[RFC PATCH 08/27] containers, vfs: Honour CONTAINER_NEW_EMPTY_FS_NS

David Howells
2019-02-15T16:08:43Z

Allow a container to be created with an empty mount namespace, as specified
by passing CONTAINER_NEW_EMPTY_FS_NS to container_create(), and allow a
root filesystem to be mounted into the container:

cfd = container_create("foo", CONTAINER_NEW_EMPTY_FS_NS);

fsfd = fsopen("ext3", 0);
fsconfig(fsfd, FSCONFIG_SET_CONTAINER, NULL, NULL, cfd);
fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "/dev/sda3", 0);
fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
...
rfd = fsmount(fsfd, 0, 0);
move_mount(rfd, "", cfd, "/",
MOVE_MOUNT_F_EMPTY_PATH | MOVE_MOUNT_T_CONTAINER_ROOT);

pfd = fsopen("proc", 0);
write(pfd, "n c=<cfd>");
...
procfd = fsmount(pfd, 0, 0);
move_mount(procfd, "", cfd, "proc", MOVE_MOUNT_F_EMPTY_PATH);

Signed-off-by: David Howells <dhowells@redhat.com>
---

fs/namespace.c | 95 +++++++++++++++++++++++++++++++++++++++-----
include/uapi/linux/mount.h | 3 +
kernel/container.c | 6 +++
kernel/fork.c | 6 ++-
4 files changed, 97 insertions(+), 13 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index cc5d56f7ae29..22cf4a8f8065 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3513,6 +3513,63 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags,
return ret;
}

+/*
+ * Create a mount namespace for a container and set the root mount in it.
+ */
+static int set_container_root(struct path *path, int fd)
+{
+ struct mnt_namespace *mnt_ns;
+ struct container *container;
+ struct mount *mnt;
+ struct fd f;
+ int ret;
+
+ f = fdget(fd);
+ if (!f.file)
+ return -EBADF;
+ ret = -EINVAL;
+ if (!is_container_file(f.file))
+ goto out_fd;
+
+ ret = -EBUSY;
+ container = f.file->private_data;
+ if (container->ns->mnt_ns)
+ goto out_fd;
+
+ mnt_ns = alloc_mnt_ns(container->cred->user_ns, false);
+ if (IS_ERR(mnt_ns)) {
+ ret = PTR_ERR(mnt_ns);
+ goto out_fd;
+ }
+
+ mnt = real_mount(path->mnt);
+ mnt_add_count(mnt, 1);
+ mnt->mnt_ns = mnt_ns;
+ mnt_ns->root = mnt;
+ mnt_ns->mounts++;
+ list_add(&mnt->mnt_list, &mnt_ns->list);
+
+ ret = -EBUSY;
+ spin_lock(&container->lock);
+ if (!container->ns->mnt_ns) {
+ container->ns->mnt_ns = mnt_ns;
+ write_seqcount_begin(&container->seq);
+ container->root.mnt = path->mnt;
+ container->root.dentry = path->dentry;
+ write_seqcount_end(&container->seq);
+ path_get(&container->root);
+ mnt_ns = NULL;
+ ret = 0;
+ }
+ spin_unlock(&container->lock);
+
+ if (ret < 0)
+ put_mnt_ns(mnt_ns);
+out_fd:
+ fdput(f);
+ return ret;
+}
+
/*
* Move a mount from one place to another. In combination with
* fsopen()/fsmount() this is used to install a new mount and in combination
@@ -3528,6 +3585,7 @@ SYSCALL_DEFINE5(move_mount,
{
struct path from_path, to_path;
unsigned int lflags;
+ char buf[2];
int ret = 0;

if (!may_mount())
@@ -3536,6 +3594,17 @@ SYSCALL_DEFINE5(move_mount,
if (flags & ~MOVE_MOUNT__MASK)
return -EINVAL;

+ if (flags & MOVE_MOUNT_T_CONTAINER_ROOT) {
+ if (flags & (MOVE_MOUNT_T_SYMLINKS |
+ MOVE_MOUNT_T_AUTOMOUNTS |
+ MOVE_MOUNT_T_EMPTY_PATH))
+ return -EINVAL;
+ if (strncpy_from_user(buf, to_pathname, 2) < 0)
+ return -EFAULT;
+ if (buf[0] != '/' || buf[1] != '\0')
+ return -EINVAL;
+ }
+
/* If someone gives a pathname, they aren't permitted to move
* from an fd that requires unmount as we can't get at the flag
* to clear it afterwards.
@@ -3549,20 +3618,24 @@ SYSCALL_DEFINE5(move_mount,
if (ret < 0)
return ret;

- lflags = 0;
- if (flags & MOVE_MOUNT_T_SYMLINKS) lflags |= LOOKUP_FOLLOW;
- if (flags & MOVE_MOUNT_T_AUTOMOUNTS) lflags |= LOOKUP_AUTOMOUNT;
- if (flags & MOVE_MOUNT_T_EMPTY_PATH) lflags |= LOOKUP_EMPTY;
+ if (flags & MOVE_MOUNT_T_CONTAINER_ROOT) {
+ ret = set_container_root(&from_path, to_dfd);
+ } else {
+ lflags = 0;
+ if (flags & MOVE_MOUNT_T_SYMLINKS) lflags |= LOOKUP_FOLLOW;
+ if (flags & MOVE_MOUNT_T_AUTOMOUNTS) lflags |= LOOKUP_AUTOMOUNT;
+ if (flags & MOVE_MOUNT_T_EMPTY_PATH) lflags |= LOOKUP_EMPTY;

- ret = user_path_at(to_dfd, to_pathname, lflags, &to_path);
- if (ret < 0)
- goto out_from;
+ ret = user_path_at(to_dfd, to_pathname, lflags, &to_path);
+ if (ret < 0)
+ goto out_from;

- ret = security_move_mount(&from_path, &to_path);
- if (ret < 0)
- goto out_to;
+ ret = security_move_mount(&from_path, &to_path);
+ if (ret < 0)
+ goto out_to;

- ret = do_move_mount(&from_path, &to_path);
+ ret = do_move_mount(&from_path, &to_path);
+ }

out_to:
path_put(&to_path);
diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index f60bbe6f4099..cfaa75fa0594 100644
--- a/include/uapi/linux/mount.h
+++ b/include/uapi/linux/mount.h
@@ -70,7 +70,8 @@
#define MOVE_MOUNT_T_SYMLINKS 0x00000010 /* Follow symlinks on to path */
#define MOVE_MOUNT_T_AUTOMOUNTS 0x00000020 /* Follow automounts on to path */
#define MOVE_MOUNT_T_EMPTY_PATH 0x00000040 /* Empty to path permitted */
-#define MOVE_MOUNT__MASK 0x00000077
+#define MOVE_MOUNT_T_CONTAINER_ROOT 0x00000080 /* Set as container root */
+#define MOVE_MOUNT__MASK 0x000000f7

/*
* fsopen() flags.
diff --git a/kernel/container.c b/kernel/container.c
index fd3b2a6849a1..360284db959b 100644
--- a/kernel/container.c
+++ b/kernel/container.c
@@ -21,6 +21,7 @@
#include <linux/printk.h>
#include <linux/security.h>
#include <linux/proc_fs.h>
+#include <linux/mnt_namespace.h>
#include "namespaces.h"

struct container init_container = {
@@ -400,6 +401,11 @@ static struct container *create_container(const char __user *name, unsigned int
fs->root.mnt = NULL;
fs->root.dentry = NULL;

+ if (flags & CONTAINER_NEW_EMPTY_FS_NS) {
+ put_mnt_ns(ns->mnt_ns);
+ ns->mnt_ns = NULL;
+ }
+
ret = security_container_alloc(c, flags);
if (ret < 0)
goto err_fs;
diff --git a/kernel/fork.c b/kernel/fork.c
index 09de5f35d312..6ec507a5f739 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2374,7 +2374,11 @@ SYSCALL_DEFINE1(fork_into_container, int, containerfd)
if (is_container_file(f.file)) {
struct container *dest_container = f.file->private_data;

- ret = _do_fork(SIGCHLD, 0, 0, NULL, NULL, 0, dest_container);
+ if (!dest_container->ns->mnt_ns)
+ ret = -ENOENT;
+ else
+ ret = _do_fork(SIGCHLD, 0, 0, NULL, NULL, 0,
+ dest_container);
}
fdput(f);
return ret;