cpufreq: apple-soc: Fix possible null pointer dereference

Check if policy is NULL before dereferencing it.

This is similar to the commit cf7de25878a1 ("cppc_cpufreq: Fix possible null pointer dereference").

This is found by our static analysis tool KNighter.

Signed-off-by Chenyuan Yang <[email protected]>

Fixes 6286bbb40576 ("cpufreq: apple-soc: Add new driver to control Apple SoC CPU P-states")

---
 drivers/cpufreq/apple-soc-cpufreq.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

drivers/cpufreq/apple-soc-cpufreq.c +1 -1

index 4994c86feb57..3de9bb2b0f22 100644
@@ -135,10 +135,14 @@ static const struct of_device_id apple_soc_cpufreq_of_match[] __maybe_unused = {
 static unsigned int apple_soc_cpufreq_get_rate(unsigned int cpu)
 {
 	struct cpufreq_policy *policy = cpufreq_cpu_get_raw(cpu);
-	struct apple_cpu_priv *priv = policy->driver_data;
+	struct apple_cpu_priv *priv;

Quoted 6 lines · Chenyuan Yang

Check if policy is NULL before dereferencing it.

This is similar to the commit cf7de25878a1 ("cppc_cpufreq: Fix possible null pointer dereference").

No, it's not similar. The patch you refer to actually introduces bugs by returning -ENODEV in functions that have an unsigned return type.

Quoted 1 line

This is found by our static analysis tool KNighter.

I'm surprised that your tool hasn't found the above, because it should be a pretty easy thing to do.

Irrespective of this, it would be good to describe under which circumstances this can occur, because I can't see *how* this can trigger. The policy is directly provided by the core code and provide its association with a cpu, and is never NULL at the point of init.

And if it can trigger, why only fix this one particular case? Dereferences of policy are all over the map, and would be just as wrong.

Quoted 17 lines

Signed-off-by Chenyuan Yang <[email protected]>

Fixes 6286bbb40576 ("cpufreq: apple-soc: Add new driver to control Apple SoC CPU P-states")

---
 drivers/cpufreq/apple-soc-cpufreq.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

drivers/cpufreq/apple-soc-cpufreq.c +1 -1

index 4994c86feb57..3de9bb2b0f22 100644
@@ -135,10 +135,14 @@ static const struct of_device_id apple_soc_cpufreq_of_match[] __maybe_unused = {
 static unsigned int apple_soc_cpufreq_get_rate(unsigned int cpu)
 {
      struct cpufreq_policy *policy = cpufreq_cpu_get_raw(cpu);
-     struct apple_cpu_priv *priv = policy->driver_data;
+     struct apple_cpu_priv *priv;

drivers/cpufreq/apple-soc-cpufreq.c static const struct of_device_id apple_soc_cpufreq_of_match[] __maybe_unused = { · line 148

Quoted 10 lines

drivers/cpufreq/apple-soc-cpufreq.c +4 -0

      struct cpufreq_frequency_table *p;
      unsigned int pstate;
 
+     if (!policy)
+             return 0;
+     priv = policy->driver_data;
+
      if (priv->info->cur_pstate_mask) {
              u32 reg = readl_relaxed(priv->reg_base + APPLE_DVFS_STATUS);
 

So while this is not wrong, I don't think this serves any real purpose.

Thanks,

        M.

--

Quoted 15 lines · Marc Zyngier

Quoted 6 lines · Chenyuan Yang

Check if policy is NULL before dereferencing it.

This is similar to the commit cf7de25878a1 ("cppc_cpufreq: Fix possible null pointer dereference").

No, it's not similar. The patch you refer to actually introduces bugs by returning -ENODEV in functions that have an unsigned return type.

Quoted 1 line

This is found by our static analysis tool KNighter.

Quoted 7 lines

I'm surprised that your tool hasn't found the above, because it should be a pretty easy thing to do.

Irrespective of this, it would be good to describe under which circumstances this can occur, because I can't see *how* this can trigger. The policy is directly provided by the core code and provide its association with a cpu, and is never NULL at the point of init.

Our tool currently identifies bug patterns by analyzing patches. For example, in the similar function cppc_cpufreq_get_rate(), a patch was applied to add a null check for the policy. Therefore, we assume that a similar check should be implemented here

Quoted 4 lines

And if it can trigger, why only fix this one particular case? Dereferences of policy are all over the map, and would be just as wrong.

It appears that similar checks are implemented in other areas—such as in acpi-cpufreq.c, cppc_cpufreq.c, drivers/cpufreq/cpufreq_ondemand.c, and drivers/cpufreq/cpufreq.c.
However, I'm not sure if apple_soc should adopt the same checking style.

Irrespective of this, it would be good to describe under which circumstances this can occur, because I can't see *how* this can trigger. The policy is directly provided by the core code and provide its association with a cpu, and is never NULL at the point of init.

And if it can trigger, why only fix this one particular case? Dereferences of policy are all over the map, and would be just as wrong.

So while this is not wrong, I don't think this serves any real purpose.

Quoted 11 lines · Marc Zyngier

Irrespective of this, it would be good to describe under which circumstances this can occur, because I can't see *how* this can trigger. The policy is directly provided by the core code and provide its association with a cpu, and is never NULL at the point of init.

And if it can trigger, why only fix this one particular case? Dereferences of policy are all over the map, and would be just as wrong.

So while this is not wrong, I don't think this serves any real purpose.

I have applied such patches in the past, considering the same as good practice. But I do understand your inputs.

And so I tried to see if there is actually a way to trigger this.

- Platform with two cpufreq policies (freq domains) with one CPU in each of them.
- Boot the kernel, policies will initialize for both the domains.
- Hotplug out CPU1, that will remove the policy as well.
- Call cpufreq_quick_get(1), this will call the ->get() callback for CPU1, for which there is no policy available.

But this is the case only for drivers with setpolicy callback, this shouldn't happen on apple-soc.

I am not sure now if we should just apply this patch to be safe, or leave it as is. The cpufreq core may change in the future and call the get callback for all drivers.