Re: cpufreq_stats NULL deref on second system suspend

Viresh Kumar
2013-09-11T10:44:37Z

On 11 September 2013 15:51, Srivatsa S. Bhat
<srivatsa.bhat@linux.vnet.ibm.com> wrote:
> On 09/11/2013 04:04 AM, Rafael J. Wysocki wrote:
>> On Tuesday, September 10, 2013 02:53:01 PM Stephen Warren wrote:

>>> Sure, it's due to 5302c3f "cpufreq: Perform light-weight init/teardown
>>> during suspend/resume".

Sorry Stephen, I was away on vacations and came back yesterday only..
And was badly stuck in something other CPUFreq bugs until now :)

> Sure, Rafael. Thanks for CC'ing me.

Thanks for jumping in and helping us out buddy!!!.

> Stephen, I went through the code and I think I found out what is going wrong.
> Can you please try the following patch?
>
> Regards,
> Srivatsa S. Bhat
>
> ----------------------------------------------------------------------------
>
>
> From: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
> Subject: [PATCH] cpufreq: Fix crash in cpufreq-stats during suspend/resume
>
> Stephen Warren reported that the cpufreq-stats code hits a NULL pointer
> dereference during the second attempt to suspend a system. He also
> pin-pointed the problem to commit 5302c3f "cpufreq: Perform light-weight
> init/teardown during suspend/resume".
>
> That commit actually ensured that the cpufreq-stats table and the
> cpufreq-stats sysfs entries are *not* torn down (ie., not freed) during
> suspend/resume, which makes it all the more surprising. However, it turns
> out that the root-cause is not that we access an already freed memory, but
> that the reference to the allocated memory gets moved around and we lose
> track of that during resume, leading to the reported crash in a subsequent
> suspend attempt.
>
> In the suspend path, during CPU offline, the value of policy->cpu is updated
> by choosing one of the surviving CPUs in that policy, as long as there is
> atleast one CPU in that policy. And cpufreq_stats_update_policy_cpu() is
> invoked to update the reference to the stats structure by assigning it to
> the new CPU. However, in the resume path, during CPU online, we end up
> assigning a fresh CPU as the policy->cpu, without letting cpufreq-stats
> know about this. Thus the reference to the stats structure remains
> (incorrectly) associated with the old CPU. So, in a subsequent suspend attempt,
> during CPU offline, we end up accessing an incorrect location to get the
> stats structure, which eventually leads to the NULL pointer dereference.
>
> Fix this by letting cpufreq-stats know about the update of the policy->cpu
> during CPU online in the resume path. (Also, move the update_policy_cpu()
> function higher up in the file, so that __cpufreq_add_dev() can invoke
> it).

Observation looks good..

> Reported-by: Stephen Warren <swarren@nvidia.com>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
> ---
>
> drivers/cpufreq/cpufreq.c | 37 ++++++++++++++++++++++++-------------
> 1 file changed, 24 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 5a64f66..62bdb95 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -947,6 +947,18 @@ static void cpufreq_policy_free(struct cpufreq_policy *policy)
> kfree(policy);
> }
>
> +static void update_policy_cpu(struct cpufreq_policy *policy, unsigned int cpu)
> +{
> + policy->last_cpu = policy->cpu;
> + policy->cpu = cpu;
> +
> +#ifdef CONFIG_CPU_FREQ_TABLE
> + cpufreq_frequency_table_update_policy_cpu(policy);
> +#endif
> + blocking_notifier_call_chain(&cpufreq_policy_notifier_list,
> + CPUFREQ_UPDATE_POLICY_CPU, policy);
> +}
> +
> static int __cpufreq_add_dev(struct device *dev, struct subsys_interface *sif,
> bool frozen)
> {
> @@ -1000,7 +1012,18 @@ static int __cpufreq_add_dev(struct device *dev, struct subsys_interface *sif,
> if (!policy)
> goto nomem_out;
>
> - policy->cpu = cpu;
> +
> + /*
> + * In the resume path, since we restore a saved policy, the assignment
> + * to policy->cpu is like an update of the existing policy, rather than
> + * the creation of a brand new one. So we need to perform this update
> + * by invoking update_policy_cpu().
> + */
> + if (frozen && cpu != policy->cpu)
> + update_policy_cpu(policy, cpu);
> + else
> + policy->cpu = cpu;
> +
> policy->governor = CPUFREQ_DEFAULT_GOVERNOR;
> cpumask_copy(policy->cpus, cpumask_of(cpu));
>
> @@ -1092,18 +1115,6 @@ static int cpufreq_add_dev(struct device *dev, struct subsys_interface *sif)
> return __cpufreq_add_dev(dev, sif, false);
> }
>
> -static void update_policy_cpu(struct cpufreq_policy *policy, unsigned int cpu)
> -{
> - policy->last_cpu = policy->cpu;
> - policy->cpu = cpu;
> -
> -#ifdef CONFIG_CPU_FREQ_TABLE
> - cpufreq_frequency_table_update_policy_cpu(policy);
> -#endif
> - blocking_notifier_call_chain(&cpufreq_policy_notifier_list,
> - CPUFREQ_UPDATE_POLICY_CPU, policy);
> -}
> -

But I would have solved it differently :)

We don't really need to call update_policy_cpu() again and again
as we don't really need to update policy->cpu...

Rather it would be better to just move following inside