netprio_cgroup: Fix unlimited memory leak of v2 cgroups

hulk inclusion
category: bugfix
bugzilla: 34583
CVE: NA

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If systemd is configured to use hybrid mode which enables the use of
both cgroup v1 and v2, systemd will create new cgroup on both the default
root (v2) and netprio_cgroup hierarchy (v1) for a new session and attach
task to the two cgroups. If the task does some network thing then the v2
cgroup can never be freed after the session exited.

One of our machines ran into OOM due to this memory leak.

In the scenario described above when sk_alloc() is called cgroup_sk_alloc()
thought it's in v2 mode, so it stores the cgroup pointer in sk->sk_cgrp_data
and increments the cgroup refcnt, but then sock_update_netprioidx() thought
it's in v1 mode, so it stores netprioidx value in sk->sk_cgrp_data, so the
cgroup refcnt will never be freed.

Currently we do the mode switch when someone writes to the ifpriomap cgroup
control file. The easiest fix is to also do the switch when a task is attached
to a new cgroup.

Fixes: bd1060a1 ("sock, cgroup: add sock->sk_cgroup")
Reported-by: Yang Yingliang <yangyingliang@huawei.com>
Tested-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Zefan Li <lizefan@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>