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

[Michael Kerrisk (man-pages)]

Hello Andrei,

On 24 December 2016 at 14:16, Andrei Vagin <avagin@xxxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxx>

Thanks for looking the patches over.

> Here is one minor comment bellow
>
>> Signed-off-by: Michael Kerrisk <mtk-manpages@xxxxxxxxx>
>>
>> ---
>> 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

Fair enough.

Should in fact the type for the ioctl() argument be changed? I mean,
make the following change to the line before:

-       unsigned int __user *argp;
+       uid_t __user *argp;

And further below, change:

-             argp = (unsigned int __user *) arg;
+             argp = (uid_t __user *) arg;

?

Happy New Year!

Cheers,

Michael

>>       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
>>



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/