Re: [PATCH bpf 2/4] nsfs: add an ioctl to discover the network namespace cookie

[Christian Brauner]

On Wed, Feb 10, 2021 at 12:04:23PM +0000, Lorenz Bauer wrote:
> Network namespaces have a globally unique non-zero identifier aka a
> cookie, in line with socket cookies. Add an ioctl to retrieve the
> cookie from user space without going via BPF.
> 
> Cc: linux-api@xxxxxxxxxxxxxxx
> Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
> ---
>  fs/nsfs.c                   |  9 +++++++++
>  include/net/net_namespace.h | 11 +++++++++++
>  include/uapi/linux/nsfs.h   |  2 ++
>  3 files changed, 22 insertions(+)
> 
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index 800c1d0eb0d0..d7865e39c049 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -11,6 +11,7 @@
>  #include <linux/user_namespace.h>
>  #include <linux/nsfs.h>
>  #include <linux/uaccess.h>
> +#include <net/net_namespace.h>
>  
>  #include "internal.h"
>  
> @@ -191,6 +192,8 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
>  	struct user_namespace *user_ns;
>  	struct ns_common *ns = get_proc_ns(file_inode(filp));
>  	uid_t __user *argp;
> +	struct net *net_ns;
> +	u64 cookie;
>  	uid_t uid;
>  
>  	switch (ioctl) {
> @@ -209,6 +212,12 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
>  		argp = (uid_t __user *) arg;
>  		uid = from_kuid_munged(current_user_ns(), user_ns->owner);
>  		return put_user(uid, argp);
> +	case NS_GET_COOKIE:
> +		if (ns->ops->type != CLONE_NEWNET)
> +			return -EINVAL;
> +		net_ns = container_of(ns, struct net, ns);
> +		cookie = net_gen_cookie(net_ns);
> +		return put_user(cookie, (u64 __user *)arg);

Hey Lorenz,

Just to make sure: is it intentional that any user can retrieve the
cookie associated with any network namespace, i.e. you don't require any
form of permission checking in the owning user namespace of the network
namespace?

Christian