linux-toradex.git/kernel/user_namespace.c, branch v4.9.16

Merge branch 'nsfs-ioctls' into HEAD

2016-09-23T01:00:36+00:00

From: Andrey Vagin 

Each namespace has an owning user namespace and now there is not way
to discover these relationships.

Pid and user namepaces are hierarchical. There is no way to discover
parent-child relationships too.

Why we may want to know relationships between namespaces?

One use would be visualization, in order to understand the running
system.  Another would be to answer the question: what capability does
process X have to perform operations on a resource governed by namespace
Y?

One more use-case (which usually called abnormal) is checkpoint/restart.
In CRIU we are going to dump and restore nested namespaces.

There [1] was a discussion about which interface to choose to determing
relationships between namespaces.

Eric suggested to add two ioctl-s [2]:
> Grumble, Grumble.  I think this may actually a case for creating ioctls
> for these two cases.  Now that random nsfs file descriptors are bind
> mountable the original reason for using proc files is not as pressing.
>
> One ioctl for the user namespace that owns a file descriptor.
> One ioctl for the parent namespace of a namespace file descriptor.

Here is an implementaions of these ioctl-s.

$ man man7/namespaces.7
...
Since  Linux  4.X,  the  following  ioctl(2)  calls are supported for
namespace file descriptors.  The correct syntax is:

      fd = ioctl(ns_fd, ioctl_type);

where ioctl_type is one of the following:

NS_GET_USERNS
      Returns a file descriptor that refers to an owning user names‐
      pace.

NS_GET_PARENT
      Returns  a  file descriptor that refers to a parent namespace.
      This ioctl(2) can be used for pid  and  user  namespaces.  For
      user namespaces, NS_GET_PARENT and NS_GET_USERNS have the same
      meaning.

In addition to generic ioctl(2) errors, the following  specific  ones
can occur:

EINVAL NS_GET_PARENT was called for a nonhierarchical namespace.

EPERM  The  requested  namespace  is outside of the current namespace
      scope.

[1] https://lkml.org/lkml/2016/7/6/158
[2] https://lkml.org/lkml/2016/7/9/101

Changes for v2:
* don't return ENOENT for init_user_ns and init_pid_ns. There is nothing
  outside of the init namespace, so we can return EPERM in this case too.
  > The fewer special cases the easier the code is to get
  > correct, and the easier it is to read. // Eric

Changes for v3:
* rename ns->get_owner() to ns->owner(). get_* usually means that it
  grabs a reference.

Cc: "Eric W. Biederman" 
Cc: James Bottomley 
Cc: "Michael Kerrisk (man-pages)" 
Cc: "W. Trevor King" 
Cc: Alexander Viro 
Cc: Serge Hallyn

nsfs: add ioctl to get a parent namespace

2016-09-23T00:59:41+00:00

Pid and user namepaces are hierarchical. There is no way to discover
parent-child relationships.

In a future we will use this interface to dump and restore nested
namespaces.

Acked-by: Serge Hallyn 
Signed-off-by: Andrei Vagin 
Signed-off-by: Eric W. Biederman

kernel: add a helper to get an owning user namespace for a namespace

2016-09-23T00:59:39+00:00

Return -EPERM if an owning user namespace is outside of a process
current user namespace.

v2: In a first version ns_get_owner returned ENOENT for init_user_ns.
    This special cases was removed from this version. There is nothing
    outside of init_user_ns, so we can return EPERM.
v3: rename ns->get_owner() to ns->owner(). get_* usually means that it
grabs a reference.

Acked-by: Serge Hallyn 
Signed-off-by: Andrei Vagin 
Signed-off-by: Eric W. Biederman

userns: When the per user per user namespace limit is reached return ENOSPC

2016-09-22T18:25:56+00:00

The current error codes returned when a the per user per user
namespace limit are hit (EINVAL, EUSERS, and ENFILE) are wrong.  I
asked for advice on linux-api and it we made clear that those were
the wrong error code, but a correct effor code was not suggested.

The best general error code I have found for hitting a resource limit
is ENOSPC.  It is not perfect but as it is unambiguous it will serve
until someone comes up with a better error code.

Signed-off-by: "Eric W. Biederman"

userns: Generalize the user namespace count into ucount

2016-08-08T19:41:52+00:00

The same kind of recursive sane default limit and policy
countrol that has been implemented for the user namespace
is desirable for the other namespaces, so generalize
the user namespace refernce count into a ucount.

Acked-by: Kees Cook 
Signed-off-by: "Eric W. Biederman"

userns: Make the count of user namespaces per user

2016-08-08T19:40:30+00:00

Add a structure that is per user and per user ns and use it to hold
the count of user namespaces.  This makes prevents one user from
creating denying service to another user by creating the maximum
number of user namespaces.

Rename the sysctl export of the maximum count from
/proc/sys/userns/max_user_namespaces to /proc/sys/user/max_user_namespaces
to reflect that the count is now per user.

Signed-off-by: "Eric W. Biederman"

userns: Add a limit on the number of user namespaces

2016-08-08T18:41:24+00:00

Export the export the maximum number of user namespaces as
/proc/sys/userns/max_user_namespaces.

Acked-by: Kees Cook 
Signed-off-by: "Eric W. Biederman"

userns: Add per user namespace sysctls.

2016-08-08T18:18:58+00:00

Limit per userns sysctls to only be opened for write by a holder
of CAP_SYS_RESOURCE.

Add all of the necessary boilerplate for having per user namespace
sysctls.

Acked-by: Kees Cook 
Signed-off-by: "Eric W. Biederman"

userns: Free user namespaces in process context

2016-08-08T14:17:18+00:00

Add the necessary boiler plate to move freeing of user namespaces into
work queue and thus into process context where things can sleep.

This is a necessary precursor to per user namespace sysctls.

Signed-off-by: "Eric W. Biederman"

fs: Limit file caps to the user namespace of the super block

2016-06-24T15:40:31+00:00

Capability sets attached to files must be ignored except in the
user namespaces where the mounter is privileged, i.e. s_user_ns
and its descendants. Otherwise a vector exists for gaining
privileges in namespaces where a user is not already privileged.

Add a new helper function, current_in_user_ns(), to test whether a user
namespace is the same as or a descendant of another namespace.
Use this helper to determine whether a file's capability set
should be applied to the caps constructed during exec.

--EWB Replaced in_userns with the simpler current_in_userns.

Acked-by: Serge Hallyn 
Signed-off-by: Seth Forshee 
Signed-off-by: Eric W. Biederman