Hi Andrei,
On 12/21/2016 04:13 AM, Andrei Vagin wrote:
> 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.
Good point. So, we could instead return the UID via a buffer pointed to
by the ioctl() arg. That would seem better, right?
> off-topic: What is about user_ns->group? I can't find where it is used...
I've no idea. Like you, I can't see any place where it's being used.
Cheers,
Michael
>> 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
>>
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
--
To unsubscribe from this list: send the line "unsubscribe linux-api" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
