[PATCH v3] cpufreq: schedutil: Fix rate limit overflow

[Hui Su]

rate_limit_us is an unsigned int, while NSEC_PER_USEC is defined as
1000L. On 32-bit systems, the multiplication is therefore performed
using 32-bit unsigned arithmetic before the result is assigned to
freq_update_delay_ns.

For example, writing 4294968 to rate_limit_us wraps the delay from
4294968000 ns to 704 ns. This makes schedutil update far more often
than configured.

Add sugov_update_rate_limit_us() to widen rate_limit_us to s64 before
converting it to nanoseconds. Use the helper when updating the tunable
through sysfs and when starting the governor, so both paths perform the
conversion without overflow.

Fixes: 9bdcb44e391d ("cpufreq: schedutil: New governor based on scheduler utilization data")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Hui Su <sh_def@xxxxxxx>
Reviewed-by: Zhongqiu Han <zhongqiu.han@xxxxxxxxxxxxxxxx>
---
Changes in v3:
- Add a comment explaining why rate_limit_us must be cast before
  multiplication.

v2: https://lore.kernel.org/r/20260806072656.1386351-1-sh_def@xxxxxxx

Changes in v2:
- Clarify why the multiplication uses 32-bit unsigned arithmetic on
  32-bit systems.
- Cast rate_limit_us to s64 to match freq_update_delay_ns.
- Add Zhongqiu's Reviewed-by tag.

v1: https://lore.kernel.org/r/20260805143942.805176-1-sh_def@xxxxxxx

 kernel/sched/cpufreq_schedutil.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index dff4ee04694c..614ff0d33c01 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -61,6 +61,17 @@ static DEFINE_PER_CPU(struct sugov_cpu, sugov_cpu);
 
 /************************ Governor internals ***********************/
 
+static void sugov_update_rate_limit_us(struct sugov_policy *sg_policy)
+{
+	/*
+	 * Cast rate_limit_us before multiplication to force 64-bit arithmetic.
+	 * Otherwise, on 32-bit platforms, both operands are converted to
+	 * 32-bit unsigned long and the multiplication may overflow.
+	 */
+	sg_policy->freq_update_delay_ns =
+		(s64)sg_policy->tunables->rate_limit_us * NSEC_PER_USEC;
+}
+
 static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
 {
 	s64 delta_ns;
@@ -606,7 +617,7 @@ rate_limit_us_store(struct gov_attr_set *attr_set, const char *buf, size_t count
 	tunables->rate_limit_us = rate_limit_us;
 
 	list_for_each_entry(sg_policy, &attr_set->policy_list, tunables_hook)
-		sg_policy->freq_update_delay_ns = rate_limit_us * NSEC_PER_USEC;
+		sugov_update_rate_limit_us(sg_policy);
 
 	return count;
 }
@@ -848,7 +859,7 @@ static int sugov_start(struct cpufreq_policy *policy)
 	void (*uu)(struct update_util_data *data, u64 time, unsigned int flags);
 	unsigned int cpu;
 
-	sg_policy->freq_update_delay_ns	= sg_policy->tunables->rate_limit_us * NSEC_PER_USEC;
+	sugov_update_rate_limit_us(sg_policy);
 	sg_policy->last_freq_update_time	= 0;
 	sg_policy->next_freq			= 0;
 	sg_policy->work_in_progress		= false;
-- 
2.43.0