On Mon, Dec 19, 2016 at 03:38:35PM +0100, Michael Kerrisk (man-pages) wrote:
> # Some open questions about this patch below.
> #
> One of the rules regarding capabilities is:
>
> A process that resides in the parent of the user namespace and
> whose effective user ID matches the owner of the namespace has
> all capabilities in the namespace.
>
> Therefore, in order to write code that discovers whether process X has
> capabilities in namespace Y, we need a way to find out who the creator
> of a user namespace is. This patch adds an NS_GET_CREATOR_UID ioctl()
> that returns the (munged) UID of the creator of the user namespace
> referred to by the specified file descriptor.
>
> If the supplied file descriptor does not refer to a user namespace,
> the operation fails with the error EINVAL.
>
> Signed-off-by: Michael Kerrisk <mtk-manpages@xxxxxxxxx>
> ---
> fs/nsfs.c | 6 ++++++
> include/uapi/linux/nsfs.h | 8 +++++---
> 2 files changed, 11 insertions(+), 3 deletions(-)
>
> Open questions:
>
> * Would it be preferabe to separate the logic for NS_GET_CREATOR_UID
> into a small helper function?
> * Is this a correct use of container_of()? I did not immediately
> see another way to get to the user_namespace struct, but I
> may well have missed something.
>
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index 5d53476..26f6d94 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -163,6 +163,7 @@ int open_related_ns(struct ns_common *ns,
> static long ns_ioctl(struct file *filp, unsigned int ioctl,
> unsigned long arg)
> {
> + struct user_namespace *user_ns;
> struct ns_common *ns = get_proc_ns(file_inode(filp));
>
> switch (ioctl) {
> @@ -174,6 +175,11 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
> return open_related_ns(ns, ns->ops->get_parent);
> case NS_GET_NSTYPE:
> return ns->ops->type;
> + case NS_GET_CREATOR_UID:
> + if (ns->ops->type != CLONE_NEWUSER)
> + return -EINVAL;
> + user_ns = container_of(ns, struct user_namespace, ns);
> + return from_kuid_munged(current_user_ns(), user_ns->owner);
uid_t is "unsigned int", ioctl() returns long, so it may be hard to
distinguish user id-s from errors on x32.
off-topic: What is about user_ns->group? I can't find where it is used...
> default:
> return -ENOTTY;
> }
> diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
> index 2b48df1..b3c6c78 100644
> --- a/include/uapi/linux/nsfs.h
> +++ b/include/uapi/linux/nsfs.h
> @@ -6,11 +6,13 @@
> #define NSIO 0xb7
>
> /* Returns a file descriptor that refers to an owning user namespace */
> -#define NS_GET_USERNS _IO(NSIO, 0x1)
> +#define NS_GET_USERNS _IO(NSIO, 0x1)
> /* Returns a file descriptor that refers to a parent namespace */
> -#define NS_GET_PARENT _IO(NSIO, 0x2)
> +#define NS_GET_PARENT _IO(NSIO, 0x2)
> /* Returns the type of namespace (CLONE_NEW* value) referred to by
> file descriptor */
> -#define NS_GET_NSTYPE _IO(NSIO, 0x3)
> +#define NS_GET_NSTYPE _IO(NSIO, 0x3)
> +/* Get creator UID for a user namespace */
> +#define NS_GET_CREATOR_UID _IO(NSIO, 0x4)
>
> #endif /* __LINUX_NSFS_H */
> --
> 2.5.5
>
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