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

[Tejun Heo <tj@kernel.org>]

Hello,

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.

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

Michal, does this also fix the original bug you were trying to fix? For now,
I didn't add Fixes / stable tags. If ppl are okay with the patchset, I'll
route it through cgroup/for-5.16-fixes.

diffstat follows. Thanks.

 kernel/cgroup/cgroup-internal.h              |   14 ++++
 kernel/cgroup/cgroup-v1.c                    |    7 +-
 kernel/cgroup/cgroup.c                       |   82 ++++++++++++++++++-------
 tools/testing/selftests/cgroup/cgroup_util.c |    2 
 tools/testing/selftests/cgroup/test_core.c   |  165 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 244 insertions(+), 26 deletions(-)

--
tejun