For review: user_namespace(7) man page

Michael Kerrisk (man-pages) mtk.manpages at gmail.com
Mon Sep 1 16:58:12 UTC 2014


On 08/22/2014 11:12 PM, Serge E. Hallyn wrote:
> Quoting Michael Kerrisk (man-pages) (mtk.manpages at gmail.com):
>> Hello Eric et al.,
>>
>> For various reasons, my work on the namespaces man pages 
>> fell off the table a while back. Nevertheless, the pages have
>> been close to completion for a while now, and I recently restarted,
>> in an effort to finish them. As you also noted to me f2f, there have
>> been recently been some small namespace changes that you may affect
>> the content of the pages. Therefore, I'll take the opportunity to
>> send the namespace-related pages out for further (final?) review.
>>
>> So, here, I start with the user_namespaces(7) page, which is shown 
>> in rendered form below, with source attached to this mail. I'll
>> send various other pages in follow-on mails.
>>
>> Review comments/suggestions for improvements / bug fixes welcome.
>>
>> Cheers,
>>
>> Michael
>>
>> ==
>>
>> NAME
>>        user_namespaces - overview of Linux user_namespaces
>>
>> DESCRIPTION
>>        For an overview of namespaces, see namespaces(7).
>>
>>        User   namespaces   isolate   security-related   identifiers  and
>>        attributes, in particular, user IDs and group  IDs  (see  creden‐
>>        tials(7), the root directory, keys (see keyctl(2)), and capabili‐
>>        ties (see capabilities(7)).  A process's user and group  IDs  can
>>        be different inside and outside a user namespace.  In particular,
>>        a process can have a normal unprivileged user ID outside  a  user
>>        namespace while at the same time having a user ID of 0 inside the
>>        namespace; in other words, the process has  full  privileges  for
>>        operations  inside  the  user  namespace, but is unprivileged for
>>        operations outside the namespace.
>>
>>    Nested namespaces, namespace membership
>>        User namespaces can be nested;  that  is,  each  user  namespace—
>>        except  the  initial  ("root") namespace—has a parent user names‐
>>        pace, and can have zero or more child user namespaces.  The  par‐
>>        ent user namespace is the user namespace of the process that cre‐
>>        ates the user namespace via a call to unshare(2) or clone(2) with
>>        the CLONE_NEWUSER flag.
>>
>>        The kernel imposes (since version 3.11) a limit of 32 nested lev‐
>>        els of user namespaces.  Calls to  unshare(2)  or  clone(2)  that
>>        would cause this limit to be exceeded fail with the error EUSERS.
>>
>>        Each  process  is  a  member  of  exactly  one user namespace.  A
>>        process created via fork(2) or clone(2) without the CLONE_NEWUSER
>>        flag  is  a  member  of the same user namespace as its parent.  A
>>        process can join another user namespace with setns(2) if  it  has
>>        the  CAP_SYS_ADMIN  in  that namespace; upon doing so, it gains a
>>        full set of capabilities in that namespace.
>>
>>        A call to clone(2) or  unshare(2)  with  the  CLONE_NEWUSER  flag
>>        makes  the  new  child  process (for clone(2)) or the caller (for
>>        unshare(2)) a member of the new user  namespace  created  by  the
>>        call.
>>
>>    Capabilities
>>        The child process created by clone(2) with the CLONE_NEWUSER flag
>>        starts out with a complete set of capabilities in  the  new  user
>>        namespace.  Likewise, a process that creates a new user namespace
>>        using unshare(2)  or  joins  an  existing  user  namespace  using
>>        setns(2)  gains a full set of capabilities in that namespace.  On
>>        the other hand, that process has no capabilities  in  the  parent
>>        (in  the case of clone(2)) or previous (in the case of unshare(2)
>>        and setns(2)) user namespace, even if the new namespace  is  cre‐
>>        ated  or  joined by the root user (i.e., a process with user ID 0
>>        in the root namespace).
>>
>>        Note that a call to execve(2) will cause a process  to  lose  any
>>        capabilities that it has, unless it has a user ID of 0 within the
>>        namespace.  See the discussion of user  and  group  ID  mappings,
>>        below.
> 
> The above is an approximation, but a bit misleading.  On exec, the task
> capability set is recalculated according to the usual rules.  So if the
> file being executed has file capabilities, the result task may end up
> with capabilities even if it is not root (even if it is uid -1).
> 
> Perhaps it should be phrased as:
> 
>         Note that a call to execve(2) will cause a process' capabilities
> 	to be recalculated (see capabilities(7)), so that usually, unless
> 	it has a user ID of 0 within the namespace, it will lose all
> 	capabilities.  See the discussion of user  and  group  ID  mappings,
>         below.

Thanks, Serge. Changed as you suggest.

Cheers,

Michael



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


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