[PATCH v2 1/2] nsfs: Add an ioctl() to return the namespace type

[PATCH v2 1/2] nsfs: Add an ioctl() to return the namespace type

[Michael Kerrisk (man-pages)]

Linux 4.9 added two ioctl() operations that can be used to discover:

* the parental relationships for hierarchical namespaces (user and PID)
  [NS_GET_PARENT]
* the user namespaces that owns a specified non-user-namespace
  [NS_GET_USERNS]

For no good reason that I can glean, NS_GET_USERNS was made synonymous
with NS_GET_PARENT for user namespaces. It might have been better if
NS_GET_USERNS had returned an error if the supplied file descriptor
referred to a user namespace, since it suggests that the caller may be
confused. More particularly, if it had generated an error, then I wouldn't
need the new ioctl() operation proposed here. (On the other hand, what
I propose here may be more generally useful.)

I would like to write code that discovers namespace relationships for
the purpose of understanding the namespace setup on a running system.
In particular, given a file descriptor (or pathname) for a namespace,
N, I'd like to obtain the corresponding user namespace.  Namespace N
might be a user namespace (in which case my code would just use N) or
a non-user namespace (in which case my code will use NS_GET_USERNS to
get the user namespace associated with N). The problem is that there
is no way to tell the difference by looking at the file descriptor
(and if I try to use NS_GET_USERNS on an N that is a user namespace, I
get the parent user namespace of N, which is not what I want).

This patch therefore adds a new ioctl(), NS_GET_NSTYPE, which, given
a file descriptor that refers to a user namespace, returns the
namespace type (one of the CLONE_NEW* constants).

Signed-off-by: Michael Kerrisk <mtk-manpages@gmail.com>
---
 fs/nsfs.c                 | 2 ++
 include/uapi/linux/nsfs.h | 3 +++
 2 files changed, 5 insertions(+)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 8718af8..9f24b47 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -172,6 +172,8 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
 		if (!ns->ops->get_parent)
 			return -EINVAL;
 		return open_related_ns(ns, ns->ops->get_parent);
+	case NS_GET_NSTYPE:
+		return ns->ops->type;
 	default:
 		return -ENOTTY;
 	}
diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
index 3af6172..2b48df1 100644
--- a/include/uapi/linux/nsfs.h
+++ b/include/uapi/linux/nsfs.h
@@ -9,5 +9,8 @@
 #define NS_GET_USERNS	_IO(NSIO, 0x1)
 /* Returns a file descriptor that refers to a parent namespace */
 #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)
 
 #endif /* __LINUX_NSFS_H */
-- 
2.5.5

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

[Michael Kerrisk (man-pages)]

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.

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;
 
 	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

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

[Andrei Vagin]

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
>