[SRU Bionic/Focal] netprio_cgroup: Fix unlimited memory leak of v2 cgroups

[Thadeu Lima de Souza Cascardo]

From: Zefan Li <lizefan at huawei.com>

BugLink: https://bugs.launchpad.net/bugs/1886859

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: bd1060a1d671 ("sock, cgroup: add sock->sk_cgroup")
Reported-by: Yang Yingliang <yangyingliang at huawei.com>
Tested-by: Yang Yingliang <yangyingliang at huawei.com>
Signed-off-by: Zefan Li <lizefan at huawei.com>
Acked-by: Tejun Heo <tj at kernel.org>
Signed-off-by: Jakub Kicinski <kuba at kernel.org>
(cherry picked from commit 090e28b229af92dc5b40786ca673999d59e73056)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 net/core/netprio_cgroup.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/netprio_cgroup.c b/net/core/netprio_cgroup.c
index b9057478d69c..239786608ee4 100644
--- a/net/core/netprio_cgroup.c
+++ b/net/core/netprio_cgroup.c
@@ -240,6 +240,8 @@ static void net_prio_attach(struct cgroup_taskset *tset)
 	struct task_struct *p;
 	struct cgroup_subsys_state *css;
 
+	cgroup_sk_alloc_disable();
+
 	cgroup_taskset_for_each(p, css, tset) {
 		void *v = (void *)(unsigned long)css->cgroup->id;
 
-- 
2.27.0