ns_capable(user_ns, CAP_SYS_ADMIN))
        struct ns_common *ns = inode->i_private;
        |                return -EPERM;
        clear_inode(inode);                                                    |
        ns->ops->put(ns);
        |        cred = prepare_creds();
}

Gotcha, thanks.