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Tejun Heo authored
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, afcf6c8b ("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: afcf6c8b ("cgroup: add cgroup_subsys->free() method and use it to fix pids controller")
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