Re: [PATCH v2 2/2] nsfs: Add an ioctl() to return owner UID of a userns

[Andrei Vagin <avagin-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>]

On Fri, Dec 23, 2016 at 10:54:53AM +0100, Michael Kerrisk (man-pages) wrote:
> I'd like to write code that discovers the user namespace hierarchy on
> a running system, and also shows who owns the various user namespaces.
> Currently, there is no way of getting the owner UID of a user
> namespace. Therefore, this patch adds an NS_GET_CREATOR_UID ioctl()
> that fetches 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.
>

Acked-by: Andrey Vagin <avagin-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>

Here is one minor comment bellow

> Signed-off-by: Michael Kerrisk <mtk-manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> 
> ---
> V2 changes:
> * Renamed ioctl() from NS_CREATOR_UID to NS_OWNER_UID, at the
>   suggestion of Eric Biederman.
> * Make ioctl() return UID via buffer pointed to by argp. (Returning
>   the UID via the result value could lead to problems since a large
>   unsigned int UID might be misinterpreted as an error.) Thanks to
>   Andrei Vagin for pointing this out.
> ---
>  fs/nsfs.c                 | 11 +++++++++++
>  include/uapi/linux/nsfs.h |  8 +++++---
>  2 files changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index 9f24b47..9c9277c 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -7,6 +7,7 @@
>  #include <linux/seq_file.h>
>  #include <linux/user_namespace.h>
>  #include <linux/nsfs.h>
> +#include <linux/uaccess.h>
>  
>  static struct vfsmount *nsfs_mnt;
>  
> @@ -163,7 +164,10 @@ static 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));
> +	unsigned int __user *argp;
> +	unsigned int uid;

I think it is better to use uid_t here

>  
>  	switch (ioctl) {
>  	case NS_GET_USERNS:
> @@ -174,6 +178,13 @@ 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_OWNER_UID:
> +		if (ns->ops->type != CLONE_NEWUSER)
> +			return -EINVAL;
> +		user_ns = container_of(ns, struct user_namespace, ns);
> +		argp = (unsigned int __user *) arg;
> +		uid = from_kuid_munged(current_user_ns(), user_ns->owner);
> +		return put_user(uid, argp);
>  	default:
>  		return -ENOTTY;
>  	}
> diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
> index 2b48df1..c4a925e 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 owner UID for a user namespace */
> +#define NS_GET_OWNER_UID	_IO(NSIO, 0x4)
>  
>  #endif /* __LINUX_NSFS_H */
> -- 
> 2.5.5
>