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Subject[PATCH 3/5] nsfs: add ioctl to get an owning user namespace for ns file descriptor
Date
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.

Link: https://lkml.org/lkml/2016/7/6/158
Signed-off-by: Andrey Vagin <avagin@openvz.org>
---
fs/nsfs.c | 94 ++++++++++++++++++++++++++++++++++++++++-------
include/uapi/linux/nsfs.h | 9 +++++
2 files changed, 90 insertions(+), 13 deletions(-)
create mode 100644 include/uapi/linux/nsfs.h

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 8f20d60..1e5d2d0 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 +92,88 @@ 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;
+}
+
+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 *parent;
+
+ parent = get_ns(ns);
+ if (IS_ERR(parent)) {
+ put_unused_fd(fd);
+ return PTR_ERR(parent);
+ }
+
+ err = __ns_get_path(&path, parent);
+ 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..7a09ede
--- /dev/null
+++ b/include/uapi/linux/nsfs.h
@@ -0,0 +1,9 @@
+#ifndef __LINUX_NSFS_H
+#define __LINUX_NSFS_H
+
+#include <linux/ioctl.h>
+
+#define NSIO 0xb7
+#define NS_GET_USERNS _IO(NSIO, 0x1)
+
+#endif /* __LINUX_NSFS_H */
--
2.5.5
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