[PATCHSET v2 cgroup/for-5.16-fixes] cgroup: Use open-time creds and namespace for migration perm checks

[Tejun Heo <tj@kernel.org>]

Hello,

v2: 0002-cgroup-Allocate-cgroup_file_ctx-for-kernfs_open_file updated to
    drop the union and embed procs.iter as suggested by Linus.

cgroup process migration permission checks are performed at write time as
whether a given operation is allowed or not is dependent on the content of
the write - the PID. This currently uses current's credentials and cgroup
namespace which is a potential security weakness as it may allow scenarios
where a less privileged process tricks a more privileged one into writing
into a fd that it created.

This patchset make the perm checks use credentials and cgroup namespace
stored at the time of open and contains the following patches.

 0001-cgroup-Use-open-time-credentials-for-process-migrato.patch
 0002-cgroup-Allocate-cgroup_file_ctx-for-kernfs_open_file.patch
 0003-cgroup-Use-open-time-cgroup-namespace-for-process-mi.patch
 0004-selftests-cgroup-Make-cg_create-use-0755-for-permiss.patch
 0005-selftests-cgroup-Test-open-time-credential-usage-for.patch
 0006-selftests-cgroup-Test-open-time-cgroup-namespace-usa.patch

The patchset is also available in the following git branch. If there's no
objetion, I will apply to cgroup/for-5.16-fixes in a few days.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git review-migration-perms-1

diffstat follows. Thank you.

 kernel/cgroup/cgroup-internal.h              |   13 ++
 kernel/cgroup/cgroup-v1.c                    |    7 -
 kernel/cgroup/cgroup.c                       |   88 +++++++++-----
 tools/testing/selftests/cgroup/cgroup_util.c |    2 
 tools/testing/selftests/cgroup/test_core.c   |  165 +++++++++++++++++++++++++++
 5 files changed, 243 insertions(+), 32 deletions(-)

--
tejun