[PATCH 0/2] cgroup: fix cgroup.kill for exited thread group leader

[Kir Kolyshkin <kolyshkin@gmail.com>]

cgroup.kill fails to kill a process whose thread group leader has
exited while other threads are still running, leaving the cgroup
populated. This can be used by a process in a container to survive
the container being killed, including the case of a container with a
private PID namespace, where such a process is the container's init.

Patch 1 fixes __cgroup_kill() to walk all threads and to send a
thread-group-directed SIGKILL. Patch 2 adds selftests for both
scenarios described in patch 1; they fail without the fix.

Tested on v7.3-rc7 (x86_64 VM): test_kill passes (30 runs), other
cgroup selftests show no changes compared to the unpatched kernel.

The issue was found in crun, see patch 1 for details.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
---
Kir Kolyshkin (2):
      cgroup: fix cgroup.kill for exited thread group leader
      selftests/cgroup: test cgroup.kill with an exited thread group leader

 kernel/cgroup/cgroup.c                     |   4 +-
 tools/testing/selftests/cgroup/test_kill.c | 158 +++++++++++++++++++++++++++++
 2 files changed, 160 insertions(+), 2 deletions(-)
---
base-commit: f909e534f5e34feb2ea2701d54df63e21aa506fc
change-id: 20261018-cgroup-kill-exited-leader-a15159e8c230

Best regards,
-- 
Kir Kolyshkin <kolyshkin@gmail.com>