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Subject[PATCH cgroup/for-4.4-fixes] cgroup: make css_set pin its css's to avoid use-afer-free
A css_set represents the relationship between a set of tasks and
css's. css_set never pinned the associated css's. This was okay
because tasks used to always disassociate immediately (in RCU sense) -
either a task is moved to a different css_set or exits and never
accesses css_set again.

Unfortunately, afcf6c8b7544 ("cgroup: add cgroup_subsys->free() method
and use it to fix pids controller") and patches leading up to it made
a zombie hold onto its css_set and deref the associated css's on its
release. Nothing pins the css's after exit and it might have already
been freed leading to use-after-free.

general protection fault: 0000 [#1] PREEMPT SMP
task: ffffffff81bf2500 ti: ffffffff81be4000 task.ti: ffffffff81be4000
RIP: 0010:[<ffffffff810fa205>] [<ffffffff810fa205>] pids_cancel.constprop.4+0x5/0x40
...
Call Trace:
<IRQ>
[<ffffffff810fb02d>] ? pids_free+0x3d/0xa0
[<ffffffff810f8893>] cgroup_free+0x53/0xe0
[<ffffffff8104ed62>] __put_task_struct+0x42/0x130
[<ffffffff81053557>] delayed_put_task_struct+0x77/0x130
[<ffffffff810c6b34>] rcu_process_callbacks+0x2f4/0x820
[<ffffffff810c6af3>] ? rcu_process_callbacks+0x2b3/0x820
[<ffffffff81056e54>] __do_softirq+0xd4/0x460
[<ffffffff81057369>] irq_exit+0x89/0xa0
[<ffffffff81876212>] smp_apic_timer_interrupt+0x42/0x50
[<ffffffff818747f4>] apic_timer_interrupt+0x84/0x90
<EOI>
...
Code: 5b 5d c3 48 89 df 48 c7 c2 c9 f9 ae 81 48 c7 c6 91 2c ae 81 e8 1d 94 0e 00 31 c0 5b 5d c3 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 <f0> 48 83 87 e0 00 00 00 ff 78 01 c3 80 3d 08 7a c1 00 00 74 02
RIP [<ffffffff810fa205>] pids_cancel.constprop.4+0x5/0x40
RSP <ffff88001fc03e20>
---[ end trace 89a4a4b916b90c49 ]---
Kernel panic - not syncing: Fatal exception in interrupt
Kernel Offset: disabled
---[ end Kernel panic - not syncing: Fatal exception in interrupt

Fix it by making css_set pin the associate css's until its release.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Dave Jones <davej@codemonkey.org.uk>
Reported-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Link: http://lkml.kernel.org/g/20151120041836.GA18390@codemonkey.org.uk
Link: http://lkml.kernel.org/g/5652D448.3080002@bmw-carit.de
Fixes: afcf6c8b7544 ("cgroup: add cgroup_subsys->free() method and use it to fix pids controller")
---
kernel/cgroup.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -754,9 +754,11 @@ static void put_css_set_locked(struct cs
if (!atomic_dec_and_test(&cset->refcount))
return;

- /* This css_set is dead. unlink it and release cgroup refcounts */
- for_each_subsys(ss, ssid)
+ /* This css_set is dead. unlink it and release cgroup and css refs */
+ for_each_subsys(ss, ssid) {
list_del(&cset->e_cset_node[ssid]);
+ css_put(cset->subsys[ssid]);
+ }
hash_del(&cset->hlist);
css_set_count--;

@@ -1056,9 +1058,13 @@ static struct css_set *find_css_set(stru
key = css_set_hash(cset->subsys);
hash_add(css_set_table, &cset->hlist, key);

- for_each_subsys(ss, ssid)
+ for_each_subsys(ss, ssid) {
+ struct cgroup_subsys_state *css = cset->subsys[ssid];
+
list_add_tail(&cset->e_cset_node[ssid],
- &cset->subsys[ssid]->cgroup->e_csets[ssid]);
+ &css->cgroup->e_csets[ssid]);
+ css_get(css);
+ }

spin_unlock_bh(&css_set_lock);


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