RFC Per user namespace rlimits

Preface
-------
These patches are for binding the rlimits to a user in the user namespace. This patch set can be applied on top of:

git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git v5.8-2-g43e210d68200

Problem
-------
Some rlimits are set per user: RLIMIT_NPROC, RLIMIT_MEMLOCK, RLIMIT_SIGPENDING, RLIMIT_MSGQUEUE. When several containers are created from one user then the processes inside the containers influence each other.

Eric W. Biederman mentioned this issue [1][2][3].

Introduced changes
------------------
To fix this problem, you can bind the counter of the specified rlimits to the user within the user namespace. By default, to preserve backward compatibility, only the initial user namespace is used. This patch adds one more prctl parameter to change the binding to the user namespace.

This will not cause the user to take more resources than allowed in the parent user namespace because it only virtualizes the rlimit counter. Limits in all parent user namespaces are taken into account.

For example, this allows us to run multiple containers by the same user and set the RLIMIT_NPROC to 1 inside.

ToDo
----
* RLIMIT_MEMLOCK, RLIMIT_SIGPENDING and RLIMIT_MSGQUEUE are not implemented.
* No documentation.
* No tests.

[1] https://lore.kernel.org/containers/[email protected]/
[2] https://lists.linuxfoundation.org/pipermail/containers/2020-August/042096.html
[3] https://lists.linuxfoundation.org/pipermail/containers/2020-October/042524.html

Changelog
---------
v1:
* After discussion with Eric W. Biederman, I increased the size of ucounts to atomic_long_t.
* Added ucount_max to avoid the fork bomb.

--

Alexey Gladkov (4):
Increase size of ucounts to atomic_long_t
Move the user's process counter to ucounts
Do not allow fork if RLIMIT_NPROC is exceeded in the user namespace
tree
Allow to change the user namespace in which user rlimits are counted

 fs/exec.c                      | 13 ++++++---
 fs/io-wq.c                     | 25 +++++++++++++-----
 fs/io-wq.h                     |  1 +
 fs/io_uring.c                  |  1 +
 include/linux/cred.h           |  8 ++++++
 include/linux/sched.h          |  3 +++
 include/linux/sched/user.h     |  1 -
 include/linux/user_namespace.h | 12 +++++++--
 include/uapi/linux/prctl.h     |  5 ++++
 kernel/cred.c                  | 44 ++++++++++++++++++++++++-------
 kernel/exit.c                  |  2 +-
 kernel/fork.c                  | 13 ++++++---
 kernel/sys.c                   | 26 ++++++++++++++++--
 kernel/ucount.c                | 48 +++++++++++++++++++++++++++++-----
 kernel/user.c                  |  3 ++-
 kernel/user_namespace.c        |  3 +++
 16 files changed, 171 insertions(+), 37 deletions(-)

2.25.4