[PATCH] cpufreq: zero-initialize policy cpumask before sysfs publication

[Zhongqiu Han]

cpufreq_policy_alloc() allocates policy->cpus with alloc_cpumask_var(),
i.e. without __GFP_ZERO, unlike the sibling related_cpus and real_cpus
masks. With CONFIG_CPUMASK_OFFSTACK=y the mask is a separate
kmalloc_node() allocation, so its bitmap holds whatever the slab allocator
left behind:

  cpufreq_online()
    cpufreq_policy_alloc()
      alloc_cpumask_var(&policy->cpus)    /* bitmap is uninitialized */
      kobject_init_and_add()              /* policy%u/ appears in sysfs */
    cpufreq_policy_online()
      cpumask_copy(policy->cpus, cpumask_of(cpu))  /* first valid value */

This leaves a window in which the sysfs attributes are already reachable
while policy->cpus is still garbage. show()/store() gate on
policy_is_inactive(), i.e. cpumask_empty(policy->cpus), so a non-zero
bitmap makes them run the attribute callbacks on a policy that is not
initialized yet.

Fix this by using zalloc_cpumask_var() for policy->cpus.

Fixes: 2fc3384dc75b ("cpufreq: Initialize policy->kobj while allocating policy")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Zhongqiu Han <zhongqiu.han@xxxxxxxxxxxxxxxx>
---
This patch is based on https://lore.kernel.org/all/83f87db8-fa13-41d0-ae52-544d22e228e6@xxxxxxxxxxxxxxxx/

 drivers/cpufreq/cpufreq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 0d0df986fa3d..3b1b2ed86cc3 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1249,7 +1249,7 @@ static struct cpufreq_policy *cpufreq_policy_alloc(unsigned int cpu)
 	if (!policy)
 		return NULL;
 
-	if (!alloc_cpumask_var(&policy->cpus, GFP_KERNEL))
+	if (!zalloc_cpumask_var(&policy->cpus, GFP_KERNEL))
 		goto err_free_policy;
 
 	if (!zalloc_cpumask_var(&policy->related_cpus, GFP_KERNEL))
-- 
2.43.0