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

["Rafael J. Wysocki" <rafael@kernel.org>]

On Tue, Oct 15, 2019 at 1:46 PM Viresh Kumar <viresh.kumar@linaro.org> wrote:
>
> 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.

The same problem will appear if the original policy CPU goes offline, won't it?

> 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).

This isn't sufficient for the offline case.

> - 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.

- Move the PM QoS requests and notifiers to the new policy CPU on all
changes of that.  That is, when cpufreq_offline() nominates the new
"leader", all of the QoS stuff for the policy needs to go to this one.

Of course, the reverse order of unregistration in the subsys layer
would help to avoid some useless churn related to that.