[RFC PATCH 0/6] RLIMIT_NPROC in ucounts fixups

[Michal Koutný]

This series is a result of looking deeper into breakage of
tools/testing/selftests/rlimits/rlimits-per-userns.c after 
https://lore.kernel.org/r/20220204181144.24462-1-mkoutny@xxxxxxxx/
is applied.

The description of the original problem that lead to RLIMIT_NPROC et al.
ucounts rewrite could be ambiguously interpretted as supporting either
the case of:
- never-fork service or
- fork (RLIMIT_NPROC-1) times service.

The scenario is weird anyway given existence of pids controller.

The realization of that scenario relies not only on tracking number of
processes per user_ns but also newly allows the root to override limit through
set*uid. The commit message didn't mention that, so it's unclear if it
was the intention too.

I also noticed that the RLIMIT_NPROC enforcing in fork seems subject to TOCTOU
race (check(nr_tasks),...,nr_tasks++) so the limit is rather advisory (but
that's not a new thing related to ucounts rewrite).

This series is RFC to discuss relevance of the subtle changes RLIMIT_NPROC to
ucounts rewrite introduced.

Michal Koutný (6):
  set_user: Perform RLIMIT_NPROC capability check against new user
    credentials
  set*uid: Check RLIMIT_PROC against new credentials
  cred: Count tasks by their real uid into RLIMIT_NPROC
  ucounts: Allow root to override RLIMIT_NPROC
  selftests: Challenge RLIMIT_NPROC in user namespaces
  selftests: Test RLIMIT_NPROC in clone-created user namespaces

 fs/exec.c                                     |   2 +-
 include/linux/cred.h                          |   2 +-
 kernel/cred.c                                 |  29 ++-
 kernel/fork.c                                 |   2 +-
 kernel/sys.c                                  |  20 +-
 kernel/ucount.c                               |   3 +
 kernel/user_namespace.c                       |   2 +-
 .../selftests/rlimits/rlimits-per-userns.c    | 233 +++++++++++++++---
 8 files changed, 229 insertions(+), 64 deletions(-)

-- 
2.34.1