Re: [PATCH V7 5/7] cpufreq: Register notifiers with the PM QoS framework

[Viresh Kumar]

On 22-09-19, 23:12, Dmitry Osipenko wrote:
> Hello Viresh,
> 
> This patch causes use-after-free on a cpufreq driver module reload. Please take a look, thanks in advance.
> 
> 
> [   87.952369] ==================================================================
> [   87.953259] BUG: KASAN: use-after-free in notifier_chain_register+0x4f/0x9c
> [   87.954031] Read of size 4 at addr e6abbd0c by task modprobe/243
> 
> [   87.954901] CPU: 1 PID: 243 Comm: modprobe Tainted: G        W
> 5.3.0-next-20190920-00185-gf61698eab956-dirty #2408
> [   87.956077] Hardware name: NVIDIA Tegra SoC (Flattened Device Tree)
> [   87.956807] [<c0110aad>] (unwind_backtrace) from [<c010bb71>] (show_stack+0x11/0x14)
> [   87.957709] [<c010bb71>] (show_stack) from [<c0d37b25>] (dump_stack+0x89/0x98)
> [   87.958616] [<c0d37b25>] (dump_stack) from [<c02937e1>]
> (print_address_description.constprop.0+0x3d/0x340)
> [   87.959785] [<c02937e1>] (print_address_description.constprop.0) from [<c0293c6b>]
> (__kasan_report+0xe3/0x12c)
> [   87.960907] [<c0293c6b>] (__kasan_report) from [<c014988f>] (notifier_chain_register+0x4f/0x9c)
> [   87.962001] [<c014988f>] (notifier_chain_register) from [<c01499b5>]
> (blocking_notifier_chain_register+0x29/0x3c)
> [   87.963180] [<c01499b5>] (blocking_notifier_chain_register) from [<c06f7ee9>]
> (dev_pm_qos_add_notifier+0x79/0xf8)
> [   87.964339] [<c06f7ee9>] (dev_pm_qos_add_notifier) from [<c092927d>] (cpufreq_online+0x5e1/0x8a4)
> [   87.965351] [<c092927d>] (cpufreq_online) from [<c09295c9>] (cpufreq_add_dev+0x79/0x80)
> [   87.966247] [<c09295c9>] (cpufreq_add_dev) from [<c06eb9d3>] (subsys_interface_register+0xc3/0x100)
> [   87.967297] [<c06eb9d3>] (subsys_interface_register) from [<c0926e53>]
> (cpufreq_register_driver+0x13b/0x1ec)
> [   87.968476] [<c0926e53>] (cpufreq_register_driver) from [<bf800435>]
> (tegra20_cpufreq_probe+0x165/0x1a8 [tegra20_cpufreq])

Hi Dmitry,

Thanks for the bug report and I was finally able to reproduce it at my end and
this was quite an interesting debugging exercise :)

When a cpufreq driver gets registered, we register with the subsys interface and
it calls cpufreq_add_dev() for each CPU, starting from CPU0. And so the QoS
notifiers get added to the first CPU of the policy, i.e. CPU0 in common cases.

When the cpufreq driver gets unregistered, we unregister with the subsys
interface and it calls cpufreq_remove_dev() for each CPU, starting from CPU0
(should have been in reverse order I feel). We remove the QoS notifier only when
cpufreq_remove_dev() gets called for the last CPU of the policy, lets call it
CPUx. Now this has a different notifier list as compared to CPU0.

In short, we are adding the cpufreq notifiers to CPU0 and removing them from
CPUx. When we try to add it again by inserting the module for second time, we
find a node in the notifier list which is already freed but still in the list as
we removed it from CPUx's list (which doesn't do anything as the node wasn't
there in the first place).

@Rafael: How do you see we solve this problem ? Here are the options I could
think of:

- Update subsys layer to reverse the order of devices while unregistering (this
  will fix the current problem, but we will still have corner cases hanging
  around, like if the CPU0 is hotplugged out, etc).

- Update QoS framework with the knowledge of related CPUs, this has been pending
  until now from my side. And this is the thing we really need to do. Eventually
  we shall have only a single notifier list for all CPUs of a policy, at least
  for MIN/MAX frequencies.

- ??

-- 
viresh